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CWE-187 Variante Incomplet
Partial String Comparison
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…
Définition
What is CWE-187?
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.
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.
Impact réel
Real-world CVEs caused by CWE-187
-
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.
Comment les attaquants l'exploitent
Parcours de l'attaquant étape par étape
- 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.
- 2
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.
- 3
As a result, this partial comparison leads to improper authentication (CWE-287).
- 4
Any of these passwords would still cause authentication to succeed for the "admin" user:
- 5
This significantly reduces the search space for an attacker, making brute force attacks more feasible.
Exemple de code vulnérable
Vulnerable C
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");
}
} Charge utile de l'attaquant
Any of these passwords would still cause authentication to succeed for the "admin" user:
p
pa
pas
pass Exemple de code sécurisé
Secure pseudo
// Validate, sanitize, or use a safe API before reaching the sink.
function handleRequest(input) {
const safe = validateAndEscape(input);
return executeWithGuards(safe);
} What changed: the unsafe sink is replaced (or the input is validated/escaped) so the same payload no longer triggers the weakness.
Liste de contrôle de prévention
How to prevent CWE-187
- Testing Thoroughly test the comparison scheme before deploying code into production. Perform positive testing as well as negative testing.
Signaux de détection
How to detect CWE-187
Exécuter des tests de sécurité applicative dynamique (DAST) contre le point de terminaison en ligne.
Surveiller les journaux runtime pour détecter des traces d'exception inhabituelles, des entrées malformées ou des tentatives de contournement d'autorisation.
Revue de code : signaler tout nouveau code qui traite les entrées de cette surface sans utiliser les helpers du framework validés.
CWE-187
Don't catalog this weakness. Prove it's reachable.
Plexicus turns CWE catalogs into evidence: every CWE-pattern is matched against your real code graph, reach is proven on a sandbox clone, and verified findings ship as reviewed PRs.
Questions fréquentes Qu'est-ce que CWE-187 ?
Quelle est la gravité de CWE-187 ?
Quels langages ou plateformes sont affectés par CWE-187 ?
Comment puis-je prévenir CWE-187 ?
Comment Plexicus détecte et corrige CWE-187 ?
Où puis-je en savoir plus sur CWE-187 ?
Frequently asked questions
Qu'est-ce que CWE-187 ?
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.
Quelle est la gravité de CWE-187 ?
MITRE n'a pas publié de note de probabilité d'exploitation pour cette faiblesse. Traitez-la comme un impact moyen jusqu'à ce que votre modèle de menace prouve le contraire.
Quels langages ou plateformes sont affectés par CWE-187 ?
MITRE n'a pas spécifié les plateformes affectées pour ce CWE — il peut s'appliquer à la plupart des stacks applicatives.
Comment puis-je prévenir CWE-187 ?
Thoroughly test the comparison scheme before deploying code into production. Perform positive testing as well as negative testing.
Comment Plexicus détecte et corrige CWE-187 ?
Le moteur SAST de Plexicus reconnaît la signature de flux de données de CWE-187 à chaque commit. Lorsqu'une correspondance est trouvée, notre agent Codex Remedium ouvre une PR de correction avec le code corrigé, les tests et un résumé d'une ligne pour le relecteur.
Où puis-je en savoir plus sur CWE-187 ?
MITRE publie la définition canonique à https://cwe.mitre.org/data/definitions/187.html. Vous pouvez également consulter la documentation OWASP et NIST pour des conseils adjacents.
Faiblesses associées
Weaknesses related to CWE-187
CWE-1023 Parent
Incomplete Comparison with Missing Factors
This weakness occurs when a program compares two items but fails to check all the necessary attributes that define their true…
CWE-184 Frère
Incomplete List of Disallowed Inputs
This vulnerability occurs when a security filter or validation mechanism relies on a 'denylist'—a predefined list of forbidden inputs—but…
CWE-478 Frère
Missing Default Case in Multiple Condition Expression
This vulnerability occurs when code with multiple conditional branches, like a switch statement, lacks a default case to handle unexpected…
CWE-839 Frère
Numeric Range Comparison Without Minimum Check
This vulnerability occurs when software validates that a number is within an acceptable range by only checking that it's less than or…
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