Use of sizeof() on a Pointer Type

Description

The use of sizeof() on a pointer can sometimes generate useful information. An obvious case is to find out the wordsize on a platform. More often than not, the appearance of sizeof(pointer) indicates a bug.

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

Care should be taken to ensure sizeof returns the size of the data structure itself, and not the size of the pointer to the data structure.

In this example, sizeof(foo) returns the size of the pointer.

double *foo;
...
foo = (double *)malloc(sizeof(foo));

In this example, sizeof(*foo) returns the size of the data structure and not the size of the pointer.

double *foo;
...
foo = (double *)malloc(sizeof(*foo));

Example Two

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) {

  printf("Sizeof username = %d\n", sizeof(username));
  printf("Sizeof pass = %d\n", sizeof(pass));

  if (strncmp(username, inUser, sizeof(username))) {
    printf("Auth failure of username using sizeof\n");
    return(AUTH_FAIL);
  }
  /* Because of CWE-467, the sizeof returns 4 on many platforms and architectures. */

  if (! strncmp(pass, inPass, sizeof(pass))) {
    printf("Auth success of password using sizeof\n");
    return(AUTH_SUCCESS);
  }
  else {
    printf("Auth fail of password using sizeof\n");
    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) {
    ExitError("Authentication failed");
  }
  else {
    DoAuthenticatedTask(argv[1]);
  }

}

In AuthenticateUser(), because sizeof() is applied to a parameter with an array type, the sizeof() call might return 4 on many modern architectures. As a result, the strncmp() call only checks the first four characters of the input password, resulting in a partial comparison (CWE-187), leading to improper authentication (CWE-287).

Because of the partial comparison, any of these passwords would still cause authentication to succeed for the "admin" user:

pass5
passABCDEFGH
passWORD

Because only 4 characters are checked, 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 "adminXYZ" and "administrator" will succeed for the username.

See Also