# Cwe 776 XML Entity Expansion

> Use this skill when you need to remediate CWE-776 (XML Entity Expansion (Billion Laughs)) vulnerabilities in Java code. Triggers on SAST findings, security reviews, or when fixing xml entity expansion (billion laughs) issues.

- Skill: `developerscoffee/cwe-776-xml-entity-expansion` (Agent Skill)
- Install (CLI): `npx skillmds@latest add developerscoffee/cwe-776-xml-entity-expansion`
- Raw SKILL.md: https://api.skillmd.com/api/skills/developerscoffee/cwe-776-xml-entity-expansion/raw
- Safety review: pending (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Security
- License: MIT
- Author: DevelopersCoffee (https://skillmd.com/u/developerscoffee)
- Updated: 2026-09-10
- Page: https://skillmd.com/skills/developerscoffee/cwe-776-xml-entity-expansion

---


# CWE-776 XML Entity Expansion (Billion Laughs)

## Description

XML Entity Expansion (Billion Laughs)

Reference:
https://cwe.mitre.org/data/definitions/776.html


**OWASP Category**: A05:2021 – Security Misconfiguration


---

## Vulnerable Pattern


### ❌ Example 1: Vulnerable Pattern

```java
// VULNERABLE: DTD enabled allows entity expansion
DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
// Default settings allow entity expansion attack
Document doc = factory.newDocumentBuilder().parse(xmlInput);
```


**Why it's vulnerable:** This pattern is vulnerable to XML Entity Expansion (Billion Laughs)




---

## Deterministic Fix


### ✅ Secure Implementation: Secure Implementation

```java
// SECURE: Disable DTD and entity expansion
DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
factory.setFeature("http://apache.org/xml/features/disallow-doctype-decl", true);
factory.setFeature("http://javax.xml.XMLConstants/feature/secure-processing", true);
factory.setExpandEntityReferences(false);
Document doc = factory.newDocumentBuilder().parse(xmlInput);
```


**Why it's secure:** Implements proper protection against XML Entity Expansion (Billion Laughs)




---

## Detection Pattern

Look for these patterns in your codebase:


```bash
# Find XML parsing without security features
grep -rn "DocumentBuilderFactory\\|SAXParser" --include="*.java" | grep -v "disallow-doctype"
```



---

## Remediation Steps


1. Disable DOCTYPE declarations

2. Enable secure processing feature

3. Set entity expansion limit

4. Disable entity reference expansion


---

## Key Imports

```java

import javax.xml.parsers.DocumentBuilderFactory;

import javax.xml.XMLConstants;

```

---

## Verification

After remediation:


- Run SAST scanner to confirm vulnerability is resolved

- Review all instances of the vulnerable pattern

- Add unit tests that verify the secure implementation

- Check for similar patterns in related code


---

## Trigger Examples

```
Fix CWE-776 vulnerability
Resolve XML Entity Expansion (Billion Laughs) issue
Secure this Java code against xml entity expansion (billion laughs)
SAST reports CWE-776
```

---

## Common Vulnerable Locations

| Layer | Files | Patterns |
|-------|-------|----------|

| Controller | `*Controller.java` | User input handling |

| Service | `*Service.java` | Business logic |

| Repository | `*Repository.java` | Data access |


---

## References


- [CWE-776: XML Entity Expansion](https://cwe.mitre.org/data/definitions/776.html)


---

**Source**: Generated by [Java CWE Security Skills Generator](https://github.com/DevelopersCoffee/java-cwe-security-skills)
**Last Updated**: 2026-03-07