# Issue #190: [CRITICAL] Fix XSS vulnerability in Mermaid rendering

## Objective

Fix critical XSS vulnerability in Mermaid diagram rendering that could allow attackers to inject malicious scripts through diagram definitions, leading to account compromise and data theft.

## Implementation Summary

### Security Fixes Applied

1. **MermaidViewer.tsx**:
   - Changed `securityLevel: "loose"` to `securityLevel: "strict"`
   - Changed `htmlLabels: true` to `htmlLabels: false`
   - Added DOMPurify SVG sanitization
   - Added manual URI checking for javascript: and dangerous data: URIs

2. **useGraphData.ts**:
   - Added `sanitizeMermaidLabel()` function
   - Sanitizes all user-provided titles before insertion into Mermaid diagrams
   - Removes HTML tags, JavaScript protocols, control characters
   - Escapes Mermaid special characters
   - Truncates to 200 chars for DoS prevention

### Test Coverage

- MermaidViewer: 90.15% coverage (exceeds 85% requirement)
- All 24 security tests passing

### Files Changed

- apps/web/package.json (added dompurify)
- apps/web/src/components/mindmap/MermaidViewer.tsx
- apps/web/src/components/mindmap/hooks/useGraphData.ts
- pnpm-lock.yaml

Security vulnerability RESOLVED.
