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SPROG(7)             BSD Miscellaneous Information Manual             SPROG(7)

NAME
     sprog -- secure programming practices

DESCRIPTION
     Security issues have crept into many systems over the years.  This docu-
     ment is a guide for programming practices that prevent these problems.

   Overview
     Writing secure applications takes a very scrutinous and pessimistic out-
     look.  Applications should be run with the principle of ``least
     privilege'' so that no process is ever running with more than the bare
     minimum access it needs to accomplish its function.  Previously tested
     code should be reused whenever possible.  Generally, anything beyond the
     control of a program should never be trusted.  This includes all forms of
     user input, system resources, interprocess communication, and the timing
     of events.

   Buffer Overflows
     One of the most common types of security problems is the buffer overflow.
     In short, if a program is not careful with the data it receives, it may
     be possible for this data to be written across memory, overwriting the
     return address for a function call, and the program will be forced to run
     code that does unfriendly things.

     A good number of functions in the standard C library make it difficult or
     even impossible to prevent buffer overflows when used.  These include
     fscanf(3), gets(3), getwd(3), realpath(3), scanf(3), sprintf(3),
     strcat(3), strcpy(3), vscanf(3), and vsprintf(3).

     Many other functions that deal with strings can also open up a potential
     buffer overflow when not used carefully.  For example, strncat(3) does
     not go out of its way to provide NUL character termination.  Of course,
     the proper length must always be specified.  Usage of strlcat(3) and
     strlcpy(3) ensure that strings are null terminated and of the specified
     length.

     Functions that receive a string format must also be used carefully.  It
     is possible for a string to contain additional format specifiers, which
     open up another possibility for a buffer overflow.  Never pass a string
     with untrusted data without using '%s'.  Always use the proper secure
     idiom:

           function("%s", string);

     There are mechanisms that provide a backstop for these problems at the
     library and compiler levels, however, there is no substitute for simply
     writing good code.

   Set-user-ID Issues
     In many cases, it may be necessary for a program to operate with an
     increased set of permissions.  Reasons for this include binding to pro-
     tected sockets, reading and writing certain files and directories, and
     access to various resources.  Using a setuid program is frequently the
     solution.  However, it is important that programs give up these privi-
     leges as soon as possible.  For example, if a program is binding to a
     protected socket, it should give up its privileges as soon as it has fin-
     ished binding to that socket.  This is accomplished with the setuid(2)
     family of system calls.

   Limited Environments
     The traditional method of restricting a process is with the chroot(2)
     system call.  This system call changes the root directory from which all
     other paths are referenced for a process and any child processes.  Of
     course, the process must have access to this path to begin with.  The new
     environment does not actually take effect until chdir(2) is called to
     place the process into the new environment.  Unfortunately, a process can
     break out of this environment if root access is obtained.

     Often, jail(2) can be used to create a more complete and enclosed envi-
     ronment than chroot(2) can provide.  A jail limits all processes inside
     that environment, including processes with superuser privileges.

     Fine grained privileges, as described by POSIX.1e extensions, are cur-
     rently a work in progress, and the focus of the TrustedBSD Project.  More
     information can be found at http://www.TrustedBSD.org/.

   Trust
     Programs should not make assumptions about the environment in which they
     are running.  This includes user input, signals, environment variables,
     system resources, interprocess communications, and shared memory, amongst
     other things that are beyond the control of the program.  They should not
     assume that all forms of invalid data can be detected either.  Instead,
     they should use positive filtering, and only allow a specific subset of
     inputs that are known to be safe.  This is the same logic that an admin-
     istrator should apply to a firewall, that is, deny by default and specify
     what is to be accepted.

   Race Conditions
     A race condition is anomalous behavior caused by the relative timing of
     events.  Programs should not assume that a particular event will occur
     before another.  The most common causes of race conditions are signals,
     access checks, and file reads.  Signals are asynchronous by nature, so
     special care must be taken while dealing with them.  Attempting to check
     access with sequential non-atomic operations is a very bad idea, as files
     can be moved and changed at any given time.  Instead of using a sequence
     of access(2) and open(2), use seteuid(2) and then call open(2) directly.
     Set umask(2) properly beforehand.

SEE ALSO
     jail(2), setuid(2), strlcat(3), strlcpy(3)

AUTHORS
     Eric Melville <ericATFreeBSD.org> originally wrote this document based on
     a chapter of the FreeBSD Developer's Handbook written by Murray Stokely
     <murrayATFreeBSD.org>.

BSD                              June 3, 2001                              BSD