CWE-187 Variante Incompleto

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…

Definición

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.
Impacto en el mundo real

Real-world CVEs caused by CWE-187

  • CVE-2014-6394

    Product does not prevent access to restricted directories due to partial string comparison with a public directory

  • 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.

  • 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.

  • CVE-2002-1374

    One-character password by attacker checks only against first character of real password.

  • CVE-2000-0979

    One-character password by attacker checks only against first character of real password.

Cómo lo explotan los atacantes

Ruta del atacante paso a paso

  1. 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. 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. 3

    As a result, this partial comparison leads to improper authentication (CWE-287).

  4. 4

    Any of these passwords would still cause authentication to succeed for the "admin" user:

  5. 5

    This significantly reduces the search space for an attacker, making brute force attacks more feasible.

Ejemplo de código vulnerable

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.

Vulnerable 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");
  		}
  }
Payload del atacante

Any of these passwords would still cause authentication to succeed for the "admin" user:

Payload del atacante 
p
  pa
  pas
  pass
Ejemplo de código seguro

Secure pseudo

Seguro 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.
Lista de prevención

How to prevent CWE-187

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

How to detect CWE-187

SAST High

Ejecuta análisis estático (SAST) sobre el código buscando el patrón inseguro en el flujo de datos.

DAST Moderate

Ejecuta pruebas dinámicas de seguridad de aplicaciones (DAST) contra el endpoint en vivo.

Runtime Moderate

Vigila los logs en tiempo de ejecución para detectar trazas de excepción inusuales, entradas malformadas o intentos de bypass de autorización.

Code review Moderate

Revisión de código: marca cualquier código nuevo que maneje entrada desde esta superficie sin usar los helpers validados del framework.

Auto-corrección de Plexicus

Plexicus detecta automáticamente CWE-187 y abre un PR de corrección en menos de 60 segundos.

Codex Remedium escanea cada commit, identifica esta debilidad concreta y entrega un pull request listo para revisión con el parche. Sin tickets. Sin traspasos.

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Preguntas frecuentes

Frequently asked questions

¿Qué es 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.

¿Qué gravedad tiene CWE-187?

MITRE no ha publicado una calificación de probabilidad de explotación para esta debilidad. Trátala como de impacto medio hasta que tu modelo de amenazas demuestre lo contrario.

¿Qué lenguajes o plataformas se ven afectados por CWE-187?

MITRE no ha especificado plataformas afectadas para esta CWE — puede aplicar a la mayoría de los stacks de aplicaciones.

¿Cómo puedo prevenir CWE-187?

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

¿Cómo detecta y corrige Plexicus CWE-187?

El motor SAST de Plexicus detecta la firma de flujo de datos para CWE-187 en cada commit. Cuando hay coincidencia, nuestro agente Codex Remedium abre un PR de corrección con el código corregido, las pruebas y un resumen de una línea para el revisor.

¿Dónde puedo aprender más sobre CWE-187?

MITRE publica la definición canónica en https://cwe.mitre.org/data/definitions/187.html. También puedes consultar la documentación de OWASP y NIST para guías relacionadas.

Debilidades relacionadas

Weaknesses related to CWE-187

CWE-1023 Padre

Incomplete Comparison with Missing Factors

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CWE-184 Hermano

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 Hermano

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 Hermano

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…

Recursos

Further reading

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