ENV00-A. Do not store the pointer to the string returned by getenv()

The getenv() function searches an environment list, provided by the host environment, for a string that matches a specified name. The getenv() function returns a pointer to a string associated with the matched list member. It is best not to store this pointer as it may be overwritten by a subsequent call to the getenv() function [[ISO/IEC 9899-1999]] or invalidated as a result of changes made to the environment list through calls to putenv(), setenv(), or other means. Storing the pointer for later use could result in a dangling pointer or a pointer to incorrect data.

According to C99 [[ISO/IEC 9899-1999]]:

The getenv function returns a pointer to a string associated with the matched list member. The string pointed to shall not be modified by the program, but may be overwritten by a subsequent call to the getenv function.

This allows an implementation, for example, to copy the environmental variable to an internal static buffer and return a pointer to that buffer.

If you do not immediately use and discard this string, make a copy of the referenced string returned by getenv() so that this copy may be safely referenced at a later time. The getenv() function is not thread-safe. Make sure to address any possible race conditions resulting from the use of this function.

Implementation Details

According to the Microsoft Visual Studio 2005/.NET Framework 2.0 help pages:

The getenv function searches the list of environment variables for varname. getenv is not case sensitive in the Windows operating system. getenv and _putenv use the copy of the environment pointed to by the global variable _environ to access the environment. getenv operates only on the data structures accessible to the run-time library and not on the environment "segment" created for the process by the operating system. Therefore, programs that use the envp argument to main or wmain may retrieve invalid information.

Non-Compliant Coding Example

This non-compliant code example compares the value of the TMP and TEMP environment variables to determine if they are the same. This code example is non-compliant because the string referenced by tmpvar may be overwritten as a result of the second call to getenv() function. As a result, it is possible that both tmpvar and tempvar will compare equal even if the two environment variables have different values.

char *tmpvar;
char *tempvar;
  
tmpvar = getenv("TMP");
if (!tmpvar) return -1;
tempvar = getenv("TEMP");
if (!tempvar) return -1;

if (strcmp(tmpvar, tempvar) == 0) {
  puts("TMP and TEMP are the same.\n");
}
else {
  puts("TMP and TEMP are NOT the same.\n");    
}

Compliant Solution (Windows)

Microsoft Visual Studio 2005 provides provides the getenv_s() and _wgetenv_s() functions for getting a value from the current environment.

char *tmpvar;
char *tempvar;
size_t requiredSize;

getenv_s(&requiredSize, NULL, 0, "TMP");
tmpvar= malloc(requiredSize * sizeof(char));
if (!tmpvar) {
   /* handle error condition */
}
getenv_s(&requiredSize, tmpvar, requiredSize, "TMP" );

getenv_s(&requiredSize, NULL, 0, "TEMP");
tempvar= malloc(requiredSize * sizeof(char));
if (!tempvar) {
   /* handle error condition */
}
getenv_s(&requiredSize, tempvar, requiredSize, "TEMP" );

if (strcmp(tmpvar, tempvar) == 0) {
  puts("TMP and TEMP are the same.\n");
}
else {
  puts("TMP and TEMP are NOT the same.\n");    
}

Compliant Solution (Windows)

Microsoft Visual Studio 2005 provides provides the _dupenv_s() and _wdupenv_s() functions for getting a value from the current environment. [Microsoft Visual Studio 2005/.NET Framework 2.0 help pages].

The _dupenv_s() function searches the list of environment variables for a specified name. If the name is found, a buffer is allocated, the variable's value is copied into the buffer, and the buffer's address and number of elements are returned. By allocating the buffer itself, _dupenv_s() provides a more convenient alternative to getenv_s(), _wgetenv_s().

It is the calling program's responsibility to free the memory by calling free().

char *tmpvar;
char *tempvar;
size_t len;

errno_t err = _dupenv_s(&tmpvar, &len, "TMP");
if (err) return -1;
errno_t err = _dupenv_s(&tempvar, &len, "TEMP");
if (err) {
  free(tmpvar);
  return -1;
}

if (strcmp(tmpvar, tempvar) == 0) {
  puts("TMP and TEMP are the same.\n");
}
else {
  puts("TMP and TEMP are NOT the same.\n");    
}
free(tmpvar);
free(tempvar);

Compliant Solution (POSIX)

The following compliant solution depends on the POSIX strdup() function to make a copy of the environment variable string.

char *tmpvar = strdup(getenv("TMP"));
char *tempvar = strdup(getenv("TEMP"));
if (!tmpvar) return -1;
if (!tempvar) return -1;

if (strcmp(tmpvar, tempvar) == 0) {
  puts("TMP and TEMP are the same.\n");
}
else {
  puts("TMP and TEMP are NOT the same.\n");    
}

If an environmental variable does not exist, the call to getenv() returns a null pointer. In these cases, the call to strdup() should also return a null pointer, but it is important to verify this as this behavior is not guaranteed by POSIX [[Open Group 04]]

Compliant Solution

This compliant solution is fully portable.

char *tmpvar;
char *tempvar;
char *temp;

if ( (temp = getenv("TMP")) != NULL) {
  tmpvar= malloc(strlen(temp)+1);
  if (tmpvar != NULL) {
    strcpy(tmpvar, temp);
  }
  else {
    /* handle error condition */
  }
}
else {
  return -1;
}

if ( (temp = getenv("TEMP")) != NULL) {
  tempvar= malloc(strlen(temp)+1);
  if (tempvar != NULL) {
    strcpy(tempvar, temp);
  }
  else {
    /* handle error condition */
  }
}
else {
  return -1;
}

if (strcmp(tmpvar, tempvar) == 0) {
  puts("TMP and TEMP are the same.\n");
}
else {
  puts("TMP and TEMP are NOT the same.\n");    
}

Risk Assessment

Examples of vulnerabilities resulting from the violation of this recommendation can be found on the CERT website.

References

[[ISO/IEC 9899-1999]] Section 7.20.4, "Communication with the environment"
[[Open Group 04]] Chapter 8, "Environment Variables", strdup
[[Viega 03]] Section 3.6, "Using Environment Variables Securely"