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

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. Due to the manner in which environment variables are stored, multiple environment variables with the same name can cause unexpected results.

Non-Compliant Code Example

In this non-compliant code example, the getenv function is used to retrieve a value from the environment.

char *temp;
char *copy;

if ((temp = getenv("TEST_ENV")) != NULL) {
  copy = (char *)malloc(strlen(temp) + 1);
  if (copy != NULL) {
    strcpy(copy, temp);
  }
  else {
    /* handle error condition */
  }

  copy[0] = 'a';
  setenv("TEST_ENV", copy, 1);
}
else {
  return -1;
}

Compliant Solution (glibc)

Depending on the implementation, a program may not consistently choose the same value if there are multiple environment variables with the same name. 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.

Compliant Solution (POSIX)

In this compliant solution, the environ array is manually searched for duplicate key entries. Any duplicates may indicate an attack.

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() {
  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;
}

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"

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