Example 1
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.
(Bad Code)
Example Languages: C and C++
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.
(Good Code)
Example Languages: C and C++
double *foo;
...
foo = (double *)malloc(sizeof(*foo));
Example 2
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.
Description
