ENV02-A. Beware of multiple environment variables with the same effective name

Section 7.20.4.5 of C99 says that [[ISO/IEC 9899-1999]]

The set of environment names and the method for altering the environment list are implementation-defined.

The getenv() function searches an environment list for a string that matches a specified name, and returns a pointer to a string associated with the matched list member. Depending on the implementation, multiple environment variables with the same name may be allowed and can cause unexpected results if a program cannot consistently choose the same value. The GNU glibc library addresses this issue in getenv() and setenv() by always using the first variable it encounters and ignoring the rest. Other implementations are following suit, although it is unwise to rely on this.

One common difference between implementations is whether or not environment variables are case sensitive. While UNIX-like implementations are generally case sensitive, environment variables are "not case sensitive in Windows 98/Me and Windows NT/2000/XP". [[MSDN]]

Duplicate Environment Variable Detection (POSIX)

Here is a function that uses the environ array (specified in POSIX) to manually search for duplicate key entries. Any duplicate environment variables are considered an attack, and so the program immediately terminates.

extern char ** environ;

int main(void) {
  if(multiple_vars_with_same_name()) {
    printf("Someone may be tampering.\n");
    return 1;
  }

  /* ... */

  return 0;
}

int multiple_vars_with_same_name(void) {
  size_t i;
  size_t j;
  size_t k;
  size_t l;
  size_t len_i;
  size_t len_j;

  for(i = 0; environ[i] != NULL; i++) {
    for(j = i; environ[j] != NULL; j++) {
      if(i != j) {
        k = 0;
        l = 0;

        len_i = strlen(environ[i]);
        len_j = strlen(environ[j]);

        while(k < len_i && l < len_j) {
          if(environ[i][k] != environ[j][l])
            break;

          if(environ[i][k] == '=')
            return 1;

          k++;
          l++;
        }
      }
    }
  }
  return 0;
}

Non-Compliant Code Example

This code behaves differently when compiled under Linux and Windows.

char *temp;

if (putenv("TEST_ENV=foo") != 0) {
  /* Handle Error */
}
if (putenv("Test_ENV=bar") != 0) {
  /* Handle Error */
}

temp = getenv("TEST_ENV");

if (temp == NULL) {
  /* Handle Error */
}

printf("%s\n",temp);

On a test IA-32 Linux machine with GCC Compiler Version 3.4.4, this code prints:

foo

Whereas, on a test IA-32 Windows XP machine with Microsoft Visual C++ 2008 Express, it prints:

bar

Compliant Solution

Portable code should use environment variables that differ by more than capitalization.

char *temp;

if (putenv("TEST_ENV=foo") != 0) {
  /* Handle Error */
}
if (putenv("OTHER_ENV=bar") != 0) {
  /* Handle Error */
}

temp = getenv("TEST_ENV");

if (temp == NULL) {
  /* Handle Error */
}

printf("%s\n",temp);

Risk Assessment

An adversary can create multiple environment variables with the same name. If the program checks one copy but uses another, security checks may be circumvented.

Related Vulnerabilities

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

References

[[ISO/IEC 9899-1999]] Section 7.20.4, "Communication with the environment"
[[MSDN]] getenv()

ENV01-A. Do not make assumptions about the size of an environment variable      10. Environment (ENV)       ENV03-A. Sanitize the environment when invoking external programs