diff --git a/docs/scratchpads/190-fix-mermaid-xss.md b/docs/scratchpads/190-fix-mermaid-xss.md
new file mode 100644
index 0000000..a3351d7
--- /dev/null
+++ b/docs/scratchpads/190-fix-mermaid-xss.md
@@ -0,0 +1,36 @@
+# 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.