"Don't do it. Most of the time, it's not necessary."^[17]

^[17] Thanks to Patrick H. Wood and Stephen G. Kochan, Unix System Security (Hayden Books, 1985) for this insightful remark.

Avoid writing SUID shell scripts.

If you are using SUID to access a special set of files, don't. Instead, create a special group for your files and make the program SGID to that group. If you must use SUID, create a special user for the purpose.

If your program needs to perform some functions as superuser, but generally does not require SUID permissions, consider putting the SUID part in a different program, and constructing a carefully controlled and monitored interface between the two.

If you need SUID or SGID permissions, use them for their intended purpose as early in the program as possible, and then revoke them by returning the effective, and real, UIDs and GIDs to those of the process that invoked the program.

If you have a program that absolutely must run as SUID, try to avoid equipping the program with a general-purpose interface that allows users to specify much in the way of commands or options.

Erase the execution environment, if at all possible, and start fresh. Many security problems have been caused because there was a significant difference between the environment in which the program was run by an attacker and the environment in which the program was developed.

If your program must spawn processes, use only the execve( ), execv( ), or execl( ) calls, and use them with great care. Avoid the execlp( ) and execvp( ) calls because they use the PATH environment variable to find an executable, and you might not run what you think you are running. Avoid system( ) and popen( ) at all costs.

If you must provide a shell escape, be sure to setgid(getgid( )) and setuid(getuid( )) before executing the user's command—and use them in the correct order! You must reset the group ID before you reset the user ID, or the call will fail.

In general, use the setuid( ) and setgid( ) functions and their friends to bracket the sections of your code that require superuser privileges. For example:

/* setuid program is effectively superuser so it can open the master file */
fd = open("/etc/masterfile",O_RDONLY);
assert(seteuid(getuid(  )) == 0);                                                
/* Give up superuser now, but we can get it back.*/
assert(geteuid() == getuid(  ));/* Insure that the euid is what we expect. */
if(fd<0) error_open(  );     /* Handle errors. */

Not all versions of Unix allow you to switch UIDs in this way; moreover, the semantics of the various versions of setuid( ), seteuid( ), and setreuid( ) have been shown to vary between Unix flavors, and even be misimplemented. It's also crucial both to check their return values and to separately test to ensure that the UIDs are as you expect them. Read Chen, Wagner, and Dean's paper "Setuid Demystified" (http://www.cs.berkeley.edu/~daw/papers/setuid-usenix02.pdf) before you even think about writing code that tries to save and restore privileges.

If you must use pipes or subshells, be especially careful with the environment variables PATH and IFS. One approach is to erase these variables and set them to safe values. For example:

putenv("PATH=/bin:/usr/bin:/usr/ucb");
putenv("IFS= \t\n");

Then, examine the environment to be certain that there is only one instance of the variable: the one you set. An attacker can run your code from another program that creates multiple instances of an environment variable. Without an explicit check, you may find the first instance, but not the others; such a situation could result in problems later on. In particular, step through the elements of the environment yourself rather than depending on the library getenv( ) function.

Another approach, simpler but more drastic, is to create an empty environment and fill it with only those variables that you know are OK. This environment can then be passed to execve( ):

char *env[MAX_ENV];
int mysetenv(const char *name, const char *value) {
        static char count = 0;
        char buff[255];
        if (count == MAX_ENV) return 0;
        if (!name || !value) return 0;
        if (snprintf(buff, sizeof(buff), "%s=%s", name, value) < 0) return 0;
        if (env[count] = strdup(buff)) {
                count++;
                return 1;
        }
        return 0;
}

...And then in the program...

if (mysetenv("PATH", "/bin:/usr/bin") &&
mysetenv("SHELL", "/bin/sh") &&
mysetenv("TERM", "vt100") &&
mysetenv("USER", getenv("USER")) &&
mysetenv("LOGNAME", getenv("LOGNAME")) &&
mysetenv("HOME", getenv("HOME"))) {
        execve(myprogram,NULL,env);
        perror(myprogram);
} else {
        perror("Unable to establish safe environment");
}

Use the full pathname for all files that you open. Do not make any assumptions about the current directory. (You can enforce this requirement by doing a chdir("/tmp/root/") as one of the first steps in your program, but be sure to check the return code!)

Consider statically linking your program. If a user can substitute a different module in a dynamic library, even carefully coded programs are vulnerable. (We have some serious misgivings about the trend in commercial systems towards completely shared, dynamic libraries. (See our comments in Section 23.6.2 in Chapter 23.)

Consider using perl -T or taintperl for your SUID programs and scripts. Perl's tainting features often make Perl more suited than C to SUID programming. For example, taintperl insists that you set the PATH environment variable to a known "safe value" before calling system( ). The program also requires that you "untaint" any variable that is input from the user before using it (or any variable dependent on that variable) as an argument for opening a file.

However, note that you can still get yourself in a great deal of trouble with taintperl if you circumvent its checks or if you are careless in writing code. Also note that using taintperl introduces dependence on another large body of code working correctly: we suggest you skip using taintperl if you believe that you can code at least as well as Larry Wall.^[18]

^[18] Hint: if you think you can, you are probably wrong.