SETUID(7) BSD Miscellaneous Information Manual SETUID(7)NAME
setuid — checklist for security of setuid programs
DESCRIPTION
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include, even though parts of it are outdated.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‐
tic...
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.
· 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.
· 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.
· 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.
· 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.
· 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‐
tems.
· 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.
· 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.
· Similarly, programs executed at user request must not receive other
sensitive resources, notably file descriptors. Use of fcntl(2)
F_CLOSEM, FILENO_STDERR + 1 (close all fd's greater than stderr)
and/or fcntl(2) F_SETFD, FD_CLOEXEC (close-on-exec) arrangements on
systems which have them is recommended.
Other resources should also be examined for sanity and possibly set
to desired settings, such as the current working directory, signal
disposition, resource limits, environment, umask, group membership,
chroot.
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 per‐
forming work on behalf of the system as opposed to a specific user.
· 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.
· 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.
· 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.
· 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)).
· 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.
· Environment variables in general cannot be trusted. Their contents
should never be taken for granted.
· 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.
· 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
solution.
· 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).
· The value of argc might be 0.
· 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.
· 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).
· 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.
· Descriptors shared by other processes (i.e., any that are open on
startup) may be manipulated in strange ways by said processes.
· 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‐
fully.
· 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.
· 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
ignored.
· On some systems there may be an alarm(3) signal pending on startup.
· The program may have children it did not create. This is normal when
the process is part of a pipeline.
· 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.
· 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.
· 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.
· 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.
· There might be many files open, exhausting the supply of descriptors.
Running fcntl(2) F_CLOSEM on systems which have it, is recommended.
· There might be many arguments.
· 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.
SEE ALSOpasswd(1), pwd(1), access(2), chdir(2), chroot(2), execve(2), fcntl(2),
fork(2), getlogin(2), link(2), setegid(2), seteuid(2), setgid(2),
setgroups(2), setrlimit(2), setuid(2), sigaction(2), umask(2), alarm(3),
creat(3), execvp(3), popen(3), printf(3), ttyname(3)HISTORY
Written by Henry Spencer, and based on additional outside contributions.
AUTHORS
Henry Spencer ⟨henry@spsystems.net⟩
BUGS
The list really is rather long... and probably incomplete.
BSD February 26, 2009 BSD
