CWE-187: Partial String Comparison

Description

The product performs a comparison that only examines a portion of a factor before determining whether there is a match, such as a substring, leading to resultant weaknesses.

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

Potential Impact

Integrity, Access Control

Alter Execution Logic, Bypass Protection Mechanism

Demonstrative Examples

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.

Bad

/* 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:

Attack

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.

Mitigations & Prevention

Testing

Thoroughly test the comparison scheme before deploying code into production. Perform positive testing as well as negative testing.

Real-World CVE Examples

CVE ID	Description
CVE-2014-6394	Product does not prevent access to restricted directories due to partial string comparison with a public directory
CVE-2004-1012	Argument parser of an IMAP server treats a partial command "body[p" as if it is "body.peek", leading to index error and out-of-bounds corruption.
CVE-2004-0765	Web browser only checks the hostname portion of a certificate when the hostname portion of the URI is not a fully qualified domain name (FQDN), which allows remote attackers to spoof trusted certifica
CVE-2002-1374	One-character password by attacker checks only against first character of real password.
CVE-2000-0979	One-character password by attacker checks only against first character of real password.

Taxonomy Mappings

PLOVER: — Partial Comparison

Frequently Asked Questions

What is CWE-187?

CWE-187 (Partial String Comparison) is a software weakness identified by MITRE's Common Weakness Enumeration. It is classified as a Variant-level weakness. The product performs a comparison that only examines a portion of a factor before determining whether there is a match, such as a substring, leading to resultant weaknesses.

How can CWE-187 be exploited?

Attackers can exploit CWE-187 (Partial String Comparison) to alter execution logic, bypass protection mechanism. This weakness is typically introduced during the Implementation phase of software development.

How do I prevent CWE-187?

Key mitigations include: Thoroughly test the comparison scheme before deploying code into production. Perform positive testing as well as negative testing.

What is the severity of CWE-187?

CWE-187 is classified as a Variant-level weakness (Low-Medium abstraction). It has been observed in 5 real-world CVEs.