Partial String Comparison

Description

For example, an attacker might succeed in authentication by providing a small password that matches the associated portion of the larger, correct password.

Demonstrations

The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

This example defines a fixed username and password. The AuthenticateUser() function is intended to accept a username and a password from an untrusted user, and check to ensure that it matches the username and password. If the username and password match, AuthenticateUser() is intended to indicate that authentication succeeded.

/* Ignore CWE-259 (hard-coded password) and CWE-309 (use of password system for authentication) for this example. */

char *username = "admin";
char *pass = "password";

int AuthenticateUser(char *inUser, char *inPass) {
  if (strncmp(username, inUser, strlen(inUser))) {
    logEvent("Auth failure of username using strlen of inUser");
    return(AUTH_FAIL);
  }
  if (! strncmp(pass, inPass, strlen(inPass))) {
    logEvent("Auth success of password using strlen of inUser");
    return(AUTH_SUCCESS);
  }
  else {
    logEvent("Auth fail of password using sizeof");
    return(AUTH_FAIL);
  }
}

int main (int argc, char **argv) {

  int authResult;

  if (argc < 3) {
    ExitError("Usage: Provide a username and password");
  }
  authResult = AuthenticateUser(argv[1], argv[2]);
  if (authResult == AUTH_SUCCESS) {
    DoAuthenticatedTask(argv[1]);
  }
  else {
    ExitError("Authentication failed");
  }

}

In AuthenticateUser(), the strncmp() call uses the string length of an attacker-provided inPass parameter in order to determine how many characters to check in the password. So, if the attacker only provides a password of length 1, the check will only examine the first byte of the application's password before determining success.

As a result, this partial comparison leads to improper authentication (CWE-287).

Any of these passwords would still cause authentication to succeed for the "admin" user:

p
pa
pas
pass

This significantly reduces the search space for an attacker, making brute force attacks more feasible.

The same problem also applies to the username, so values such as "a" and "adm" will succeed for the username.

While this demonstrative example may not seem realistic, see the Observed Examples for CVE entries that effectively reflect this same weakness.

See Also