Partial Comparison
Weakness ID: 187 (Weakness Base)Status: Incomplete
+ 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

Languages

All

+ 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: C 
/* 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 check 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)
 
p
pa
pas
pass

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
ReferenceDescription
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.
+ Potential Mitigations

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

+ Weakness Ordinalities
OrdinalityDescription
Primary
(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
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
Submissions
Submission DateSubmitterOrganizationSource
PLOVERExternally Mined
Modifications
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
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