ENV31-C. Do not rely on an environment pointer following an operation that may invalidate it

Some environments provide environment pointers that are valid when main() is called but may be invalided by operations that modify the environment.

Subclause J.5.1 of the C Standard [ISO/IEC 9899:2011] states:

In a hosted environment, the main function receives a third argument, char *envp[], that points to a null-terminated array of pointers to char, each of which points to a string that provides information about the environment for this execution of the program.

Consequently, under a hosted environment, it is possible to access the environment through a modified form of main():

main(int argc, char *argv[], char *envp[])

However, modifying the environment by any means may cause the environment memory to be reallocated with the result that envp now references an incorrect location.

For example, when compiled with GCC 4.8.1 and run on a 32-bit Intel GNU/Linux machine, the following code,

#include <stdio.h>
#include <stdlib.h>
 
extern char **environ;

int main(int argc, const char *argv[], const char *envp[]) {
  printf("environ:  %p\n", environ);
  printf("envp:     %p\n", envp);
  setenv("MY_NEW_VAR", "new_value", 1);
  puts("--Added MY_NEW_VAR--");
  printf("environ:  %p\n", environ);
  printf("envp:     %p\n", envp);
  return 0;
}

yields

% ./envp-environ
environ: 0xbf8656ec
envp:    0xbf8656ec
--Added MY_NEW_VAR--
environ: 0x804a008
envp:    0xbf8656ec

It is evident from these results that the environment has been relocated as a result of the call to setenv().

Noncompliant Code Example (POSIX)

After a call to the POSIX setenv() function or to another function that modifies the environment, the envp pointer may no longer reference the environment. Standard for Information Technology—Portable Operating System Interface (POSIX®), Base Specifications, Issue 7 [IEEE Std 1003.1:2013] states:

Unanticipated results may occur if setenv( ) changes the external variable environ. In particular, if the optional envp argument to main( ) is present, it is not changed, and thus may point to an obsolete copy of the environment (as may any other copy of environ).

This noncompliant code example accesses the envp pointer after calling setenv():

#include <stdio.h>
#include <stdlib.h>
 
int main(int argc, const char *argv[], const char *envp[]) {
  if (setenv("MY_NEW_VAR", "new_value", 1) != 0) {
    /* Handle error */
  }
  if (envp != NULL) {
    for (size_t i = 0; envp[i] != NULL; ++i) {
      if (puts(envp[i]) == EOF) {
        /* Handle error */
      }
    }
  }
  return 0;
}

Because envp may no longer point to the current environment, this program has undefined behavior.

Compliant Solution (POSIX)

Use environ in place of envp when defined:

#include <stdio.h>
#include <stdlib.h>
 
extern char **environ;

int main(int argc, const char *argv[]) {
  if (setenv("MY_NEW_VAR", "new_value", 1) != 0) {
    /* Handle error */
  }
  if (environ != NULL) {
    for (size_t i = 0; environ[i] != NULL; ++i) {
      if (puts(environ[i]) == EOF) {
        /* Handle error */
      }
    }
  }
  return 0;
}

Noncompliant Code Example (Windows)

After a call to the Windows _putenv_s() function or to another function that modifies the environment, the envp pointer may no longer reference the environment.

According to the Visual C++ reference [MSDN],

The environment block passed to main and wmain is a "frozen" copy of the current environment. If you subsequently change the environment via a call to _putenv or _wputenv, the current environment (as returned by getenv / _wgetenv and the _environ / _wenviron variable) will change, but the block pointed to by envp will not change.

This noncompliant code example accesses the envp pointer after calling _putenvs():

#include <stdio.h>
#include <stdlib.h>
 
int main(int argc, const char *argv[], const char *envp[]) {
  if (_putenv_s("MY_NEW_VAR", "new_value") != 0) {
    /* Handle error */
  }
  if (envp != NULL) {
    for (size_t i = 0; envp[i] != NULL; ++i) {
      if (puts(envp[i]) == EOF) {
        /* Handle error */
      }
    }
  }
  return 0;
}

Because envp no longer points to the current environment, this program fails to print the value of MY_NEW_VAR.

Compliant Solution (Windows)

Use _environ in place of envp when defined:

#include <stdio.h>
#include <stdlib.h>
 
_CRTIMP extern char **_environ;

int main(int argc, const char *argv[]) {
  size_t i;
  if (_putenv_s("MY_NEW_VAR", "new_value") != 0) {
    /* Handle error */
  }
  if (_environ != NULL) {
    for (i = 0; _environ[i] != NULL; ++i) {
      if (puts(_environ[i]) == EOF) {
        /* Handle error */
      }
    }
  }
  return 0;
}

Compliant Solution

If you have a great deal of unsafe envp code, you can save time in your remediation by replacing

int main(int argc, char *argv[], char *envp[]) {
  /* ... */
}

with

#if  defined (_POSIX_) || defined (__USE_POSIX)
  extern char **environ;
  #define envp environ
#elif defined(_WIN32)
  _CRTIMP extern char **_environ;
  #define envp _environ
#endif

int main(int argc, char *argv[]) {
  /* ... */
}

Risk Assessment

Using the envp environment pointer after the environment has been modified can result in undefined behavior.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography