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Secure UNIX Programming FAQ

Archive-name: unix-faq/programmer/secure-programming
Posting-Frequency: Every 15 days.

                 Secure UNIX Programming FAQ 

  Version 0.5 

  Sun May 16 21:31:40 PDT 1999 

  The master copy of this FAQ is currently kept at 

  The webpage has a more spiffy version of the FAQ in html. 

  This FAQ is also posted to (c.s.u) , 
  comp.answers , news.answers. 

  Please do not mirror this FAQ without prior permission. Due to the 
  high volume of readers I'm worried that old versions of the FAQ are 
  left to grow stale, consequently receive email based on fixed 

Additional Resources 

After receiving many comments, and suggestions I decided to make some 
more SUP FAQ related resources available. 

  A change log can be found at: 

  A moderated mailing list has been setup for the discussion of 
  Secure UNIX programming. You can find a copy of the announcement 

  I'm currently working on a terse reference guide. It will be made 
  available Real Soon Now in PostScript format. The reference can be 
  printed out and kept handy next to your can of cola. It contains, 
  tables, diagrams, and step by step instructions for various 
  operations mentioned in the FAQ. It will not be posted to Usenet, 
  and downloadable from the FAQ's website. Its "Real Soon Now" status 
  is very Real Soon Now. 


I, Thamer Al-Herbish reserve a collective copyright on this FAQ. 
Individual contributions made to this FAQ are the intellectual 
property of the contributor. 

I am responsible for the validity of all information found in this 

This FAQ may contain errors, or inaccurate material. Use it at your 
own risk. Although an effort is made to keep all the material 
presented here accurate, the contributors and maintainer of this FAQ 
will not be held responsible for any damage -- direct or indirect -- 
which may result from inaccuracies. 

You may redistribute this document as long as you keep it in its 
current form, without any modifications. Read -- keep it up to date 
please!! :-) 


This FAQ answers questions about secure programming in the UNIX 
environment. It is a guide for programmers and not administrators. 
Keep this in mind because I do not tackle any administrative issues. 
Try to read it as a guide if possible. I'm sorry it sounds like a bad 
day on jeopardy. 

At the risk of sounding too philosophical, this FAQ is also a call to 
arms. Over almost the last decade, a good six years, a movement took 
place where security advisories would hit mailing lists and other 
forums at astonishing speed. I think the veterans are all to familiar 
with the repetitive nature of these security advisories, and the 
small amount of literature that has been published to help avoid 
insecure programming. This text is a condensation of this movement 
and a contribution made to it, placed in a technical context to 
better serve the UNIX security community. As the Usenet phrase goes: 
"Hope this helps." 

Additions and Contributions 

The current FAQ is not complete. I will continue to work on it as I 
find time. Feel free to send in material for the Todo sections, and 
for the small notes I've left around. Also, compatibility is an issue 
I struggle with sometimes. The best I can do for some UNIX flavors is 
read man pages. Corrections/addendums for compatibility notes is 
greatly appreciated, and easily done as a collective effort. All 
contributions, comments, and criticisms can be sent to: 

Secure UNIX Programming FAQ <> 

Please don't send them to my personal mailbox, because I can keep 
things organized better with the above e-mail address. Also please 
try to be as concise as possible. Remember I will usually quote you 
directly if you have something to add. 

Finally, although the contributors list is currently short, the 
material in this FAQ did not pop out of my head in a pig-flying 
fashion. Attribution is given where applicable. If you feel any of 
this is unfair to something you have published, do let me know. The 
bibliography is found at the end. 

Special thanks to John W. Temples, Darius Bacon, Brian Spilsbury, 
Elias Levy, who had looked at some of the drafts of past material 
that made it into this FAQ. As usual, all mistakes are mine and only 

Also kudos to the people at for hosting Bugtraq all 
these years. The archive is invaluable to my research. 

Table of Contents 

  1) General Questions: 
    1.1) What is a secure program? 
    1.2) What is a security hole? 
    1.3) How do I find security holes? 
    1.4) What types of attacks exist? 
    1.5) How do I fix a security hole? 

  2) The Flow Of Information: 
    2.1) What is the flow of information? 
    2.2) What is trust? 
    2.3) What is validation? 

  3) Privileges and Credentials 
    3.1) What is a privilege and a credential? 
    3.2) What is the least privilege principle? 
    3.3) How do I apply the least privilege principle safely? 

  4) Local Process Interaction: 
    4.1) What is process attribute inheritance? Or why should I not 
    write SUID programs? 
    4.2) How can I limit access to a SUID/SGID process-image safely? 
    4.3) How do I authenticate a parent process? 
    4.4) How do I authenticate a non-parent process? 

  5) Accessing The File System Securely: 
    5.1) How do I avoid race condition attacks? 
    5.2) How do I create/open files safely? 
    5.3) How do I delete files safely? 
    5.4) Is chroot() safe? 

  6) Handling Input: 
    6.1) What is a "buffer overrun/overflow attack" and how do I 
    avoid it? 
    6.2) How do I hand integer values safely? 
    6.3) How do I safely pass input to an external program? 

  7) Handling Resources Limits: [ Todo ] 
  8) Bibliography 
  9) List of Contributors 

    1) General Questions 

      1.1) What is a secure program? 

      The simplest definition would be : a program that is capable of 
      performing its task withstanding any attempts to subvert it. 
      This extends to the attribute of "robustness." Most importantly 
      the program should be able to perform its task without 
      jeopardizing the security policies of the system it is running 
      on. This is done by making sure it adheres to local security 
      policies at all times. To draw an analogy, a locksmith will 
      install a lock, and the home owner will decide whether or not 
      he will lock the door at any given time. It is the lock smith's 
      responsibility to make sure the lock performs its function of 
      keeping an intruder out. It is just as much the responsibility 
      of the programmer to make sure the program adheres to the local 
      security policies. Thus returning to the introduction, this FAQ 
      is about the programmer's responsibilities and not the 

      The problem with that analogy is that when it is translated 
      back into UNIX terms one thinks of an authentication program. 
      By all means 'login' needs to be secure, but so do all the 
      system components. To quote the U.S. Department of Defense 
      Trusted Computer System Evaluation Criteria (a.k.a The Orange 

      "No computer system can be considered truly secure if the basic 
      hardware and software mechanisms that enforce the security 
      policy are themselves subject to unauthorized modification or 

      Unfortunately this doesn't really help because we are sadly 
      left thinking of firewalls, access control lists, persistent 
      authentication systems etc. and we miss out on the other system 
      components that must also be considered. So the quote can be 
      re-written as such: 

      "No computer system can be considered truly secure if the basic 
      hardware and software mechanisms that _can affect_ the security 
      policies are themselves subject to unauthorized modification or 

      This gives us a much better view of what a secure program is, 
      and places a distinction between a secure program and a 
      security program. The security program enforces security 
      policies; however, the secure program does not enforce any 
      policies but must also co-exist with the security policies. 
      This allows a much broader view of every program on the system. 
      All the applications, and all the servers, and all the clients 
      must be implemented securely. Granted that this approach is a 
      bit extremist, it is actually quite reasonable. Programming 
      securely should always be done as will be seen by some of the 
      points brought up in this FAQ. 

      Finally, to finish this definition, consider a Mail User Agent 
      (MUA), such a 'pine' or 'elm.' Both have to be written securely 
      because they can affect the security policies if they were not. 
      In light of an advisory posted to Bugtraq (Zalewski 1999), pine 
      was reported to have a security hole. Even though it is not 
      enforcing security policies it still failed to adhere to them. 

      1.2) What is a security hole? 

      The term is somewhat colloquial but it has been used in 
      technical context enough times to warrant common usage in 
      security advisories. It just means the program has a flaw that 
      allows an attacker to "exploit it." Thus comes the "exploit" 
      that denotes a program, or technique to take advantage of the 
      flaw, or "vulnerability." The terms mentioned here will be 
      found in many advisories, and in this FAQ so familiarity with 
      them is essential. 

      1.3) How do I find security holes? 

      Careful auditing of source code is usually the way. One way of 
      doing it is going through this FAQ in its treatment toward 
      specific security holes and attempt to find them throughout the 
      code in question. I will attempt to give tips toward finding a 
      said security hole where applicable. 

      However, if you really really need to find that security hole, 
      disassemble the binary image of the program, grok the asm 
      output into your head, run it slowly but carefully keeping 
      track of registers, stacks etc -- and yes grasshopper, that is 
      the One True Way. 

      1.4) What types of attacks exist? 

      There are three main types of attacks (Saltzer 1975): 

          Unauthorized release of privileged information. 

          Unauthorized modification of privileged information. 

          Denial of service. 

      The word unauthorized speaks for itself. If information can be 
      read, or modified when it should not have been, security has 
      been breached. A denial of service attack is any attack that 
      stops a program from performing its function. When considering 
      whether a program is secure from its design, provisions for 
      these three attacks need to be accounted for. 

      Obviously these attacks are aggregates of the more specific 
      ones that exploit security holes. But that should give you an 
      idea of what you're looking out for. 

      1.5) How do I fix a security hole? 

      Traditionally there are three approaches to fixing a security 
      hole. At the risk of going slightly off topic, let us go back 
      to the heyday of the SYN flood attack (daemon9 1997). 

      SYN flooding is when a host sends out a large number of TCP/IP 
      packets with an unreachable source address, and a TCP flag of 
      SYN. The receiving host responds and awaits for the SYN-ACK to 
      complete the three-way handshake. Since the source address is 
      unreachable the receiving host never receives a response to 
      complete the handshake. Instead it is left in a "half open" 
      state till it times out. The problem is that there is a finite 
      number of "slots" per connection received on the listening 
      socket. This is because the host needs to store information in 
      order to recognize the last part of the TCP three-way 
      handshake. This results in a denial of service where the 
      receiving host would simply stop accepting new connections till 
      the bogus half-open connections timed out. They are called 
      half-open connections because the handshake is never completed. 

      Interestingly enough several different approaches were used to 
      solve this problem: 

      Cisco Systems Inc., implemented a TCP Intercept feature on its 
      routers. The router would act as a transparent TCP proxy 
      between the real server, and the client. When a connection 
      request was made from the client, the router would complete the 
      handshake for the server, and open the real connection only 
      after the handshake has completed. This allowed the router to 
      impose a very aggressive strategy for accepting new 
      connections. It would place a threshold on the amount of 
      connection requests it would handle: If the amount of half-open 
      connections exceeded the threshold it would lower the timeout 
      period interval, thus dropping the half-open connections 
      faster. The real servers were completely shielded while the 
      routers took the brunt and handled it aggressively. 

      The OpenBSD developers implemented a work-around that caused 
      old half-open TCP connections to be randomly dropped when new 
      connection requests arrived on a full backlog. This allowed new 
      connections to be established even with a constant SYN-flood 
      taking place. The old bogus connections would be dropped at the 
      behest of a new connection, legitimate or not. The randomness 
      was implemented to be fair to all incoming connections. 
      Although arguably with a large enough flood this technique may 
      fail, it did have good results as tested by the developers. 

      Dan Bernstein (Bernstein 1996; Schenk 1996) proposed SYN 
      cookies, which would eliminate the need to store information 
      per half open connection. When a connection is initiated, the 
      server generates a cookie with the initial TCP packet 
      information. The server would then respond with the cookie. 
      When the client responded with a SYN-ACK to complete the 
      handshake, the server would redo the hash, this time with the 
      information taken from the recent SYN-ACK packet. This would of 
      course entail decrementing the sequence numbers since they have 
      been incremented in the client response. If the new hash 
      matched that of the returned sequence numbers, the server would 
      have completed the three-way handshake. Only the secret was 
      stored, the rest of the information is gathered from the 
      incoming packets during the handshake. This meant only one 
      datum for all incoming connections. Thus an infinite amount of 
      half open connections could exist. 

      Three different methods were used. Cisco used a "wrapper." The 
      actual UNIX system was completely unconcerned with what the 
      router did and required no modification. This is good for a 
      scalable solution, but does not remove the problem entirely. 
      The wrapper just acts as a canvas. 

      The OpenBSD solution was to fix the problem in the 
      implementation itself. This is usually the case with most 
      security holes, especially the less complicated ones. 

      The solution presented by Dan Bernstein was more of a design 
      change. The system's responses were changed, but remained 
      reasonably well in conformance with the TCP standard. Some 
      compromises were made however (see Schenk 1996). 

      There is no one True Way of fixing security holes. Approach the 
      problem first in the code, then design, and finally by wrapping 
      it if you really must. 

    2) The Flow Of Information 

  Although what is presented here is a bit cross platform and not 
  UNIX dependent, it is so essential that I had to put it in its own 

      2.1) What is the flow of information? 

      Every program can be considered to follow a simple design: it 
      accepts input, processes it, and produces output. Input may 
      come from the keyboard, a file, or the network. As long as it 
      is gained from an external source that is not part of the 
      program, it is considered input. Output is not necessarily 
      information printed on the screen, or in a log file, it may be 
      an affect like the creation of a file. The processing may be 
      any work from simple arithmetic, to parsing strings. 
      Mathematically speaking, at least, your program should really 
      be a function taking variables and producing results. This 
      cycle may happen more than once during a program's lifetime. 

      Most programmers, for purposes of keeping things simple, will 
      make assumptions about input. Particularly its format, and 
      whether to apply sanity checks. There are probably entire books 
      on doing this correctly: designing your program correctly, 
      picking the right data formats and so on. This FAQ isn't 
      interested in that aspect of processing information. Instead it 
      is interested in three things: trust, validation, and acting on 

      2.2) What is trust? 

      When trust is given to an external source of input, a program 
      accepts information from it while considering the information 
      valid. Secure programs need to be very untrusting and always 
      validate information gained from external sources. Some 
      programs, such as Dan Bernstein's 'qmail' distrust information 
      gained from within. Usually trust is only given to an external 
      source after it has been authenticated. Take the login program 
      under UNIX. Once authenticated the user is trusted to do 
      whatever he wishes to do under his own credentials. Although 
      this example fails because the login program vanishes and is 
      replaced by a shell, you get the idea. 

      As a general rule: any information than an attacker can 
      manipulate cannot be given trust. For example: 

      In March 1994 Sun Microsystems released a security update for 
      SunOS 4.1.x that fixed a security hole related to "/etc/utmp". 
      The file acted as a database that keeps track of current users 
      logged onto the system with additional information such as the 
      terminal, and time of login. Certain daemons such as comsat, 
      and talkd, would access the file to retrieve the terminal name 
      associated with a user. The terminal name would consist of the 
      path to the terminal device. The daemons would open the file 
      specified by the path, and write to it. Users could modify the 
      file, because it was world writable, and set arbitrary file 
      names for the terminal. This resulted in potentially having the 
      daemons open sensitive files while running with special 
      privileges, and writing to them at the behest of the attacker. 
      This is a good example of trusting information that can be 
      manipulated by an attacker. 

      2.3) What is validation? 

      When information is received from an untrusted source it must 
      be validated prior to processing it. In the case of the 
      aforementioned talkd hole, the daemon should have made sure the 
      path to the terminal file was indeed correct. This could have 
      been done by simply checking the password database, making sure 
      the ownership matched, and that the terminal path did indeed 
      point to a terminal. Later in the FAQ, the concept of the least 
      privilege principle is explained, and it would have worked 
      wonders with the aforementioned security hole. 

      There are several ways you can validate information depending 
      on what it is supposed to be. A good place to start is by 
      defining its attributes. Is it supposed to hold a file name? 
      Does the file exist? Is the user allowed access to that file? 
      That as mentioned previously is what the talkd daemon should 
      have done. In the "Handling Input" section a security hole 
      found in SSH(van der Wijk 1997; Al-Herbish 1997)) will be 
      brought up where privileged ports could be bound to by normal 
      users. In that particular case the function binding to a port 
      did not properly check to make sure the port number was not > 
      1024, and as such the attacker could bind to privileged ports; 
      however, the security hole entailed another error on the part 
      of the program that is discussed in more detail in the coming 

      2.4) What do you mean by "acting on input"? 

      [ I need a more formal term for it. Unfortunately I'm lost for 
      words. ] 

      When you pass input directly to a system call, external 
      program, memory copying routine etc. Basically you perform an 
      operation with the aid of the information. In the 
      aforementioned talkd hole the pathname read from the utmp 
      database was passed to a file opening system call directly. The 
      program assumed it was valid, and would not be malicious. This 
      is a wrong assumption. 

      PERL supports "tainting" (Wall, Schwartz 1992). All input 
      passed from an external source is tainted unless explicitly 
      untainted. Any tainted input that is passed directly to a 
      system call results in an error. This method of validation is 
      quite ingenious. Regardless of whether or not you are using 
      PERL, the methodology is a good one to follow. 

    3) Privileges and Credentials 

      3.1) What are privileges and credentials? 

      Every process under UNIX has three sets of credentials: Real 
      credentials, effective credentials, and saved credentials. The 
      credentials are split into two groups, user and group 
      credentials. Additionally the process has a list of 
      supplemental group credentials. The different "set*id()" system 
      calls allow a process to change the values in these sets. Only 
      the root user can change them arbitrarily. Non-root users are 
      limited to what they may change their credentials to. 

      It is essential to know how the system calls work on the 
      credential sets (see Stevens 1992 for a more exhaustive 
      reference). The following table lists each system call, what 
      credential set it affects, and what credentials it will allow 
      the process to change into. The credential sets are abbreviated 
      with RUID standing for real user ID, EGID for effective group 
      ID, SVUID for the saved user ID. ( Self explanatory really.) 

      System Call  Changes                  Can change to            

      setuid       RUID EUID SVUID          RUID EUID SVUID          

      setgid       RGID RGID SVGID          RGID RGID SVGID          

      setreuid     RUID EUID                RUID EUID                

      setregid     RGID EGID                RGID EGID                

      setruid      RUID                     RUID EUID                

      setrgid      GUID                     RGID EGID                

      seteuid      EUID                     RUID EUID                

      setegid      EGID                     RGID EGID                

      Make sure you've read the man pages, and just use the table for 
      reference. When changing credentials make sure you change the 
      right ones. 

      The credentials are checked by the kernel for access control. A 
      process is considered privileged if its credentials give it 
      access to privileged information, or privileged facilities. 
      This FAQ will make use of three main privilege levels: 

          Special User -- The root user. 

          Normal User -- A local user without root privileges. 

          Anonymous User -- A user that has not been authenticated, 
          or logged, into the local system. 

      The definitions above are a bit misleading without some 
      elaboration. The root user is considered special because the 
      kernel gives him special abilities; his access to files is not 
      limited by file permissions; he can bind to privileged ports; 
      he can change resource limits; he can arbitrarily change his 
      own credentials lowering them to any other credential; he can 
      send signals to any other process, and on some UNIX flavors 
      trace any other process. Although there are some other special 
      abilities the root user has, the list consisted of some of the 
      more important abilities. However, on certain systems 
      privileged information is accessible by non-root users. For 
      example, on SCO 5.0.4 the passwd database is accessible by any 
      user in group "auth." Thus non-root users in that group can 
      still access privileged information. In the case of SCO 5.0.4 
      it is also modifiable by users in that group. The astute reader 
      will note that modifying the password database can effectively 
      lead to modifying one's credentials. So keep in mind that the 
      usage of special privileges in this FAQ is meant to encompass 
      any user who has special abilities that are not conferred upon 
      other local users. This may seem ambiguous but I hope the 
      definition serves its purpose well. 

      The normal user has been authenticated, but is regarded as 
      normal without any special privileges. The traditional UNIX 
      kernel itself without any augmentation will not recognize any 
      user except for the root user. The special user and the normal 
      user have both been authenticated, but the special user is 
      recognized to have higher privileges. 

      The anonymous user is one who has not been authenticated. It is 
      very important to recognize this user when network applications 
      are written. For example, consider an FTP server using the 
      "anonymous" user open to everyone. In the same way consider the 
      FTP client that connects to the FTP server. The client is run 
      by a local "normal user," (or special user if the admin is 
      nutty enough) but it is connecting to a completely anonymous 
      entity. It must not give the server any special abilities on 
      the local system, and allow only a set of abilities such as 
      writing to a predefined file (downloading from the server). 
      Indeed some advisories discussed the simplest of programs such 
      as 'tar' (Tarreau 1998;Der Mouse 1998) where the tar archive 
      itself could subvert the application into unauthorized 
      modification of privileged information. 

      Depending on the privilege level, the application must take 
      into account what the privilege allows the entity to do. 
      Consider a web server that allows clients to browse the entire 
      file system under a normal user ID (or the username 'www'). The 
      web server should still not allow the client to browse just any 
      file or it has given away part of the normal privileges to 
      every user on the net. 

      3.2) What is the least privilege principle? 

      When an application runs with higher privileges than the source 
      of input, it can prevent the occurrence of security holes by 
      only using the higher privileges for specific tasks. This is 
      known as the least privilege principle (Saltzer 1975), because 
      the lowest privileges are used during the program's execution. 
      If the attacker is able to trick the program into accomplishing 
      a specific task, it will do so with his privileges. 

      Most UNIX flavors come with a utility that allows the user to 
      change his personal information. It is usually called chfn. The 
      information is copied to a temporary file from the password 
      database. The utility then forks a child process which executes 
      an editor on the temporary copy. The user is subsequently given 
      control of the editor and is free to modify the copy. Once the 
      user completes modifying the copy and exits from the editor, 
      the utility reads the temporary copy, performs any sanity 
      checks on the input, and copies it back to the password 
      database. The least privilege principle must be applied in this 
      case. The child process running the editor cannot do so with 
      special privileges. The editor may allow the user to run a 
      shell, or open other files. chfn must revert the privilege to 
      that of the user in the child process before executing the 

      A security hole that was reported concerning XFree86 (plaguez 
      1997) The server would run with root privileges and read any 
      configuration file specified from a command line option. The 
      advisory demonstrated how the shadowed password database could 
      be read by pointing the server to it as its configuration file. 
      Since the server ran with root privileges it could open the 
      database, and would inadvertently output its contents as part 
      of its error reporting. Thus an attacker could read files he 
      would not normally be able to. Had the X server used the same 
      privileges as the end user when attempting to read the 
      configuration file, it would not have been able to. The 
      attacker would only be able to access files readable by him. 
      The file opening operation should have been done with the least 
      privilege principle. 

      3.3) How do I apply the least privilege principle safely? 

      The least privilege principle can be applied by either lowering 
      privileges temporarily, or completely dropping privileges so 
      that they will never be regained again. However, there are 
      viable attacks that can occur from both operations. Also 
      lowering privileges is not always enough without doing away 
      with privileged information. 

      Note on saved credentials 

      Before discussing the details of lowering credentials properly, 
      the saved credential set needs to be elaborated upon. The saved 
      credential set is initialized to the effective credentials of 
      the process at the time of its execution. So if the 
      process-image has a set-id-on-execution (SUID) or 
      set-group-id-on-execution (SGID) bit set, the saved credentials 
      will match that credential. This is very useful if the program 
      wishes to temporarily drop its effective credentials and then 
      regain them. 

      Lowering privileges temporarily entails changing one of the 
      credential sets, usually the effective credentials because they 
      are most often checked by the kernel. The seteuid() and 
      setegid() system calls allow a process to set its effective 
      credentials to its real credentials or its saved credentials. 
      This is where the switching between the two credential sets 
      becomes very useful. A SUID or SGID process can change its 
      effective credentials to its real credentials, which are 
      inherited from the parent process, and then switch them back to 
      its saved credentials which it inherits from the SUID or SGID 
      file permission. In doing so the SUID or SGID program is 
      toggling its privileges between its caller and the 
      process-image owner. 

      Because a process cannot get its saved credentials via any 
      system call, it is recommended to do a geteuid() and getegid() 
      at the beginning of execution and store them internally. This 
      works because the saved credentials are an exact copy of the 
      effective credentials at the start of a process' execution. 
      This will work: saved_uid = geteuid(); saved_gid = getegid(); 

      To change effective credentials to the saved credentials do a 
      setegid(saved_gid); seteuid(saved_uid); Now to switch them back 
      to match the real credentials do a setegid(getgid()); 
      seteuid(getuid); Simple and straight forward. 

      The second method of applying the least privilege principle is 
      to completely drop privileges and never regain them again. 
      Recall the chfn example mentioned in question 3.2? It would 
      have to drop the privileges in its child process completely 
      because it gave the user control of the child process. This is 
      done by calling setgid() and then setuid(). A common mistake is 
      to drop the user ID first, and this will fail if the process is 
      relying on the fact that it has root privileges! 

      There are, as mentioned earlier, viable attacks. The first is 
      the signal attack. BSD derived operating systems allow a 
      process to send a signal to another process if: 

          The real user ID of process A is that of the root user. 

          The real user ID of process A matches the real user ID of 
          process B. 

          The effective user ID of process A matches the effective 
          user ID of process B. 

          The real user ID of process A matches the effective user ID 
          of process B. 

          The effective user ID of process A matches the real user ID 
          of process B. 

          Both processes share the same session ID. 

      With those semantics it is obvious that if a process lowered 
      its effective credentials to that of the user, he would be able 
      to send it a signal. In the event that the process begins to 
      run with the same real credentials as the user (all SUID or 
      SGID processes start out this way), it should change its 
      credentials if it expects to trust signals. Keep in mind that 
      by lowering its effective credentials to that of the user's 
      real credentials it _is_ susceptible. This access check on 
      signals is quite a mishmash. Also, change the session ID via 

      In April 1998, a Bugtraq posting discussed the circumvention of 
      a protection scheme employed by implementations of the BSD ping 
      utility (Sanfilippo 1998) The ping utility would use the alarm 
      routine to synchronize the periodical sending of Internet 
      Control Message Protocol (ICMP) echo requests to a remote host, 
      and would not allow the normal user to send requests repeatedly 
      in a flooding manner. The protection scheme was simply there to 
      prevent abusive users from flooding other hosts with a large 
      number of ICMP echo requests. The normal user, of course, 
      cannot send an ICMP packet because performing this task 
      requires the use of a raw socket. Only the root user can open a 
      raw socket because of the security implications associated with 
      raw network access -- receiving incoming packets rawly from the 
      network, and sending raw packets into the network. Thus the 
      ping utility is normally installed as SUID to root. The 
      technique Sanfilippo used to get around ping's security 
      mechanism was to constantly send the SIGALRM signal to it, 
      subverting the protection scheme it attempted to implement. 
      Since the alarm routine would schedule an occurrence of SIGALRM 
      after a specified interval, the ping utility would have a 
      signal handler for it, that sends the ICMP echo request. 
      Obviously the process may not install handlers and act on them 
      blindly if an attacker can trigger the signal handlers. 

      Some UNIX flavors support the SA_SIGINFO option that can set 
      when setting the signal handler via 'sigaction'. This passes 
      the handler additional information with regards to who sent the 
      signal, and whether or not it is kernel generated. Another 
      method is by using internal sanity checks. In the case of 
      'ping' this could have been done by simply keeping track of the 
      time that passed in between signals being generated and not 
      honoring them unless a sufficient amount of time had passed. 

      However, a worse case would be a SIGTERM or SIGKILL that halts 
      a process when it is in between a critical state. In the case 
      of 'chfn' it would be downright despicable of a user to halt it 
      just as it was writing out the new password file. If a process 
      is in an "unclean state" it should not allow itself to be 
      halted by an attacker and retain higher privileges untill the 
      point whence it can afford to be halted. 

      A common mistake is to assume that a process with lowered 
      credentials is no longer a security hazard. In fact it just 
      might be, even with the previous attacks accounted for. 

      A well known, but ancient, technique of getting the password 
      file from an old SunOS box was to cause its ftp daemon to dump 
      core. Similar security holes were later reported (Temmingh 
      1997). If a privileged process reads the password database into 
      memory and is then caused to dump core because of a signal 
      attack, the core image may hold a copy of the password file 
      which is then easily readable by the attacker. But cleaning up 
      internal memory may not be enough. A security hole was found on 
      OpenBSD's chpass utility with file descriptor leakage (Network 
      Associates Inc. 1998). The child process was passed a 
      privileged file descriptor because the descriptor was never 
      properly closed before giving the user control over the 

      Finally, process tracing attacks may take place. FreeBSD, and 
      NetBSD both allow a process to trace any process with a 
      matching real user ID. Tracing implies complete control over 
      the process, including the file descriptors, memory, and 
      executable instructions etc; however, a process may not be 
      traced if it is SUID or SGID. 

      Here's a check list for lowering real and effective 

      Lowering Effective Credentials 

          The process should not have any cleaning up to do. The 
          state of external objects should be in a form that is 
          suitable for reuse. This includes lock files, updates to 
          databases, and even temporary files. 

          All signal handlers that may be triggered should not be 
          trusted; they must be validated for authenticity. 

          All privileged information held in the process memory 
          should be cleared so that a core dump will not contain them 
          (don't just free up dynamic privileged memory, clean it out 
          before freeing). 

      Lowering Real Credentials: 

      Previous steps must be followed as well. Additional steps take 
      into account the process tracing attacks which are not viable 
      on all systems. 

          Privileged information may not be held by the process. This 
          includes file descriptors or sockets referencing privileged 

          The effective credentials should be dropped to the real 
          credentials as well, since a process that is traced can be 
          forced to execute arbitrary code under this effective 

    4) Local Process Interaction: 

      4.1) What is process attribute inheritance? Or why should I not 
      write SUID/SGID programs? 

      Process attribute inheritance (AFAIK a term I coined), is when 
      a child process inherits attributes from the parent process' 
      environment. I did see this referred to as "state variables", 
      but I forgot by who and all searching has led nowhere. The 
      problems with process attribute inheritance were fore shadowed 
      by the 'ping' security hole mentioned in question 3.3, as well 
      as the OpenBSD 'chpass' hole mentioned in that section. 

      A child process is an exact copy of its parent except for the 
      process ID and the parent process ID. These change for obvious 
      reasons. However, all other attributes are the copied with the 
      exception of file descriptors. File descriptors, however, are 
      shared. (For a more exhaustive explanation see Stevens 1992). 

      A process is executed after a call to execve() or one of the 
      other routines in its family. This system call filters out many 
      of the process attributes, but lets some through. This is 
      considered a UNIX "feature" and is relied upon by daemons such 
      as inetd. Keep in mind that a SUID process is a child that has 
      been execve()'d, so it does inherit these attributes. Before I 
      present a list of process attributes (albeit probably an 
      incomplete list), some known security holes will be discussed 
      to illustrate the types of attacks that can occur (Bishop 

      A post was made to Bugtraq that discussed a weakness in a 
      popular Mail User Agent package elm because it trusted an 
      environment (Jensen 1994) variable. An autoreply utility was 
      packaged with elm that would run with special privileges, and 
      perform its own internal checks to prevent exploitation by the 
      user. One of the checks included making sure a user did have 
      read access to an arbitrary file before allowing him to read it 
      (can you see what's wrong with this picture?). If the full path 
      to the file was not specified, the utility simply prepended 
      whatever was in the HOME environment variable, and opened the 
      file for reading, without performing any checks. This allowed 
      users to read files with the same privileges as the autoreply 
      process. The mistake was to assume that environment variables 
      can be trusted for valid information, and that the files in a 
      user's home directory are his to read. Both these assumptions 
      are false. Environment variables are inherited from a parent 
      process, and thus can contain arbitrary information. The 
      attacker can manipulate environment variables before forking 
      the child process. It is also not true that the user would have 
      access to the files in his home directory, necessarily. This 
      was a tragic case of giving trust to information that can be 
      manipulated by an attacker. 

      In December 1993, Sun Microsystems released a security 
      bulletin, which among other subjects brought up a weakness in 
      the modload and loadmod system utilities. The weakness was 
      trusting the Internal Field Separator (IFS) environment 
      variable. Since the shell would use the IFS to split the 
      contents of variables after they are expanded, the attacker can 
      specify how the contents are split. A path name such as 
      "/bin/cat" , could indeed cause the file "bin" to be executed 
      if the IFS is set to '/'. This is because the character '/' 
      would be recognized as a field to separate the other strings in 
      the variable. Since both these utilities would call upon a 
      shell during their execution, the attacker could arbitrarily 
      trick the utilities to run his process-images with their 
      privileges by modifying the IFS variable. We might think that 
      shells should not make use of the IFS variable if the shell is 
      not run in interactive mode. This is not a solution, since this 
      only canvases the problem of passing down harmful process 
      attribute. The solution is not to pass the variable in the 
      first place. If the aforementioned utility would not have 
      called a shell, it may still have encountered problems with 
      other child processes. For example the LD_PRELOAD environment 
      variable is used by the run-time linker to load code from any 
      shared library the variable specifies. Although the run-time 
      linker will ignore such environment variables for SUID or SGID 
      processes, the child process of a privileged process may 
      inherit them nonetheless and not have such protection. Since 
      the child process in turn inherits its parent's privileges, the 
      parent is effectively compromised through the child process. So 
      remember, the SUID process may have children that are passed 
      down harmful environment variables that would not affect the 
      SUID program necessarily but affect its children. 

      At this point I must concede that there may very well be 
      implementations of run-time linkers (read: haven't done the 
      research yet) that do not make the mistake of using the 
      LD_PRELOAD variable even in the child process of a SUID or SGID 
      process. Nonetheless, why risk it on an old box? 

      Finally a reminder of the OpenBSD chpass hole, and the 
      descriptor leak. If you haven't read it in section 3.3, go and 
      read it. 

      Thus four types of attacks are viable: 

          Child process trusts process attribute to contain valid 
          information. (elm hole) 

          The process attribute affects the child process directly. 
          (ping hole) 

          A process inherits an attribute and passes it down to a 
          child process that is affected by it. This is the same as 
          the second attack, but it is the child process of the 
          secure process that is affected. (or the grandchild of the 
          attacker). (LD_PRELOAD attack) 

          The child process of the secure process is passed an 
          attribute containing privileged information. (chpass hole) 

      Now for the list of process attributes and how to go about 
      avoiding any security holes. 


          Just a review of section 3. Processes running with the same 
          credentials, or similar (see section 3!) can be attacked by 
          process tracing, or sent signals that affect them. 

          File Descriptors: 

          The 'chpass' security hole had a file descriptor leak. A 
          quick and easy way of avoiding file descriptor leaks is by 
          setting the FD_CLOEXEC flag on the descriptor (again, see 
          Stevens 1992 as he discusses this rather well). But that's 
          not the end of it. In 1998 the OpenBSD team released a 
          patch for OpenBSD which would not allow a SUID or SGID 
          program from inheriting empty file descriptors in the first 
          three slots. It would instead set them pointing to 
          /dev/null. Theo deRaadt mentioned to me one of the problems 
          that could occur: if the inherited descriptor would be 
          opened as a raw socket, and error reporting by the standard 
          C library (standard error) would be sent through it, bogus 
          packets could be sent to the socket. Although he did 
          mention for other reasons programs such as traceroute were 
          not susceptible. As a workaround for systems without this 
          security feature, doing a stat on the first three file 
          descriptors to check their availability and opening them to 
          "/dev/null" should do. It just adds bloat to your code, and 
          should really be handled by the kernel. 

          Environment Variables: 

          Don't trust environment variables to contain valid 
          information. In the case of the above mentioned 'elm' hole, 
          it would have been wiser to look up the home directory in 
          the password database. Another more subtle issue is not 
          placing privileged information in an environment variable 
          (Smith 1998). Specifically, a security hole related to 
          FreeBSD's 'ps' utility. The utility would allow users to 
          view another process' environment variables. Consequently 
          applications like pppd that accepted passwords via 
          environment variables became vulnerable to unauthorized 
          release of privileged information attacks. In fact, the 
          hole was not related to 'ps' if you think about it 
          critically. The application that places privileged 
          information in the environment variable is at fault. 

          Finally comes the security hole related to having an 
          environment variable affect an external program. IFS and 
          LD_PRELOAD, as discussed previously, are viable environment 
          variables, but a secure program should remove all 
          environment variables except the ones it chooses to use and 
          knows will not affect it. A good idea is to get rid of 
          every environment variable you don't need and keep the ones 
          you know are useful and actually have a use for. 

          File Mode Creation Mask: 

          Although it is very common for a robust program to reset 
          its file mode creation mask by calling umask(), it should 
          still be noted as a viable security concern. An attacker 
          can pass a mask that would affect the file permissions of 
          files created by open() and mkdir(). Resetting the mask to 
          0 suffices to prevent a file mode creation mask attack. 

          Working and Root Directories 

          Both the current working directory, and the root directory 
          are inherited from the parent process. The working 
          directory affects file system calls if they are not passed 
          a full path name. This can be made into an attack. Thus it 
          is advisable to both set the current working directory, and 
          use full paths when making file system calls. The root 
          directory should not be a concern. Only the root user can 
          change the root directory. 

          Resource Limits 

          The setrlimit() system call allows a process to set soft or 
          hard limits on its consumption of resources. When a process 
          reaches its soft limit a signal is sent depending on which 
          limit is reached. SIGSEV for the maximum stack size, 
          SIGXFSZ for an I/O operation that exceeded a file size 
          limit etc. The list is left till the section on resource 
          limits, but the signals if not handled will result in the 
          process being terminated. A careful attacker may trigger a 
          denial of service attack where the process is terminated in 
          the middle of performing a critical operation. When the 
          hard limit is reached, the process is prevented from 
          executing any further. The soft limits may be raised or 
          lowered at the process' own discretion. Thus by setting all 
          limits to infinity, the process can relieve itself of any 
          resource limit attacks. However, the hard limit can be 
          lowered by a normal user, but cannot be raised except by 
          root. A viable attack is to lower the hard resource limits, 
          and have the child process choke from underneath. The most 
          obvious solution is not to begin execution if the hard 
          limits are too low, and to heighten the soft limits to 
          infinity. Raising the soft limits over the hard limits will 
          not work. Thus the process will begin by specifying how 
          much of the individual resources it requires, and if the 
          setrlimit() system call returns an error it will not begin 
          execution as to avoid resource starvation. This is not very 
          helpful though, and better treatment of resource limits is 
          given in its own section below. 

          Scheduling Priority 

          The scheduling priority on a process can be modified by a 
          call to nice() or setpriority(). This is usually done to 
          tell the scheduler how important the process is. A very low 
          priority may cause the process to execute slower, which can 
          aid an attacker if he is attempting to exploit a race 
          condition.The semantics of setpriority() requires the user 
          to have an effective user ID matching the real or effective 
          user ID of the peer process. So this may not necessarily be 
          inherited. However, the priority of a process can only help 
          an attacker exploit a race condition more easily. It does 
          not really constitute a denial of resource attack unless 
          the process has time constraints. Regardless of the speed 
          of a process, a race condition can always be exploited. 
          Even with the element of luck, or by slowing down the 
          system as a whole. Race conditions need to be eliminated 
          and not made harder to exploit. Keep an eye out for this 
          scheduling priority in the rare case that it actually does 

          Interval Timers 

          Three interval timers that can be manipulated by the 
          setitimer() system call. The alarm facility is usually 
          wrapped around a call to setitimer(). Since the interval 
          timers are inherited, a parent process could set a timer to 
          send its child process SIGALRM, SIGVTALRM or the SIGPROF 
          signal. These could be used to either subvert a program if 
          it were to handle them, or terminate it. These signals 
          should be ignored at the beginning of the process' 
          execution or the timers should be reset. 

          Signal Handlers 

          Although all signal handlers are reset to their defaults, 
          blocked signals remain blocked and so do ignored signals. 
          This could be a problem if our program design is built 
          around receiving a signal before carrying on, and assumes 
          it is not blocked or ignored. Resetting the state of the 
          signal mask, and resetting signal handlers should also be 
          done without any preconceptions of default settings. 

      With all of this SUID/SGID programming is daunting at best. A 
      better technique (proposed by Thomas Ptacek on the newsgroup) 
      would be to use a server-client model where the server is 
      privileged but does not inherit the environment of its parent 
      process -- a would be attacker. That way the client runs with 
      no privileges, connects to the privileged server and passes the 
      relevant information through the IPC channel. 

      4.2) How can I limit access to a SUID/SGID process-image 

      The question may seem vague but sometimes it would seem 
      attractive to have a SUID process that is only executable by a 
      particular set of users. Some time ago I implemented a 
      distributed network monitoring package that had java clients 
      talk to it remotely, and sniffers running on different servers 
      (a very ambitious undertaking). The actual servers where run 
      under a special group called "sly" that would in turn have 
      access to a SUID process-image to do all the sniffing. The 
      child process ran as root, but could only be executed by users 
      in the group "sly." 

      At first this looks good. The actual server does not run with 
      special privileges, and it would seem that if it got exploited 
      the attacker would not gain root privileges. However, he would 
      gain privileges for the group "sly" that would let him sniff 
      the local network. If he was then able to exploit the sniffer, 
      he would gain root privileges. But he needs to exploit the 
      server to the point of executing arbitrary code on the machine. 
      Creating a file, or tricking it into sending privileged 
      information would not equate to gaining the privileges of group 
      "sly" necessarily. So this does not lessen the need for secure 
      code, but it _could_ in the long run lessen the chances of 
      complete compromise. I'm going to call this technique, at the 
      risk of coining yet another term, privilege segmentation. The 
      attacker may gain privileges to a specific group but has more 
      work to do in order to gain higher privileges. 

      Fair warning that security via "chance" or "hope" is not good. 
      I don't particularly like hearing about "risk management," and 
      the above technique is just that: risk management. 

      4.3) How do I authenticate a parent process? 

      Since the child process inherits the parent's real user ID, a 
      call to getuid() does the trick. Unfortunately due to a 
      misconception, some programmers are led to believe that 
      getuid() is not sufficient. This stems from the thinking that 
      if a user managed in exploiting a SUID process into running 
      another process, the child would have a real user ID matching 
      the parent process' effective user ID. This is not true, 
      because the real user ID keeps propagating from parent to child 
      regardless of the SUID feature. As mentioned in 2.1, the child 
      process inherits the credentials directly, a perfect copy with 
      the exception of the effective credentials if the SGID or SUID 
      feature. The saved credentials are also reset. But this 
      exception does not extend to the real credentials which are 
      directly inherited. 

      However, if the exploited SUID process was to change its real 
      user ID to match its effective user ID, which is easily done 
      with setuid(), then getuid() is not sufficient. There is a 
      logical fallacy here: if the parent process has already been 
      exploited to the point where the attacker can cause it to 
      switch credentials what's the point in faking it anymore? 
      Nonetheless, additional steps can be taken but not portably. On 
      systems where login information is stored in the kernel, and 
      not on the file system, by setlogin(), getlogin() will always 
      return the username associated with the session. [ FreeBSD 
      stores login information in kernel. ] 

      On OpenBSD, and FreeBSD the issetugid() system calls can be 
      used to find out if the current process is a SUID or GUID 
      process. This propagation continues unless a child process 
      clears all its privileges, to quote the OpenBSD man page "uid 
      != euid or gid != egid". So this system call may be used in 
      conjunction with getuid to be even more paranoid. 

      A good suggestion is to do a getuid(), check getlogin() if the 
      information is stored in the kernel, and finally do a 
      issetugid(). If all tests pass, you know you are talking to the 
      Real McCoy. In saying that, caution should not be thrown to the 
      wind. Using passwords, cookies, 
      insert-fancy-authentication-mechanism-here etc. is always 
      recommended, but the previous approach is the more light weight 
      kernel supported method. 

      4.4) How do I authenticate a non-parent process? 

      [ I could use writing on SO_PEERCRED (Linux) doors (Solaris) 
      and LOCAL_CREDS (NetBSD). Also I have some example code for the 
      techniques discussed below. If you tackle the any of the issues 
      above, you would be a very nice contributor to provide example 
      source. Also this is advanced stuff so I'll elaborate more when 
      I fix it up] 

      It is possible to authenticate processes via IPC channels 
      (Bernstein 1999). However the methods differ on different UNIX 
      flavors making it a very non-portable mess to write. 

      BSD derived systems support the concept of access rights 
      (Stevens 1990). The facility allows a process to pass a file 
      descriptor through a UNIX domain socket, and with a small hack 
      it can be used to authenticate a local process. The term hack 
      is only used because the facility was not intended for 
      authentication. However, if the client sends a descriptor 
      referencing a file that has read permissions only for its 
      owner,the receiver knows the sender is the owner. Thus the file 
      acts as an identifier. However, on systems where a user may 
      give away files with chown() this method cannot be used. The 
      attacker can simply create a file with read permissions only 
      for himself, open it, and then chown it to another user. 
      Fortunately this is a System V "feature," and on many systems 
      can be turned off as a kernel configuration option. 

      A similar technique is found under System V derived systems. 
      This is done by receiving file descriptors via a STREAMS file. 
      Only the file descriptor is discarded because the UID is passed 
      along with it. This is done by using an ioctl call with the 
      I_SENDFD and I_RECVFD flags on a streams file used as an IPC 
      channel. This technique does not suffer from the chown attack 
      because the credentials are passed _along_ with the descriptor. 

      BSD/OS, FreeBSD and other BSD derived operating systems also 
      have SCM_CREDS that sends credential information through a UNIX 
      domain socket. [ Ok, someone point me to some standard that 
      documents the semantics. Every BSD camp is doing it differently 
      ":( ] 

    5) Using The File System Securely 

  [ The first contributor to find a better solution to 5.2 gets a 
  donut ] 

  Sadly too many past security holes show that the average programmer 
  fails to note that the file system is a database with links 
  pointing to resources, and that filenames act only as identifiers. 
  File names, which are stored in directories (considered special 
  files), point to inodes. Indeed that is how we get race condition 
  attacks, and symlink attacks. Both are given treatment below, but 
  keep in mind that the principles are open to other databases that 
  follow the same model as the file system. In particular race 
  conditions may occur in non-file-system related operations. 

      5.1) How do I avoid race condition attacks? 

      A race condition occurs when two or more operations occur in an 
      undefined manner (McKusick et al. 1996). Specifically in file 
      system races the attacker attempts to change the state of the 
      file system in between two file system operations on the part 
      of the program. Usually the program expects the two operations 
      to apply to the same file, or expects the information retrieved 
      to be the same. If the file operations are not atomic, or do 
      not reference the same file this cannot be guaranteed without 
      proper care. As an added note see Bishop 1996 for a more 
      exhaustive and but more theoretical discussion. 

      Solaris 2.x's 'ps' utility had a security hole that was caused 
      by a race condition (Chasin 1995). The utility would open a 
      temporary file, and then use the chown() system call with the 
      file's full path to change its ownership to root. This was 
      easily exploitable by slowing the system down, finding the file 
      created, deleting it, and then slipping in a new SUID word 
      writable file. After the file was created with that mode and 
      chowned to root by the insecure process, the attacker simply 
      copies a shell into the file. Viola, instant root shell. (The 
      exploit itself made use of symlinks to slip in the new file, 
      but I'm leaving the concept of symlinks untill the next 5.2) 

      At first glance, to the less perceptive reader, it may seem 
      that if the original file was not created world writable the 
      attacker could not delete it. Well it was not world writable 
      and he could. The file was created under the temporary 
      directory (usually "/var/tmp" or "/tmp") which had world 
      writable permissions. Global temporary directories are setup 
      this way or they aren't usable, hence the world global. It's a 
      completely different issue to argue whether having global 
      temporary directories is a good idea. [For a similar security 
      hole see Hull 1996] 

      The problem was that the second operation used the file name 
      and not the file descriptor. If a call to fchown() would have 
      been used on the file descriptor returned from the original 
      open() operation, the security hole would have been avoided. 
      File names are _not_ unique. The file name "/tmp/foo" is really 
      just an entry in the directory "/tmp". Directories are special 
      files. If an attacker can create, and delete files from a 
      directory the program cannot trust file names taken from it. Or 
      to look at it in a more critical way: because the directory is 
      modifiable by the attacker, a program cannot trust it as a 
      source of valid input. Instead it should use file descriptors 
      to perform its operations. 

      One solution is to use the sticky bit. This will prevent the 
      attacker from removing the file, but not prevent the attacker 
      from creating files in the directory. See below for a treatment 
      toward symbolic link attacks. 

      There are other race conditions that can occur. Using stat() 
      and instead of fstat(). Using access() and thinking that the 
      information will persist untill the next few lines of code. It 
      may not persist if the directory can be modified by an 
      attacker, so don't expect it to. The only way to make sure the 
      file permissions will not change, and that you have the file 
      you want is to fstat() after an open(). 

      5.2) How do I create/open files safely? 

      Several attacks have made use of symbolic links to fool the 
      process into opening a different file. A symbolic link is a 
      convenient method of creating a file that references a 
      different file. It is not a hard link, because it does not 
      reference a particular inode. Instead it holds a path to 
      another file. This is convenient because the path could be 
      point to a non-existent file. 

      A process can be tricked into opening or creating a file it did 
      not intend to via a symbolic link attack. For example: 

      A security hole reported for SUN's license manager stemmed from 
      the creation of a file without checking for symbolic links (or 
      soft links) (Eriksson 1996). An open() call was made to either 
      create the file if it did not exist, or open it if it did 
      exist. The problem with a symbolic link is that an open call 
      will follow it and not consider the link to constitute a 
      created file. So if you had "/tmp/foo" symlinked to "/.rhosts" 
      (or "/root/.rhosts" depending on your cultural background), the 
      latter file would be transparently opened. The license manager 
      seemed to have used the O_CREAT flag with the open call making 
      it create the file if it did not exist. To make matters worse, 
      it created the file with world writable permissions. Since it 
      ran as root, the ".rhosts" file could be created, written to, 
      and root privileges attained. I'll leave it as an exercise to 
      the reader to work out how a world writable ".rhosts" file can 
      be used to get root privileges. (Back in those days a world 
      writable ".rhosts" was OK on the part of rlogind) 

      The problem is two fold, since it requires the following 
      conditions to solve it: "If file does not exist, and no 
      symbolic link exists in its place, then create file." And 
      remember: we can only use one system call to do this, in order 
      to avoid race conditions (see 5.1). At first glance it would 
      seem that using an O_EXCL should solve this problem. 
      Unfortunately it does not. Here's a quote from the FreeBSD man 

      "If O_EXCL is set and the last component of the pathname is a 
      symbolic link, open() will fail even if the symbolic link 
      points to a non-existent name." 

      Because O_EXCL, on FreeBSD, only checks the last component, 
      symbolic links between directories are still viable attacks. 
      Consider the path "/usr/foo/bar/footest" to the file "footest" 
      now make a symbolic link with "/usr/foo2" pointing to 
      "/usr/foo/bar." Opening "/usr/foo2/footest" will translate to 
      opening "/usr/foo/bar/test." The only way to avoid this is to 
      make sure that the full path does not consist of symbolic 
      links. One method to is chdir() down the path to the current 
      directory. Once the directory is reached, check to make sure it 
      matches the path specified previously. If it does, create the 
      file, knowing that you are using O_EXCL and relying on the last 
      component to be checked by the system call. 

      The semantics of O_EXCL as mentioned in the FreeBSD man page is 
      not found in the UNIX98 standard. This means some systems are 
      broken inherently. We'll proceed without taking these semantics 
      into consideration, and look for other work arounds. 

      One of the other work arounds is to create a file with write 
      only permissions for the program's real user ID. Then an 
      fstat() can be made and the program will know the full path to 
      the file it has opened. If it is the correct file, go about 
      business as usual, fchmod() if you want as well. However, if it 
      is the wrong file it needs to be deleted. See below for 
      treatment toward deleting files safely. Again, we find the 
      safest method to avoid attacks while accessing the file system 
      is to do it in a directory that is only accessible by the 
      program. If the directory can be modified by an attacker, or 
      any of its parent directories for that matter, the program may 
      be tricked into creating the wrong file. 

      A final note: when applying the aforementioned work around of 
      creating, checking, and deleting files, _do not_ use O_TRUNC or 
      you may wind up truncating the password database among other 

      5.3) How do I delete files safely? 

      You cannot delete files from a directory safely unless the 
      directory is writable only by the secure process. The problem 
      relates to the discussion in 5.1 regarding race conditions. 
      Since the unlink() system call requires a file name, and does 
      not accept a file descriptor, there is no way to atomically 
      delete files from the file system. 

      Also as mentioned in 5.2, make sure that the secure directory 
      (writable only by the secure process) is accessed from a path 
      that does not contain any symbolic links. 

      5.4) Is chroot() safe? 

      chroot() only limits the file system scope and nothing else. 
      This means that the process will not have access to files 
      outside of its root file system; however, it does have access 
      to the kernel, and all the system calls any other process has 
      access to. 

      This can lead to complications. Consider the attacks mentioned 
      in the "Process Interaction" section. If a process has the same 
      real user ID as another process it can hijack it via ptrace() 
      on certain BSD derived operating systems that allow ptrace() 
      without consent from the targeted process. Same goes for 
      kill(), setpriority(). Thus the process, even if not privileged 
      may still break out of the chroot() environment. Other attacks 
      include mknod(), but need root privileges. The aforementioned 
      attacks do not. What this boils down to is that the root 
      directory is only checked by the file system calls, and not 
      other system calls. 

      The chroot() call itself will only change the root file system 
      in the process' context. A chroot() call must be followed by a 
      chdir("/") call in order to reset the current working 

      Here are a list of recommended guidelines to make sure the 
      chroot() environment is indeed a "jail". I may have missed some 
      items, especially those that may pertain to more exotic UNIX 

          Credentials and Privileges 

          The chrooted process should not run with root credentials, 
          or have any special privileges. It should also not share 
          credentials with any other process outside of the chrooted 
          environment. If it does share credentials it will be able 
          to send signals and possibly even ptrace() other processes. 
          Other attacks may exist such as setpriority() etc. By 
          segregating the credentials, the process will not be able 
          to pass credential checks when making use of process 
          interacting system calls. 

          If there exists no SUID to root process-image in the 
          chrooted environment, after calling chroot() and dropping 
          privileges, the process cannot gain root privileges without 
          breaking out of the chroot environment. It is a highly 
          recommended to do away with SUID process images completely 
          in a chroot jail. 

          Privileged Information 

          The chrooted environment should not contain privileged 
          information. Password databases, confidential data etc. 
          should not be placed within the chrooted environment. In 
          the case of passwords, this can lead to the system being 
          compromised as a whole. 

          No devices should be present in the chrooted environment. 
          Especially devices that can be mounted as a file system, or 
          a Berkeley Packet Filter (BPF) device etc. 

        [ Send in more if I missed any. ] 

    6) Handling Input 

  This section should be read with section 2 "The Flow Of 

      6.1) What is a "buffer overrun/overflow attack" and how do I 
      avoid it? 

    The term can be misleading if one thinks about buffers filling up 
    in a modem. The problem is not lossage of data, but the ability 
    for the attacker to point the process to execute arbitrary code. 
    This FAQ will not cover the various ways of exploiting this 
    security hole, since it has become an art form in itself; 
    however, understanding how the security hole can be exploited 
    will help avoid some of the common myths circulating about work 
    arounds for it. 

    The traditional method is to pass a memory copying routine 
    (string copying included) data larger than the targeted memory, 
    which is usually holding data for an automatic variable (local 
    variable in a C function), and thus spilling the excess data on 
    the other local variables and eventually onto the instruction 
    stack itself. The end spillage would ideally cause a pointer on 
    the stack to point to arbitrary code, possibly held within an 
    environment variable. When the function returns, the process 
    executes the code pointed to by instruction stack. (Aleph One 
    1996). This is not the only method, among the others include 
    writing to heap memory (dynamic memory) and overwriting 
    structures such as stdio's FILE (Conover 1999). Like I said, it 
    has become an art form. 

    The previous paragraph was a gross over simplification, but that 
    is really the best that can be done within the scope of this FAQ. 
    The point that needs to be made is that bounds checking _must_ be 
    performed on input. Bounds checking basically means keeping track 
    of sizes and not overrunning any particular memory location with 
    more than it should hold. If the concept of bounds checking is 
    alien to you, I strongly urge you to pick up a C book. Even 
    though, by all means, the concept is not native to C alone. 

    Programs that do not perform bounds checking on internal data are 
    bugged. Programs that do not perform bounds checking on input are 
    insecure. Bugs cause programs to be insecure. So you want to 
    perform bounds checking always. 

    Obligatory warnings include: "Don't use strcpy() use strncpy()", 
    "Don't use the stdio library when receiving input that may be 
    malicious, it may be implemented without proper bounds checking." 
    Indeed, I could recite a plethora of security holes that came 
    from just this, but I'll leave the research this time as an 
    exercise for the reader. The actual principle was brought up in 
    the previous paragraphs and should be come easily to a 

    Some myths need to be dispelled now. Not returning from a 
    function and calling exit() will not act as a work around. Heap 
    attacks can still be made, local variables can still be 
    overwritten, and most importantly your program could easily be 
    crashed by a segmentation fault signal (please don't mail in with 
    "but I can catch that signal"). Using huge buffers to copy data 
    about and expect things to magically work will not do. If you 
    find a fellow programmer using these work arounds, please lock 
    them up in a padded room till they get better. 

    Certain languages provide bounds checking inherently. This is a 
    good thing; however, some will argue that bounds checking at run 
    time is too costly. This is also a good thing. If you want to 
    use, and can use a language that supports bounds checking, go 
    right ahead. 

    In C you won't have any bounds checking unless you have a 
    compiler that is patched to support it. Oddly enough there 
    doesn't seem to be any mention in any standard that would 
    outright forbid the usage of run time bounds checking, and as 
    such there exist a patch for GCC to do just this. Richard Jones 
    and Paul Kelley, who have done just this, have a page at: 

    Other work arounds include patching the kernel to _not_ execute 
    code on the stack, which prevents some exploits but not all (heap 
    attacks etc.). Several vendors and individuals have already taken 
    this initiative. A quick search on Dejanews and even the Bugtraq 
    archives should point you in the right direction. [ If I receive 
    any submissions of URLs for patches, I will be happy to add 

    Unfortunately, this question could not be answered completely and 
    thoroughly. It is too big of an exploit, and too simple of a 
    problem, but yet so wide spread it requires awareness more than 
    anything else. See Aleph One 1997 for a similar discussion on the 
    prevention on bounds checking. 

    6.2) How do I hand integer values safely? 

  A problem reported in sshd 1.2.7 (van der Wijk 1997) allowed a 
  normal user to bind to privileged ports. The daemon read the port 
  number into a 32 bit value, and did the port privilege checks on 
  the 32-bit integer. After it was satisfied that the value is not 
  under 1024 (IPPORT_RESERVED), the daemon would then place the 
  integer into a 16-bit unsigned integer ("short" on most systems). 
  The value if over 65535 could wrap under 1024. This effectively 
  allowed a user to bind to a privileged port. The fix is to check 
  the value in its 16-bit form. Thus in sshd's case, check it in the 
  sin_port member of the struct sockaddr_in. Any checks prior to that 
  should be done with the assumption that 65535 (or negative values) 
  can overflow in the 16-bit integer and not be valid. If you don't 
  quite see why, pick up a C book and go over the way casting is done 
  between different types of varying length. 

  Similar problems were reported in the Linux kernel's system call 
  interface (Solar Designer 1997). 

  The fix, as mentioned previously, is to double check that the 
  values are the same after any conversion between types. Luckily 
  this is one of the more arcane security holes that don't pop up too 

  6.3) How do I safely pass input to an external program? 

One of the biggest mistakes is to use a shell. Indeed the famous 
'phf' security hole, a cgi program that came packaged with the NCSA 
httpd distribution, had a problem involving the use of a shell to 
execute an external program (CERT 1997). The security hole stemmed 
from a library routine it used, that was packaged with the NCSA cgi 
example distribution, called escape_shell(). The routine would take a 
command line, search for characters that would be interpreted by the 
shell, and remove them so the attacker can not invoke additional 
commands to the shell. 

At first glance it seems like a completely correct way to go about 
executing an external program. Escape the shell characters, and let 
the shell do the calling. It is completely and utterly wrong. In the 
rare case where you need to use a shell, a very rare and dangerous 
case, go ahead and do just that. But by removing characters you open 
yourself to a slew of mistakes. Indeed, escape_shell() forgot to 
strip certain characters that the shell would interpret. This allowed 
the attacker to send arbitrary commands to the shell. 

Instead of checking input for shell characters, don't use the shell 

Library routines such as system() and popen() invoke a shell. It is 
more secure, from the input handling perspective, to use execve() or 
one of its wrapper routines to call the process-image directly. The 
logic is that you can't mess up checking for special shell characters 
because you are not doing that. 

Also make sure you've read the section on process attribute 
inheritance. You may leak file descriptors as per the above mentioned 
chpass hole. 

8) Bibliography 

  Aleph One, "Smashing the Stack For Fun And Profit" Phrack, Vol.7, 
  No. 49, Nov 1996, [ File 14 of 16 ] 

  Al-Herbish, Thamer "Re: More ssh fun (sshd this time)" Online 
  posting. 23 Aug. 1997. Bugtraq. 

  Bernstein, Dan "Secure Interprocess Communication" 1998. <URL: 

  Bernstein, Dan "Re: A thought on TCP SYN attacks" Online posting. 
  26 Sept. 1996. SYN-Cookies Mailing List. 

  Bishop, Matt "How to write a setuid program" login 12(1) Jan/Feb 

  Bishop Matt, and M. Dilger "Checking for Race Conditions in File 
  Accesses," Computing Systems 9(2) (Spring 1996) pp. 131-152. 

  CERT (Computer Emergency Response Team) "CERT(*) Advisory CA-96.06" 
  20 March 1996. <URL:> 

  Chasin, Scott "BUGTRAQ ALERT: Solaris 2.x vulnerability" Online 
  posting. 14 Aug. 1995. Bugtraq. 

  Conover, Matt "w00w00 on Heap overflows" Online posting. 27 Jan. 

  daemon9, route, infinity "Project Neptune "Phrack, Vol.7, No. 48, 
  July 1996, [ File 13 of 18 ] 

  Der Mouse "Re: Tar "features"" Online posting. 25 Sept. 1998. 

  Eriksson, Joel "License Manager's lockfiles (Solaris 2.5.1)" Online 
  posting. 12 Oct. 1998. Bugtraq. 

  Hull, Gregory "r00t advisory -- sol2.5 su(1M) vulnerability" Online 
  posting. 26 Aug. 1996. 

  Harrison, Roger "License Manager's lockfiles (Solaris 2.5.1)" 
  Online posting. 23 Oct. 1998. Bugtraq 

  Jensen, Geir Inge "Another autoreply security hole" Online posting, 
  12 Mar. 1994. Bugtraq. 

  Network Associates Inc. "Network Associates Inc. Advisory 
  (OpenBSD)" Online posting. 10 Aug. 1998. Bugtraq. 

  plaguez shegget "XFree86 insecurity" Online posting. 21 Nov. 1997. 

  Saltzer, J.H., and M.D. Schroeder, "The Protection of Information 
  in Computer Systems," Proc. IEEE, Vol. 63, No. 9, Sept. 1975, pp. 

  Sanfilippo, Salvatore "pingflood.c" Online posting. 9 Apr. 1998. 

  Schenk, Eric "A thought on TCP SYN attacks" Online posting. 25 
  Sept. 1996. SYN-Cookies Mailing List. 

  Solar Designer "Integer Overflows" Online posting. 28 Aug. 1997. 

  Stevens, Richard W. "UNIX Network Programming" New Jersey, Prentice 
  Hall, 1990. 

    Stevens, Richard W. "Advanced Programming In The UNIX 
    Environment" Reading, Massachusetts, Addison-Wesley, 1992. 

    Smith, Ben "ps(1) for freebsd." Online posting. 12 Aug. 1998. 

    Tarreau, William "Tar "features"" Online posting. 22 Sept. 1998. 

    Temmingh, Roelof W "FreeBSD rlogin and coredumps" Online posting. 
    17 Feb. 1997. Bugtraq 

    Wall, Larry and Schwartz, Randal L. "Programming Perl" : 
    Sebastopol, California : O'Reilly And Associates, 1992. 

    van der Wijk, Ivo "More ssh fun (sshd this time)" Online posting. 
    19 Aug. 1997. Bugtraq. 

    Zalewski, Michal "ipop3d (x2) / pine (x2) / Linux kernel (x2) / 
    Midnight Commander (x2)" Online posting. 7, March 1999. Bugtraq. 

  9) List of Contributors 

    Thamer Al-Herbish <> 

    Peter Roozemaal <> 

"Youth, Nature, and relenting Jove, 
To keep my lamp _in_ strongly strove,
But Romanelli was so stout, 
He beat all three -- _and blew it out_."

-- George Gordon Byron "My Epitaph" From "Occasional Pieces"

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