Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)
CWE-86 Variante Borrador
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 names or URI schemes, allowing malicious sequences to pass…
Definición
What is CWE-86?
This vulnerability occurs when an application fails to properly filter or escape invalid characters within web identifiers like HTML tag names or URI schemes, allowing malicious sequences to pass through.
Modern web browsers often interpret malformed identifiers in unexpected ways. For instance, a browser might silently strip out null bytes or other invalid characters, reconstructing a dangerous payload that the developer's filters missed. This creates a gap where your security checks see a harmless string, but the browser executes it as active code.
Attackers exploit this by embedding these invalid sequences—like null bytes or alternative encodings—within identifiers such as URI schemes. A common bypass involves encoding "javascript:" as something like `java%00script:`, which might bypass a simple blacklist filter but still render as executable JavaScript in the browser, leading to cross-site scripting (XSS) or other client-side attacks.
Impacto en el mundo real
Real-world CVEs caused by CWE-86
-
XSS filter doesn't filter null characters before looking for dangerous tags, which are ignored by web browsers. Multiple Interpretation Error (MIE) and validate-before-cleanse.
Cómo lo explotan los atacantes
Ruta del atacante paso a paso
- 1
Identifica una ruta de código que maneje entrada no confiable sin validación.
- 2
Crea un payload que ejercite el comportamiento inseguro — inyección, traversal, overflow o abuso de lógica.
- 3
Envía el payload a través de una solicitud normal y observa la reacción de la aplicación.
- 4
Itera hasta que la respuesta filtre datos, ejecute código del atacante o escale privilegios.
Ejemplo de código vulnerable
Vulnerable pseudo
MITRE no ha publicado un ejemplo de código para esta CWE. El patrón siguiente es ilustrativo — consulta Recursos para referencias canónicas.
// Example pattern — see MITRE for the canonical references.
function handleRequest(input) {
// Untrusted input flows directly into the sensitive sink.
return executeUnsafe(input);
} Ejemplo de código seguro
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.
Lista de prevención
How to prevent CWE-86
- 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 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-86
Plexicus detecta automáticamente CWE-86 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.
Preguntas frecuentes ¿Qué es CWE-86?
¿Qué gravedad tiene CWE-86?
¿Qué lenguajes o plataformas se ven afectados por CWE-86?
¿Cómo puedo prevenir CWE-86?
¿Cómo detecta y corrige Plexicus CWE-86?
¿Dónde puedo aprender más sobre CWE-86?
Frequently asked questions
¿Qué es CWE-86?
This vulnerability occurs when an application fails to properly filter or escape invalid characters within web identifiers like HTML tag names or URI schemes, allowing malicious sequences to pass through.
¿Qué gravedad tiene CWE-86?
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-86?
MITRE no ha especificado plataformas afectadas para esta CWE — puede aplicar a la mayoría de los stacks de aplicaciones.
¿Cómo puedo prevenir CWE-86?
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,…
¿Cómo detecta y corrige Plexicus CWE-86?
El motor SAST de Plexicus detecta la firma de flujo de datos para CWE-86 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-86?
MITRE publica la definición canónica en https://cwe.mitre.org/data/definitions/86.html. También puedes consultar la documentación de OWASP y NIST para guías relacionadas.
Debilidades relacionadas
Weaknesses related to CWE-86
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-87 Hermano
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…
CWE-184 Par
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…
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