Partial String Comparison

Incomplete Variant

Structure: Simple

Description

This weakness occurs when software checks only part of a string or token to determine a match, instead of comparing the entire value. This incomplete validation can lead to incorrect security decisions.

Extended Description

Partial string comparison happens when a system, such as an authentication module, validates only the first few characters of an input against a stored value. For example, if a password check only verifies the initial 5 characters, an attacker could gain access by providing any password that starts with those same 5 characters, regardless of the full correct password. This flaw fundamentally undermines security logic by treating partially matching inputs as fully authorized. Developers should always enforce complete, exact-string comparisons for security-critical operations like authentication, authorization tokens, or integrity checks to prevent attackers from exploiting these predictable shortcuts.

Common Consequences 1

Scope: IntegrityAccess Control

Impact: Alter Execution LogicBypass Protection Mechanism

Potential Mitigations 1

Phase: Testing

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

Demonstrative Examples 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.

Code Example:

Bad

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

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 (Improper Authentication).

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

Code Example:Attack
bash

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.

Observed Examples 5

CVE-2014-6394Product does not prevent access to restricted directories due to partial string comparison with a public directory

CVE-2004-1012Argument 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-2004-0765Web 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 certificates.

CVE-2002-1374One-character password by attacker checks only against first character of real password.

CVE-2000-0979One-character password by attacker checks only against first character of real password.

Applicable Platforms

Languages:

Not Language-Specific : Undetermined

Modes of Introduction

Implementation

Related Weaknesses

ChildOf:

Incomplete Comparison with Missing Factors (CWE-1023)

Taxonomy Mapping

PLOVER

Notes

RelationshipThis is conceptually similar to other weaknesses, such as insufficient verification and regular expression errors. It is primary to some weaknesses.