CWE-776: Improper Restriction of Recursive Entity References in DTDs ('XML Entity Expansion')

Définition

What is CWE-776?

This vulnerability occurs when an XML parser allows Document Type Definitions (DTDs) to contain recursively defined entities without proper limits, enabling malicious data structures.

Attackers can craft a malicious DTD that defines XML entities in a recursive loop—where one entity references another, which then references back to the first, creating a chain. When the parser expands these entities, what looks like a small XML file in memory explodes into a massive data structure, consuming excessive CPU and memory. This results in a classic Denial-of-Service (DoS) attack, often called 'XML Entity Expansion' or 'Billion Laughs.' To prevent it, developers should disable DTD processing entirely in their XML parsers when possible, or explicitly configure them to restrict entity expansion depth and total memory usage during parsing.

Impact réel

Real-world CVEs caused by CWE-776

CVE-2008-3281

XEE in XML-parsing library.
CVE-2011-3288

XML bomb / XEE in enterprise communication product.
CVE-2011-1755

"Billion laughs" attack in XMPP server daemon.
CVE-2009-1955

XML bomb in web server module
CVE-2003-1564

Parsing library allows XML bomb

Comment les attaquants l'exploitent

Parcours de l'attaquant étape par étape

1
Identifier un chemin de code qui traite des entrées non fiables sans validation.
2
Élaborer une charge utile qui exploite le comportement non sécurisé — injection, traversal, débordement ou abus de logique.
3
Délivrer la charge utile via une requête normale et observer la réaction de l'application.
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 XML

The DTD and the very brief XML below illustrate what is meant by an XML bomb. The ZERO entity contains one character, the letter A. The choice of entity name ZERO is being used to indicate length equivalent to that exponent on two, that is, the length of ZERO is 2^0. Similarly, ONE refers to ZERO twice, therefore the XML parser will expand ONE to a length of 2, or 2^1. Ultimately, we reach entity THIRTYTWO, which will expand to 2^32 characters in length, or 4 GB, probably consuming far more data than expected.

Vulnérable XML

<?xml version="1.0"?>
  <!DOCTYPE MaliciousDTD [
  <!ENTITY ZERO "A">
  <!ENTITY ONE "&ZERO;&ZERO;">
  <!ENTITY TWO "&ONE;&ONE;">
  ...
  <!ENTITY THIRTYTWO "&THIRTYONE;&THIRTYONE;">
  ]>
  <data>&THIRTYTWO;</data>

Charge utile de l'attaquant

The DTD and the very brief XML below illustrate what is meant by an XML bomb. The ZERO entity contains one character, the letter A. The choice of entity name ZERO is being used to indicate length equivalent to that exponent on two, that is, the length of ZERO is 2^0. Similarly, ONE refers to ZERO twice, therefore the XML parser will expand ONE to a length of 2, or 2^1. Ultimately, we reach entity THIRTYTWO, which will expand to 2^32 characters in length, or 4 GB, probably consuming far more data than expected.

Charge utile de l'attaquant XML

<?xml version="1.0"?>
  <!DOCTYPE MaliciousDTD [
  <!ENTITY ZERO "A">
  <!ENTITY ONE "&ZERO;&ZERO;">
  <!ENTITY TWO "&ONE;&ONE;">
  ...
  <!ENTITY THIRTYTWO "&THIRTYONE;&THIRTYONE;">
  ]>
  <data>&THIRTYTWO;</data>

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-776

Operation If possible, prohibit the use of DTDs or use an XML parser that limits the expansion of recursive DTD entities.
Implementation Before parsing XML files with associated DTDs, scan for recursive entity declarations and do not continue parsing potentially explosive content.

Signaux de détection

How to detect CWE-776

Automated Static Analysis High

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-776

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Questions fréquentes

Frequently asked questions

Qu'est-ce que CWE-776 ?

This vulnerability occurs when an XML parser allows Document Type Definitions (DTDs) to contain recursively defined entities without proper limits, enabling malicious data structures.

Quelle est la gravité de CWE-776 ?

MITRE évalue la probabilité d'exploitation comme Moyenne — l'exploitation est réaliste mais nécessite généralement des conditions spécifiques.

Quels langages ou plateformes sont affectés par CWE-776 ?

MITRE lists the following affected platforms: XML.

Comment puis-je prévenir CWE-776 ?

If possible, prohibit the use of DTDs or use an XML parser that limits the expansion of recursive DTD entities. Before parsing XML files with associated DTDs, scan for recursive entity declarations and do not continue parsing potentially explosive content.

Comment Plexicus détecte et corrige CWE-776 ?

Le moteur SAST de Plexicus reconnaît la signature de flux de données de CWE-776 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-776 ?

MITRE publie la définition canonique à https://cwe.mitre.org/data/definitions/776.html. Vous pouvez également consulter la documentation OWASP et NIST pour des conseils adjacents.

Faiblesses associées

Weaknesses related to CWE-776

CWE-674 Parent

Uncontrolled Recursion

This vulnerability occurs when an application fails to limit how deeply a function can call itself. Without proper controls, this…

Ressources

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Improper Restriction of Recursive Entity References in DTDs ('XML Entity Expansion')