Example - 1

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. */// 

logEvent("Auth failure of username using strlen of inUser");return(AUTH_FAIL);

logEvent("Auth success of password using strlen of inUser");return(AUTH_SUCCESS);

logEvent("Auth fail of password using sizeof");return(AUTH_FAIL);if (strncmp(username, inUser, strlen(inUser))) {}if (! strncmp(pass, inPass, strlen(inPass))) {}else {}

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

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:

ppapaspass

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.