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SETUID(7)            BSD Miscellaneous Information Manual            SETUID(7)

     setuid -- checklist for security of setuid programs

     Please note: This manual page was written long ago, and is in need of
     updating to match today's systems.  We think it is valuable enough to
     include, even though parts of it are outdated.  A carefully-researched
     updated version would be very useful, if anyone is feeling enthusias-

     Writing a secure setuid (or setgid) program is tricky.  There are a num-
     ber of possible ways of subverting such a program.  The most conspicuous
     security holes occur when a setuid program is not sufficiently careful to
     avoid giving away access to resources it legitimately has the use of.
     Most of the other attacks are basically a matter of altering the pro-
     gram's environment in unexpected ways and hoping it will fail in some
     security-breaching manner.  There are generally three categories of envi-
     ronment manipulation: supplying a legal but unexpected environment that
     may cause the program to directly do something insecure, arranging for
     error conditions that the program may not handle correctly, and the spe-
     cialized subcategory of giving the program inadequate resources in hopes
     that it won't respond properly.

     The following are general considerations of security when writing a
     setuid program.

     o   The program should run with the weakest userid possible, preferably
         one used only by itself.  A security hole in a setuid program running
         with a highly-privileged userid can compromise an entire system.
         Security-critical programs like passwd(1) should always have private
         userids, to minimize possible damage from penetrations elsewhere.

     o   The result of getlogin(2) or ttyname(3) may be wrong if the descrip-
         tors have been meddled with.  There is no foolproof way to determine
         the controlling terminal or the login name (as opposed to uid) on V7.

     o   On some systems, the setuid bit may not be honored if the program is
         run by root, so the program may find itself running as root.

     o   Programs that attempt to use creat(3) for locking can foul up when
         run by root; use of link(2) is preferred when implementing locking.
         Using chmod(2) for locking is an obvious disaster.

     o   Breaking an existing lock is very dangerous; the breakdown of a lock-
         ing protocol may be symptomatic of far worse problems.  Doing so on
         the basis of the lock being 'old' is sometimes necessary, but pro-
         grams can run for surprising lengths of time on heavily-loaded sys-

     o   Care must be taken that user requests for I/O are checked for permis-
         sions using the user's permissions, not the program's.  Use of
         access(2) is recommended.

     o   Programs executed at user request (e.g. shell escapes) must not
         receive the setuid program's permissions; use of daughter processes
         and ``setuid(getuid())'' plus ``setgid(getgid())'' after fork(2) but
         before exec(3) is vital.

     o   Similarly, programs executed at user request must not receive other
         sensitive resources, notably file descriptors.

         Programs activated by one user but handling traffic on behalf of oth-
         ers (e.g. daemons) should avoid doing ``setuid(getuid())'' or
         ``setgid(getgid())'', since the original invoker's identity is almost
         certainly inappropriate.  On systems which permit it, use of
         ``setuid(geteuid())'' and ``setgid(getegid())'' is recommended when
         performing work on behalf of the system as opposed to a specific

     o   There are inherent permission problems when a setuid program executes
         another setuid program, since the permissions are not additive.  Care
         should be taken that created files are not owned by the wrong person.
         Use of ``setuid(geteuid())'' and its gid counterpart can help, if the
         system allows them.

     o   Care should be taken that newly-created files do not have the wrong
         permission or ownership even momentarily.  Permissions should be
         arranged by using umask(2) in advance, rather than by creating the
         file wide-open and then using chmod(2).  Ownership can get sticky due
         to the limitations of the setuid concept, although using a daughter
         process connected by a pipe can help.

     o   Setuid programs should be especially careful about error checking,
         and the normal response to a strange situation should be termination,
         rather than an attempt to carry on.

     The following are ways in which the program may be induced to carelessly
     give away its special privileges.

     o   The directory the program is started in, or directories it may plau-
         sibly chdir(2) to, may contain programs with the same names as system
         programs, placed there in hopes that the program will activate a
         shell with a permissive PATH setting.  PATH should always be stan-
         dardized before invoking a shell (either directly or via popen(3) or
         execvp(3) or execlp(3)).

     o   Similarly, a bizarre IFS setting may alter the interpretation of a
         shell command in really strange ways, possibly causing a user-sup-
         plied program to be invoked.  IFS too should always be standardized
         before invoking a shell.

     o   Environment variables in general cannot be trusted.  Their contents
         should never be taken for granted.

     o   Setuid shell files (on systems which implement such) simply cannot
         cope adequately with some of these problems.  They also have some
         nasty problems like trying to run a .profile when run under a suit-
         able name.  They are terminally insecure, and must be avoided.

     o   Relying on the contents of files placed in publically-writable direc-
         tories, such as /tmp, is a nearly-incurable security problem.  Setuid
         programs should avoid using /tmp entirely, if humanly possible.  The
         sticky-directories modification (sticky bit on for a directory means
         only owner of a file can remove it) helps, but is not a complete

     o   A related problem is that spool directories, holding information that
         the program will trust later, must never be publically writable even
         if the files in the directory are protected.  Among other sinister
         manipulations that can be performed, note that on many Unixes, a core
         dump of a setuid program is owned by the program's owner and not by
         the user running it.

     The following are unusual but possible error conditions that the program
     should cope with properly (resource-exhaustion questions are considered
     separately, see below).

     o   The value of argc might be 0.

     o   The setting of the umask(2) might not be sensible.  In any case, it
         should be standardized when creating files not intended to be owned
         by the user.

     o   One or more of the standard descriptors might be closed, so that an
         opened file might get (say) descriptor 1, causing chaos if the pro-
         gram tries to do a printf(3).

     o   The current directory (or any of its parents) may be unreadable and
         unsearchable.  On many systems pwd(1) does not run setuid-root, so it
         can fail under such conditions.

     o   Descriptors shared by other processes (i.e., any that are open on
         startup) may be manipulated in strange ways by said processes.

     o   The standard descriptors may refer to a terminal which has a bizarre
         mode setting, or which cannot be opened again, or which gives end-of-
         file on any read attempt, or which cannot be read or written success-

     o   The process may be hit by interrupt, quit, hangup, or broken-pipe
         signals, singly or in fast succession.  The user may deliberately
         exploit the race conditions inherent in catching signals; ignoring
         signals is safe, but catching them is not.

     o   Although non-keyboard signals cannot be sent by ordinary users in V7,
         they may perhaps be sent by the system authorities (e.g. to indicate
         that the system is about to shut down), so the possibility cannot be

     o   On some systems there may be an alarm(3) signal pending on startup.

     o   The program may have children it did not create.  This is normal when
         the process is part of a pipeline.

     o   In some non-V7 systems, users can change the ownerships of their
         files.  Setuid programs should avoid trusting the owner identifica-
         tion of a file.

     o   User-supplied arguments and input data must be checked meticulously.
         Overly-long input stored in an array without proper bound checking
         can easily breach security.  When software depends on a file being in
         a specific format, user-supplied data should never be inserted into
         the file without being checked first.  Meticulous checking includes
         allowing for the possibility of non-ASCII characters.

     o   Temporary files left in public directories like /tmp might vanish at
         inconvenient times.

     The following are resource-exhaustion possibilities that the program
     should respond properly to.

     o   The user might have used up all of his allowed processes, so any
         attempt to create a new one (via fork(2) or popen(3)) will fail.

     o   There might be many files open, exhausting the supply of descriptors.

     o   There might be many arguments.

     o   The arguments and the environment together might occupy a great deal
         of space.

     Systems which impose other resource limitations can open setuid programs
     to similar resource-exhaustion attacks.

     Setuid programs which execute ordinary programs without reducing author-
     ity pass all the above problems on to such unprepared children.  Stan-
     dardizing the execution environment is only a partial solution.

     Written by Henry Spencer, and based on additional outside contributions.

     Henry Spencer <henryATspsystems.net>

     The list really is rather long...  and probably incomplete.

BSD                            February 10, 2003                           BSD