CWE

Common Weakness Enumeration

A Community-Developed List of Software Weakness Types

CWE/SANS Top 25 Most Dangerous Software Errors
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ID

CWE-187: Partial String Comparison

Weakness ID: 187
Abstraction: Variant
Structure: Simple
Status: Incomplete
Presentation Filter:
+ Description
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.
+ Relationships

The table(s) below shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore.

+ Relevant to the view "Research Concepts" (CWE-1000)
NatureTypeIDName
ChildOfBaseBase - a weakness that is described in an abstract fashion, but with sufficient details to infer specific methods for detection and prevention. More general than a Variant weakness, but more specific than a Class weakness.1023Incomplete Comparison with Missing Factors
ParentOfBaseBase - a weakness that is described in an abstract fashion, but with sufficient details to infer specific methods for detection and prevention. More general than a Variant weakness, but more specific than a Class weakness.839Numeric Range Comparison Without Minimum Check
PeerOfBaseBase - a weakness that is described in an abstract fashion, but with sufficient details to infer specific methods for detection and prevention. More general than a Variant weakness, but more specific than a Class weakness.625Permissive Regular Expression
CanFollowClassClass - a weakness that is described in a very abstract fashion, typically independent of any specific language or technology. More general than a Base weakness.185Incorrect Regular Expression
+ Relevant to the view "Development Concepts" (CWE-699)
NatureTypeIDName
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.171Cleansing, Canonicalization, and Comparison Errors
+ Modes Of Introduction

The different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the software life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.

PhaseNote
Implementation
+ Applicable Platforms
The listings below show possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance.

Languages

Class: Language-Independent (Undetermined Prevalence)

+ Common Consequences

The table below specifies different individual consequences associated with the weakness. The Scope identifies the application security area that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in exploiting this weakness. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a weakness will be exploited to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.

ScopeImpactLikelihood
Integrity
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");
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 code)
 
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
Product does not prevent access to restricted directories due to partial string comparison with a public directory
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
OrdinalityDescription
Primary
(where the weakness exists independent of other weaknesses)
+ Memberships
This MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources.
NatureTypeIDName
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.977SFP Secondary Cluster: Design
+ Notes

Relationship

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 DateSubmitterOrganization
PLOVER
Modifications
Modification DateModifierOrganization
2008-07-01Eric DalciCigital
updated Potential_Mitigations, Time_of_Introduction
2008-09-08CWE Content TeamMITRE
updated Description, Relationships, Other_Notes, Taxonomy_Mappings, Weakness_Ordinalities
2009-12-28CWE Content TeamMITRE
updated Demonstrative_Examples, Observed_Examples, Other_Notes, Relationship_Notes
2011-03-29CWE Content TeamMITRE
updated Relationships
2011-06-01CWE Content TeamMITRE
updated Common_Consequences, Demonstrative_Examples
2011-06-27CWE Content TeamMITRE
updated Common_Consequences
2012-05-11CWE Content TeamMITRE
updated Relationships
2012-10-30CWE Content TeamMITRE
updated Potential_Mitigations
2014-07-30CWE Content TeamMITRE
updated Relationships
2017-11-08CWE Content TeamMITRE
updated Applicable_Platforms
2018-03-27CWE Content TeamMITRE
updated Name, Observed_Examples, Relationships, Type
Previous Entry Names
Change DatePrevious Entry Name
2018-03-27Partial Comparison

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Page Last Updated: March 29, 2018