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.

Primary

Implementation

Integrity
Access Control
Alter Execution Logic
Bypass Protection Mechanism

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

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) {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:

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.

CVE-2014-6394
Product does not prevent access to restricted directories due to partial string comparison with a public directory
https://www.cve.org/CVERecord?id=CVE-2014-6394

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.
https://www.cve.org/CVERecord?id=CVE-2004-1012

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 certificates.
https://www.cve.org/CVERecord?id=CVE-2004-0765

CVE-2002-1374
One-character password by attacker checks only against first character of real password.
https://www.cve.org/CVERecord?id=CVE-2002-1374

CVE-2000-0979
One-character password by attacker checks only against first character of real password.
https://www.cve.org/CVERecord?id=CVE-2000-0979

Partial Comparison

Allowed
This CWE entry is at the Variant level of abstraction, which is a preferred level of abstraction for mapping to the root causes of vulnerabilities.
Carefully read both the name and description to ensure that this mapping is an appropriate fit. Do not try to 'force' a mapping to a lower-level Base/Variant simply to comply with this preferred level of abstraction.

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

PLOVER
2006-07-19
Draft 3
2006-07-19

Eric Dalci
Cigital
2008-07-01
updated Potential_Mitigations, Time_of_Introduction

CWE Content Team
MITRE
2008-09-08
updated Description, Relationships, Other_Notes, Taxonomy_Mappings, Weakness_Ordinalities

CWE Content Team
MITRE
2009-12-28
updated Demonstrative_Examples, Observed_Examples, Other_Notes, Relationship_Notes

CWE Content Team
MITRE
2011-03-29
updated Relationships

CWE Content Team
MITRE
2011-06-01
updated Common_Consequences, Demonstrative_Examples

CWE Content Team
MITRE
2011-06-27
updated Common_Consequences

CWE Content Team
MITRE
2012-05-11
updated Relationships

CWE Content Team
MITRE
2012-10-30
updated Potential_Mitigations

CWE Content Team
MITRE
2014-07-30
updated Relationships

CWE Content Team
MITRE
2017-11-08
updated Applicable_Platforms

CWE Content Team
MITRE
2018-03-27
updated Name, Observed_Examples, Relationships, Type

CWE Content Team
MITRE
2019-01-03
updated Relationships

CWE Content Team
MITRE
2020-02-24
updated Relationships

CWE Content Team
MITRE
2023-01-31
updated Description

CWE Content Team
MITRE
2023-04-27
updated Relationships

CWE Content Team
MITRE
2023-06-29
updated Mapping_Notes

CWE Content Team
MITRE
2025-12-11
4.19
2025-12-11
updated Applicable_Platforms

Partial Comparison