ENV34-C. Do not rely on the pointer to the string returned by getenv() following a subsequent invocation

The getenv() function searches an environment list, provided by the host environment, for a string that matches a specified name. Do not rely on the pointer to the string returned by getenv() following a subsequent invocation.

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 make a copy of the value returned by getenv(), but instead store the pointer somewhere for later use, you could end up with a dangling pointer or a different value altogether.

Implementation Details

According to the Microsoft Visual Studio 2005/.NET Framework 2.0 help pages http://msdn2.microsoft.com/en-us/library/tehxacec(VS.80).aspx:

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

char *pwd;
char *home;
  
pwd = getenv("PWD");
if (!pwd) return -1;
home = getenv("HOME");
if (!home) return -1;

if (strcmp(pwd, home) == 0) {
  puts("pwd and home are the same.\n");
}
else {
  puts("pwd and home 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. http://msdn2.microsoft.com/en-us/library/ms175774(VS.80).aspx

The ((_dupenv_s()}} function searches the list of environment variables for varname. If the variable is found, a buffer is allocated, the variable's value is copied into the buffer, and the buffer's address and length are returned in buffer and numberOfElements. 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 *pValue;
   size_t len;
   errno_t err = _dupenv_s( &pValue, &len, "pathext" );
   if ( err ) return -1;
   printf( "pathext = %s\n", pValue );
   free( pValue );
   err = _dupenv_s( &pValue, &len, "nonexistentvariable" );
   if ( err ) return -1;
   printf( "nonexistentvariable = %s\n", pValue );
   free( pValue ); // It's OK to call free with NULL

Compliant Solution (Windows)

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

#include <stdlib.h>
#include <stdio.h>

int main( void ) {
   char* libvar;
   size_t requiredSize;

   getenv_s( &requiredSize, NULL, 0, "LIB");

   libvar = (char*) malloc(requiredSize * sizeof(char));
   if (!libvar)
   {
      printf("Failed to allocate memory!\n");
      exit(1);
   }

   // Get the value of the LIB environment variable.
   getenv_s( &requiredSize, libvar, requiredSize, "LIB" );

   if( libvar != NULL )
      printf( "Original LIB variable is: %s\n", libvar );

   // Attempt to change path. Note that this only affects
   // the environment variable of the current process. The command
   // processor's environment is not changed.
   _putenv_s( "LIB", "c:\\mylib;c:\\yourlib" );

   getenv_s( &requiredSize, NULL, 0, "LIB");

   libvar = (char*) malloc(requiredSize * sizeof(char));
   if (!libvar) {
      printf("Failed to allocate memory!\n");
      exit(1);
   }

   // Get the new value of the LIB environment variable. 
   getenv_s( &requiredSize, libvar, requiredSize, "LIB" );

   if( libvar != NULL )
      printf( "New LIB variable is: %s\n", libvar );
}

There is a race condition here even after you call getenv() and before you copy. Be careful to only manipulate the process environment from a single thread at a time.

Risk Assessment

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

References

[[Dowd 06]] Chapter 10, "UNIX II: Processes"
[[ISO/IEC 9899-1999]] Section 7.20.4, "Communication with the environment"
[[Open Group 04]] Chapter 8, "Environment Variables"
[[Viega 03]] Section 3.6, "Using Environment Variables Securely"
[[Wheeler 03]] Section 5.2, "Environment Variables"