Markdown sanitization chain

Markdown Sanitization Chain

Category: Security Time Saved: 2-4 hours debugging XSS vulnerabilities Battle-tested: Yes — production incident

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The Problem

You're rendering user-supplied markdown with a diagram library. The app works great until someone submits markdown containing malicious scripts that bypass your rendering.

Why It Happens

Markdown renderers (marked.js, markdown-it) convert markdown to HTML but don't sanitize it. Diagram renderers (Mermaid, PlantUML) execute after sanitizers run, potentially introducing new attack vectors. The order of operations matters critically.

The Rule

Always: marked.js → DOMPurify → Mermaid (post-render)

1. Parse markdown to HTML (marked.js)
2. Sanitize HTML (DOMPurify)
3. Render diagrams on sanitized DOM (Mermaid.run())

Never skip the sanitizer even if content is "trusted."

Implementation

import { marked } from 'marked';
import DOMPurify from 'dompurify';
import mermaid from 'mermaid';

async function renderMarkdown(content, container) {
  // Step 1: Parse markdown to HTML
  const rawHtml = marked.parse(content);
  
  // Step 2: Sanitize (BEFORE inserting into DOM)
  const cleanHtml = DOMPurify.sanitize(rawHtml, {
    ADD_TAGS: ['mermaid'],  // Allow mermaid tags through
  });
  
  // Step 3: Insert sanitized HTML
  container.innerHTML = cleanHtml;
  
  // Step 4: Render diagrams on sanitized DOM
  await mermaid.run({ nodes: container.querySelectorAll('.mermaid') });
}

Common Mistakes

Mistake	Consequence
Skip DOMPurify ("it's internal content")	XSS from any content source
Sanitize after Mermaid renders	Mermaid-injected scripts execute
Use innerHTML without sanitization	Classic XSS
Trust localStorage/URL params	User-controlled XSS payloads

DOMPurify Configuration

const config = {
  ADD_TAGS: ['mermaid'],           // Preserve diagram tags
  ADD_ATTR: ['onclick'],           // Only if absolutely needed
  FORBID_TAGS: ['style', 'script'], // Explicit blocklist
  FORBID_ATTR: ['onerror', 'onload'],
};

Verification Checklist

• Markdown parser runs first
• DOMPurify runs before DOM insertion
• Diagram renderer runs after sanitization
• No raw innerHTML without sanitization anywhere
• Test with <img src=x onerror=alert(1)> payload

Related Skills

• allowlist-over-blocklist — Validation patterns
• shell-injection-prevention — Command execution safety