Common Weakness Enumeration

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CWE-187: Partial Comparison

Weakness ID: 187
Abstraction: Base
Status: Incomplete
Presentation Filter:
+ Description

Description Summary

The software 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.

Extended Description

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

+ Time of Introduction
  • Implementation
+ Applicable Platforms



+ Common Consequences
Access Control

Technical Impact: Alter execution logic; Bypass protection mechanism

+ Demonstrative Examples

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.

(Bad Code)
Example Language:
/* 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");
if (! strncmp(pass, inPass, strlen(inPass))) {
logEvent("Auth success of password using strlen of inUser");
else {
logEvent("Auth fail of password using sizeof");

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) {
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:


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
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.
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 certificates.
One-character password by attacker checks only against first character of real password.
One-character password by attacker checks only against first character of real password.
+ Potential Mitigations

Phase: Testing

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

+ Weakness Ordinalities
(where the weakness exists independent of other weaknesses)
+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfCategoryCategory171Cleansing, Canonicalization, and Comparison Errors
Development Concepts (primary)699
ChildOfWeakness ClassWeakness Class697Insufficient Comparison
Research Concepts (primary)1000
ChildOfCategoryCategory977SFP Secondary Cluster: Design
Software Fault Pattern (SFP) Clusters (primary)888
ParentOfWeakness BaseWeakness Base839Numeric Range Comparison Without Minimum Check
Research Concepts (primary)1000
CanFollowWeakness ClassWeakness Class185Incorrect Regular Expression
Research Concepts1000
PeerOfWeakness BaseWeakness Base625Permissive Regular Expression
Research Concepts1000
+ Relationship Notes

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

+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
PLOVERPartial Comparison
+ Content History
Submission DateSubmitterOrganizationSource
PLOVERExternally Mined
Modification DateModifierOrganizationSource
2008-07-01Eric DalciCigitalExternal
updated Potential_Mitigations, Time_of_Introduction
2008-09-08CWE Content TeamMITREInternal
updated Description, Relationships, Other_Notes, Taxonomy_Mappings, Weakness_Ordinalities
2009-12-28CWE Content TeamMITREInternal
updated Demonstrative_Examples, Observed_Examples, Other_Notes, Relationship_Notes
2011-03-29CWE Content TeamMITREInternal
updated Relationships
2011-06-01CWE Content TeamMITREInternal
updated Common_Consequences, Demonstrative_Examples
2011-06-27CWE Content TeamMITREInternal
updated Common_Consequences
2012-05-11CWE Content TeamMITREInternal
updated Relationships
2012-10-30CWE Content TeamMITREInternal
updated Potential_Mitigations
2014-07-30CWE Content TeamMITREInternal
updated Relationships

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Page Last Updated: January 18, 2017