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securitycritical PriorityID: xml-external-entity-xxe-prevention

XML External Entity (XXE) Injection Prevention & Safe Parser Configuration

Production security standard (CWE-611 / OWASP Top 10). Mandates completely disabling Document Type Definitions (DTD) and external entity resolution in XML parsers (fast-xml-parser, libxmljs, defusedxml), preventing local file disclosure (/etc/passwd) and server-side request forgery through XML payloads.

Prohibited Anti-Pattern: AI parses incoming XML documents using default configurations with external entities enabled, allowing attackers to exfiltrate server files via XXE injections.

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---
description: XML External Entity (XXE) Injection Prevention & Safe Parser Configuration - Production security standard (CWE-611 / OWASP Top 10). Mandates completely disabling Document Type Definitions (DTD) and external entity resolution in XML parsers (fast-xml-parser, libxmljs, defusedxml), preventing local file disclosure (/etc/passwd) and server-side request forgery through XML payloads.
globs: **/xml/**/*.ts, **/parsers/**/*.py, **/xml*.go
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/xml-external-entity-xxe-prevention"
---

# XML External Entity (XXE) Injection Prevention & Safe Parser Configuration

> **Severity**: CRITICAL | **Category**: security
> **Anti-Pattern Warning**: AI parses incoming XML documents using default configurations with external entities enabled, allowing attackers to exfiltrate server files via XXE injections.

## System Constraints
You are an Application Security Specialist. When parsing XML files or SAML assertions, NEVER use default XML parser configurations. Most legacy parsers resolve external entities (&xxe;) by default, allowing attackers to extract local server files or query internal network services. You MUST disable DTD processing and external entity expansion.

## Strict Guidelines
- MUST disable DTDs (disallowDoctype: true) in fast-xml-parser and XML2JS.
- MUST use defusedxml.defuse_stdlib() in Python instead of standard xml.etree.ElementTree.
- MUST NOT enable resolveExternalEntities or noent: true in libxml2 bindings.
- MUST reject XML documents containing <!DOCTYPE or <!ENTITY declarations in high-security APIs.
- SHOULD prefer JSON or Protocol Buffers over XML for modern API contracts.

## Bad vs Good Implementations
### Hardened XML Parser Disabling External Entities and DTDs

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Default XML parsers expand external entities defined in DTD headers. Attackers can supply file:/// paths or external URLs to extract passwords or scan internal networks.
```typescript
// FATAL: Insecure XML parser with default DTD and external entity resolution
import { XMLParser } from 'fast-xml-parser';

function parseUserXml(rawXml: string) {
  // VULNERABLE: If input contains <!ENTITY xxe SYSTEM "file:///etc/passwd">, parser leaks files!
  const parser = new XMLParser();
  return parser.parse(rawXml);
}
```

#### Production Standard (Correct)
> **Standard**: Disables entity expansion and rejects DOCTYPE headers, neutralizing XML External Entity (XXE) and billion-laughs quadratic entity expansion attacks.
```typescript
import { XMLParser } from 'fast-xml-parser';

export function parseSafeXml(rawXml: string) {
  const parser = new XMLParser({
    // 1. Completely disable DTD processing (kills XXE attacks at the parser gate)
    processEntities: false,
    htmlEntities: false,
    stopNodes: ['*.DOCTYPE', '*.ENTITY'],
  });

  // 2. Reject payloads explicitly containing DOCTYPE injection signatures
  if (rawXml.includes('<!DOCTYPE') || rawXml.includes('<!ENTITY')) {
    throw new Error('Security Violation: XML contains forbidden DOCTYPE/ENTITY declarations');
  }

  return parser.parse(rawXml);
}
```
Recommended MCP Servers (mcp-serverlist.com)

Equip agent with system tools to verify code for this stack.

XXE & XML Security Auditor MCP
Fuzzes XML parsers with XXE entities to verify safe parser configuration.
Recommended Agent Skills (askills.dev)

Ready-to-use executable skill functions tailored to this stack.

askills: XML Parser XXE Hardener
Configures fast-xml-parser and Python defusedxml to block external entity resolution.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: XML External Entity (XXE) Injection Prevention & Safe Parser Configuration

XML External Entity (XXE) Injection Prevention & Safe Parser Configuration defines an industrial-grade behavioral standard for security engineering. Production security standard (CWE-611 / OWASP Top 10). Mandates completely disabling Document Type Definitions (DTD) and external entity resolution in XML parsers (fast-xml-parser, libxmljs, defusedxml), preventing local file disclosure (/etc/passwd) and server-side request forgery through XML payloads. Modern software architectures require strict adherence to deterministic patterns to guarantee high availability, maintainability, and zero security vulnerabilities.

Framework Paradigm Shift & Drift MechanicsLLMs are trained on millions of open-source repositories containing outdated code, legacy idioms, and unpatched antipatterns. When coding assistants generate implementations for this stack, they default to common statistical tokens: AI parses incoming XML documents using default configurations with external entities enabled, allowing attackers to exfiltrate server files via XXE injections. Without an explicit system instruction file, developers spend hours debugging silent failures.
Failure Modes & Production Blast Radius

Hazard Analysis: What Breaks Without This Rule

The following failure modes have been cataloged from hundreds of hours of production code reviews where AI assistants produced non-compliant implementations:

Failure ModeAI Symptom (The Anti-Pattern)SeverityProduction Impact & Remedy
AI Hallucination & Deprecation DriftAI parses incoming XML documents using default configurations with external entities enabled, allowing attackers to exfiltrate server files via XXE injections.Critical
Violating this industrial standard introduces regression bugs, security risks, and unexpected runtime failures in production.
Enforce explicit behavioral system directives across all AI coding assistants in your repository.
Silent Architecture InconsistencyAI assistants produce mixed paradigms across files, creating fragmented codebase conventions.Medium
Increases cognitive load during code reviews and leads to maintenance debt across engineering teams.
Pin project conventions in .cursor/rules, CLAUDE.md, and AGENTS.md so all team agents follow identical guardrails.
Architectural Directives in Detail

Step-by-Step Production Directives

Each rule constraint below is injected into the AI agent's system prompt to establish deterministic boundaries:

1

MUST disable DTDs (disallowDoctype

MUST disable DTDs (disallowDoctype: true) in fast-xml-parser and XML2JS.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
2

MUST use defusedxml.defuse_stdlib() in Python instead of standard xml.etree.ElementTree.

MUST use defusedxml.defuse_stdlib() in Python instead of standard xml.etree.ElementTree.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
3

MUST NOT enable resolveExternalEntities or noent

MUST NOT enable resolveExternalEntities or noent: true in libxml2 bindings.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
4

MUST reject XML documents containing <!DOCTYPE or <!ENTITY declarations in high-security APIs.

MUST reject XML documents containing <!DOCTYPE or <!ENTITY declarations in high-security APIs.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
5

SHOULD prefer JSON or Protocol Buffers over XML for modern API contracts.

SHOULD prefer JSON or Protocol Buffers over XML for modern API contracts.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
Technical Questions & Answers

Frequently Asked Technical Questions

Common questions developers face when configuring AI coding assistants for this stack:

Q:Why is XML External Entity (XXE) Injection Prevention & Safe Parser Configuration critical for AI coding assistants?

Production security standard (CWE-611 / OWASP Top 10). Mandates completely disabling Document Type Definitions (DTD) and external entity resolution in XML parsers (fast-xml-parser, libxmljs, defusedxml), preventing local file disclosure (/etc/passwd) and server-side request forgery through XML payloads. Without explicit behavioral boundaries, AI coding assistants (Cursor, Claude Code, Windsurf, Codex) rely on training data averages, which frequently produce deprecated, inefficient, or vulnerable code patterns.

Q:How does adopting this rule prevent AI hallucinations?

This rule explicitly declares what the AI must avoid (AI parses incoming XML documents using default configurations with external entities enabled, allowing attackers to exfiltrate server files via XXE injections.) and prescribes verified positive examples. Grounding the LLM with negative and positive pairs eliminates guesswork.

Q:Which AI coding environments are supported?

This specification is multi-platform compatible: Cursor (.cursor/rules/*.mdc & .cursorrules), Anthropic Claude Code (CLAUDE.md), Windsurf (.windsurfrules), Codex / Cline (AGENTS.md), and GitHub Copilot (.github/copilot-instructions.md).

Q:Can I combine this rule with other technology stacks?

Yes. Use the Interactive Stack Builder or download an Agent Pack to merge this rule with other frontend, backend, database, security, and DevOps rules without syntax collision.

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