POS34-C. Do not call putenv() with a pointer to an automatic variable as the argument

The POSIX function putenv() is used to set environment variable values. The putenv() function does not create a copy of the string supplied to it as an argument; rather, it inserts a pointer to the string into the environment array. If a pointer to a buffer of automatic storage duration is supplied as an argument to putenv(), the memory allocated for that buffer may be overwritten when the containing function returns and stack memory is recycled. This behavior is noted in the Open Group Base Specifications, Issue 6 [Open Group 2004]:

A potential error is to call putenv() with an automatic variable as the argument, then return from the calling function while string is still part of the environment.

The actual problem occurs when passing a pointer to an automatic variable to putenv(). An automatic pointer to a static buffer would work as intended.

Noncompliant Code Example

In this noncompliant code example, a pointer to a buffer of automatic storage duration is used as an argument to putenv() [Dowd 2006]. The TEST environment variable may take on an unintended value if it is accessed after func() has returned and the stack frame containing env has been recycled.

Note that this example also violates DCL30-C. Declare objects with appropriate storage durations.

int func(const char *var) {
  char env[1024];
  int retval = snprintf(env, sizeof(env),"TEST=%s", var);
  if (retval < 0 || (size_t)retval >= sizeof(env)) {
    /* Handle error */
  }

  return putenv(env);
}

Compliant Solution (static)

This compliant solution uses a static array for the argument to putenv().

int func(const char *var) {
  static char env[1024];

  int retval = snprintf(env, sizeof(env),"TEST=%s", var);
  if (retval < 0 || (size_t)retval >= sizeof(env)) {
    /* Handle error */
  }

  return putenv(env);
}

Compliant Solution (Heap Memory)

This compliant solution dynamically allocates memory for the argument to putenv():

int func(const char *var) {
  static char *oldenv; 
  const char *env_format = "TEST=%s";
  const size_t len = strlen(var) + strlen(env_format);
  char *env = (char *) malloc(len);
  if (env == NULL) {
    return -1;
  }
  int retval = snprintf(env, len, env_format, var);
  if (retval < 0 || (size_t)retval >= len) {
    /* Handle error */
  }
  if (putenv(env) != 0) {
    free(env);
    return -1;
  }
  if (oldenv != NULL) {
    free(oldenv); /* avoid memory leak */ 
  }
  oldenv = env; 
  return 0;
}

The POSIX setenv() function is preferred over this function [Open Group 2004].

Compliant Solution (setenv())

The setenv() function allocates heap memory for environment variables, which eliminates the possibility of accessing volatile stack memory:

int func(const char *var) {
  return setenv("TEST", var, 1);
}

Using setenv() is easier and consequently less error prone than using putenv().

Risk Assessment

Providing a pointer to a buffer of automatic storage duration as an argument to putenv() may cause that buffer to take on an unintended value. Depending on how and when the buffer is used, it can cause unexpected program behavior or possibly allow an attacker to run arbitrary code.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography