CWE-87 Variante Brouillon

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.

Définition

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.
Impact réel

Real-world CVEs caused by CWE-87

Comment les attaquants l'exploitent

Parcours de l'attaquant étape par étape

  1. 1

    Identifier un chemin de code qui traite des entrées non fiables sans validation.

  2. 2

    Élaborer une charge utile qui exploite le comportement non sécurisé — injection, traversal, débordement ou abus de logique.

  3. 3

    Délivrer la charge utile via une requête normale et observer la réaction de l'application.

  4. 4

    Itérer jusqu'à ce que la réponse divulgue des données, exécute le code de l'attaquant ou élève les privilèges.

Exemple de code vulnérable

Vulnerable Java

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

Vulnérable Java 
public String preventXSS(String input, String mask) {
  	return input.replaceAll("script", mask);
  }
Exemple de code sécurisé

Secure pseudo

Sécurisé 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-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.
Signaux de détection

How to detect CWE-87

SAST High

Exécuter une analyse statique (SAST) sur le code source à la recherche du motif non sécurisé dans le flux de données.

DAST Moderate

Exécuter des tests de sécurité applicative dynamique (DAST) contre le point de terminaison en ligne.

Runtime Moderate

Surveiller les journaux runtime pour détecter des traces d'exception inhabituelles, des entrées malformées ou des tentatives de contournement d'autorisation.

Code review Moderate

Revue de code : signaler tout nouveau code qui traite les entrées de cette surface sans utiliser les helpers du framework validés.

Correction automatique Plexicus

Plexicus détecte automatiquement CWE-87 et ouvre une PR de correction en moins de 60 secondes.

Codex Remedium analyse chaque commit, identifie cette faiblesse précise et livre une pull request prête à être relue avec le correctif. Pas de tickets. Pas de transferts.

Obtenir une démo Essayer Plexicus gratuitement
Questions fréquentes

Frequently asked questions

Qu'est-ce que 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.

Quelle est la gravité de CWE-87 ?

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-87 ?

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-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…

Comment Plexicus détecte et corrige CWE-87 ?

Le moteur SAST de Plexicus reconnaît la signature de flux de données de CWE-87 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-87 ?

MITRE publie la définition canonique à https://cwe.mitre.org/data/definitions/87.html. Vous pouvez également consulter la documentation OWASP et NIST pour des conseils adjacents.

Faiblesses associées

Weaknesses related to CWE-87

CWE-79 Parent

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 Frère

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 Frère

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 Frère

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 Frère

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 Frère

Doubled Character XSS Manipulations

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

CWE-86 Frère

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…

Ressources

Further reading

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