CWE-87 Variante Borrador

Improper Neutralization of Alternate XSS Syntax

This vulnerability occurs when an application fails to properly sanitize user-supplied input that uses alternative methods to execute cross-site scripting (XSS) attacks.

Definición

What is CWE-87?

This vulnerability occurs when an application fails to properly sanitize user-supplied input that uses alternative methods to execute cross-site scripting (XSS) attacks.
Traditional XSS defenses often focus on blocking common vectors like `` tags or basic event handlers. However, attackers can bypass these filters by using less common syntax, encoding techniques, or obscure HTML attributes that browsers still interpret as executable code. This includes alternative event handlers, JavaScript pseudo-protocols in unexpected places, or creatively malformed tags that slip past validation routines. To prevent this, developers must adopt a positive security model that defines allowed input, rather than just blocking known bad patterns. Input validation should be combined with context-aware output encoding, ensuring user data is treated as text, not code, regardless of the syntax used. Regular security testing with tools that probe for these alternate vectors is essential for robust defense.
Impacto en el mundo real

Real-world CVEs caused by CWE-87

Cómo lo explotan los atacantes

Ruta del atacante paso a paso

  1. 1

    Identifica una ruta de código que maneje entrada no confiable sin validación.

  2. 2

    Crea un payload que ejercite el comportamiento inseguro — inyección, traversal, overflow o abuso de lógica.

  3. 3

    Envía el payload a través de una solicitud normal y observa la reacción de la aplicación.

  4. 4

    Itera hasta que la respuesta filtre datos, ejecute código del atacante o escale privilegios.

Ejemplo de código vulnerable

Vulnerable Java

In the following example, an XSS neutralization method intends to replace script tags in user-supplied input with a safe equivalent:

Vulnerable Java 
public String preventXSS(String input, String mask) {
  	return input.replaceAll("script", mask);
  }
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-87

  • Implementation Resolve all input to absolute or canonical representations before processing.
  • Implementation Carefully check each input parameter against a rigorous positive specification (allowlist) defining the specific characters and format allowed. All input should be neutralized, not just parameters that the user is supposed to specify, but all data in the request, including tag attributes, hidden fields, cookies, headers, the URL itself, and so forth. A common mistake that leads to continuing XSS vulnerabilities is to validate only fields that are expected to be redisplayed by the site. We often encounter data from the request that is reflected by the application server or the application that the development team did not anticipate. Also, a field that is not currently reflected may be used by a future developer. Therefore, validating ALL parts of the HTTP request is recommended.
  • Implementation Use and specify an output encoding that can be handled by the downstream component that is reading the output. Common encodings include ISO-8859-1, UTF-7, and UTF-8. When an encoding is not specified, a downstream component may choose a different encoding, either by assuming a default encoding or automatically inferring which encoding is being used, which can be erroneous. When the encodings are inconsistent, the downstream component might treat some character or byte sequences as special, even if they are not special in the original encoding. Attackers might then be able to exploit this discrepancy and conduct injection attacks; they even might be able to bypass protection mechanisms that assume the original encoding is also being used by the downstream component. The problem of inconsistent output encodings often arises in web pages. If an encoding is not specified in an HTTP header, web browsers often guess about which encoding is being used. This can open up the browser to subtle XSS attacks.
  • Implementation With Struts, write all data from form beans with the bean's filter attribute set to true.
  • Implementation To help mitigate XSS attacks against the user's session cookie, set the session cookie to be HttpOnly. In browsers that support the HttpOnly feature (such as more recent versions of Internet Explorer and Firefox), this attribute can prevent the user's session cookie from being accessible to malicious client-side scripts that use document.cookie. This is not a complete solution, since HttpOnly is not supported by all browsers. More importantly, XMLHTTPRequest and other powerful browser technologies provide read access to HTTP headers, including the Set-Cookie header in which the HttpOnly flag is set.
Señales de detección

How to detect CWE-87

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-87 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.

Solicitar demo Probar Plexicus gratis
Preguntas frecuentes

Frequently asked questions

¿Qué es CWE-87?

This vulnerability occurs when an application fails to properly sanitize user-supplied input that uses alternative methods to execute cross-site scripting (XSS) attacks.

¿Qué gravedad tiene CWE-87?

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-87?

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

¿Cómo puedo prevenir CWE-87?

Resolve all input to absolute or canonical representations before processing. Carefully check each input parameter against a rigorous positive specification (allowlist) defining the specific characters and format allowed. All input should be neutralized, not just parameters that the user is supposed to specify, but all data in the request, including tag attributes, hidden fields, cookies, headers, the URL itself, and so forth. A common mistake that leads to continuing XSS vulnerabilities is to…

¿Cómo detecta y corrige Plexicus CWE-87?

El motor SAST de Plexicus detecta la firma de flujo de datos para CWE-87 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-87?

MITRE publica la definición canónica en https://cwe.mitre.org/data/definitions/87.html. También puedes consultar la documentación de OWASP y NIST para guías relacionadas.

Debilidades relacionadas

Weaknesses related to CWE-87

CWE-79 Padre

Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

This vulnerability occurs when a web application fails to properly sanitize or encode user-supplied input before displaying it on a…

CWE-80 Hermano

Improper Neutralization of Script-Related HTML Tags in a Web Page (Basic XSS)

This vulnerability, commonly known as Basic Cross-Site Scripting (XSS), occurs when a web application fails to properly sanitize user…

CWE-81 Hermano

Improper Neutralization of Script in an Error Message Web Page

This vulnerability occurs when an application fails to properly sanitize user-supplied input before displaying it within an error message…

CWE-83 Hermano

Improper Neutralization of Script in Attributes in a Web Page

This vulnerability occurs when a web application fails to properly sanitize or block JavaScript URIs (like 'javascript:') within HTML tag…

CWE-84 Hermano

Improper Neutralization of Encoded URI Schemes in a Web Page

This vulnerability occurs when a web application fails to properly sanitize user-supplied input that contains malicious scripts disguised…

CWE-85 Hermano

Doubled Character XSS Manipulations

This vulnerability occurs when a web application fails to properly sanitize user input that contains doubled characters, allowing…

CWE-86 Hermano

Improper Neutralization of Invalid Characters in Identifiers in Web Pages

This vulnerability occurs when an application fails to properly filter or escape invalid characters within web identifiers like HTML tag…

Recursos

Further reading

Listo cuando tú lo estés

Deja de pagar por desarrollador.
Empieza a cerrar el bucle.

Plexicus es el ASPM nativo de IA que escanea, filtra, corrige, pentestea y explica — de forma autónoma. Desarrolladores ilimitados, repos ilimitados, acciones de IA de uso justo. Nivel gratuito real, €269/mo anual cuando estés listo.

Comenzar gratis Reservar una demo