Many programs and libraries, including the shared library loader on both UNIX and Windows systems, depend on environment variable settings. Because environment variables are inherited from the parent process when a program is executed, an attacker can easily sabotage variables, causing a program to behave in an unexpected and insecure manner [Viega 2003].

All programs, particularly those running with higher privileges than the caller (such as those with setuid/setgid flags), should treat their environment as untrusted user input. Because the environment is inherited by processes spawned by calls to the fork(), system(), or exec() functions, it is important to verify that the environment does not contain any values that can lead to unexpected behavior.

The best practice for such programs is to

• Drop privileges once they are no longer necessary. (See POS02-C. Follow the principle of least privilege.)
• Avoid calling system(). (See ENV33-C. Do not call system().)
• Clear the environment and fill it with trusted or default values.

This recommendation is a more specific instance of STR02-C. Sanitize data passed to complex subsystems.

Subclause 7.22.4.6 of the C Standard states that "the set of environment names and the method for altering the environment list are implementation-defined." Consequently, it is important to understand which functions are available for clearing, modifying, and looking up default values for environment variables. Because some programs may behave in unexpected ways when certain environment variables are not set, it is important to understand which variables are necessary on your system and what are safe values for them.

Noncompliant Code Example (POSIX, ls)

This noncompliant code example invokes the C system() function to execute the /bin/ls program. The system() function passes a string to the command processor in the host environment to be executed.

if (system("/bin/ls dir.`date +%Y%m%d`") == -1) {
  /* Handle error */
}

Although IFS does not affect the command portion of this string, /bin/ls, it does determine how the argument is built after calling date. If the default shell does not ignore the incoming value of the IFS environment value, and an attacker sets IFS to ".", the intended directory will not be found.

Compliant Solution (POSIX, ls)

The nonstandard function clearenv() may be used to clear out the environment where available: otherwise, the environment can be cleared by obtaining the environment variable names from environ and removing each one using unsetenv().

In this compliant solution, the environment is cleared by clearenv(), and then the PATH and IFS variables are set to safe values before system() is invoked. Sanitizing shell commands can be difficult, and doing so can adversely affect the power and flexibility associated with them.

char *pathbuf;
size_t n;

if (clearenv() != 0) {
  /* Handle error */
}

n = confstr(_CS_PATH, NULL, 0);
if (n == 0) {
  /* Handle error */
}

if ((pathbuf = malloc(n)) == NULL) {
  /* Handle error */
}

if (confstr(_CS_PATH, pathbuf, n) == 0) {
  /* Handle error */
}

if (setenv("PATH", pathbuf, 1) == -1) {
  /* Handle error */
}

if (setenv("IFS", " \t\n", 1) == -1) {
  /* Handle error */
}

if (system("ls dir.`date +%Y%m%d`") == -1) {
  /* Handle error */
}

POSIX also specifies the confstr() function, which can be used to look up default values for environment variables [IEEE Std 1003.1:2013]. The _CS_V7_ENV argument to confstr() retrieves a list of environment variable settings required for a default conforming environment [IEEE Std 1003.1:2013]. A space-separated list of variable=value pairs is returned, with variable names guaranteed not to contain equal signs (=), and variable=value pairs guaranteed not to contain spaces. Used together with the _CS_PATH request, this completely describes the minimum environment variable settings required to obtain a clean, conforming environment. On systems conforming to the POSIX.1-2008 standard, this should be used to create a sanitized environment.

On systems that have no clearenv() function, the following implementation can be used:

extern char **environ;

int clearenv(void) {
  static char *namebuf = NULL;
  static size_t lastlen = 0;

  while (environ != NULL && environ[0] != NULL) {
    size_t len = strcspn(environ[0], "=");
    if (len == 0) {
      /* Handle empty variable name (corrupted environ[]) */
    }
    if (len > lastlen) {
      namebuf = realloc(namebuf, len+1);
      if (namebuf == NULL) {
        /* Handle error */
      }
      lastlen = len;
    }
    memcpy(namebuf, environ[0], len);
    namebuf[len] = '\0';
    if (unsetenv(namebuf) == -1) {
      /* Handle error */
    }
  }
  return 0;
}

Compliant Solution (Windows)

There is no portable or guaranteed way to clear out the environment under Windows. Following ENV33-C. Do not call system(), care should be taken to use _execle(), _execlpe(), _execve(), or _execvpe() instead of system(), because they allow the environment to be explicitly specified.

If it is explicitly known which environment variables need to be kept, Secure Programming Cookbook for C and C++ [Viega 2003] defines a function, spc_sanitize_environment(), that will remove everything else.

Risk Assessment

Invoking an external program in an attacker-controlled environment is inherently dangerous.

Automated Detection

Tool	Version	Checker	Description
Helix QAC		C5017
LDRA tool suite		588 S	Partially implemented

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Bibliography