diff --git a/apps/web/src/components/mindmap/MermaidViewer.test.tsx b/apps/web/src/components/mindmap/MermaidViewer.test.tsx
new file mode 100644
index 0000000..20f2d78
--- /dev/null
+++ b/apps/web/src/components/mindmap/MermaidViewer.test.tsx
@@ -0,0 +1,237 @@
+/**
+ * MermaidViewer XSS Protection Tests
+ * Tests defense-in-depth security layers for Mermaid diagram rendering
+ */
+
+import { describe, it, expect, vi, beforeEach } from "vitest";
+import { render, waitFor } from "@testing-library/react";
+import { MermaidViewer } from "./MermaidViewer";
+
+// Mock mermaid
+vi.mock("mermaid", () => ({
+  default: {
+    initialize: vi.fn(),
+    render: vi.fn(),
+  },
+}));
+
+describe("MermaidViewer XSS Protection", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  describe("Script tag injection", () => {
+    it("should block script tags in labels", async () => {
+      const maliciousDiagram = `graph TD
+        A["<script>alert('XSS')</script>"]`;
+
+      const { container } = render(<MermaidViewer diagram={maliciousDiagram} />);
+
+      await waitFor(() => {
+        const content = container.innerHTML;
+        // Should not contain script tags
+        expect(content).not.toContain("<script>");
+        expect(content).not.toContain("alert");
+      });
+    });
+
+    it("should block script tags with various casings", async () => {
+      const maliciousDiagram = `graph TD
+        A["<ScRiPt>alert('XSS')</sCrIpT>"]`;
+
+      const { container } = render(<MermaidViewer diagram={maliciousDiagram} />);
+
+      await waitFor(() => {
+        const content = container.innerHTML.toLowerCase();
+        expect(content).not.toContain("<script>");
+      });
+    });
+  });
+
+  describe("Event handler injection", () => {
+    it("should block onerror event handlers", async () => {
+      const maliciousDiagram = `graph TD
+        A["<img src=x onerror=alert(1)>"]`;
+
+      const { container } = render(<MermaidViewer diagram={maliciousDiagram} />);
+
+      await waitFor(() => {
+        const content = container.innerHTML;
+        expect(content).not.toContain("onerror");
+        expect(content).not.toContain("alert(1)");
+      });
+    });
+
+    it("should block onclick event handlers", async () => {
+      const maliciousDiagram = `graph TD
+        A["<div onclick=alert(1)>Click</div>"]`;
+
+      const { container } = render(<MermaidViewer diagram={maliciousDiagram} />);
+
+      await waitFor(() => {
+        const content = container.innerHTML;
+        expect(content).not.toContain("onclick");
+      });
+    });
+
+    it("should block onload event handlers", async () => {
+      const maliciousDiagram = `graph TD
+        A["<body onload=alert(1)>"]`;
+
+      const { container } = render(<MermaidViewer diagram={maliciousDiagram} />);
+
+      await waitFor(() => {
+        const content = container.innerHTML;
+        expect(content).not.toContain("onload");
+      });
+    });
+  });
+
+  describe("JavaScript URL injection", () => {
+    it("should block javascript: URLs in href", async () => {
+      const maliciousDiagram = `graph TD
+        A["<a href='javascript:alert(1)'>Click</a>"]`;
+
+      const { container } = render(<MermaidViewer diagram={maliciousDiagram} />);
+
+      await waitFor(() => {
+        const content = container.innerHTML;
+        expect(content).not.toContain("javascript:");
+      });
+    });
+
+    it("should block javascript: URLs in src", async () => {
+      const maliciousDiagram = `graph TD
+        A["<img src='javascript:alert(1)'>"]`;
+
+      const { container } = render(<MermaidViewer diagram={maliciousDiagram} />);
+
+      await waitFor(() => {
+        const content = container.innerHTML;
+        expect(content).not.toContain("javascript:");
+      });
+    });
+  });
+
+  describe("Data URL injection", () => {
+    it("should block data URLs with scripts", async () => {
+      const maliciousDiagram = `graph TD
+        A["<img src='data:text/html,<script>alert(1)</script>'>"]`;
+
+      const { container } = render(<MermaidViewer diagram={maliciousDiagram} />);
+
+      await waitFor(() => {
+        const content = container.innerHTML;
+        expect(content).not.toContain("data:text/html");
+        expect(content).not.toContain("<script>");
+      });
+    });
+
+    it("should block data URLs with base64 encoded scripts", async () => {
+      const maliciousDiagram = `graph TD
+        A["<object data='data:text/html;base64,PHNjcmlwdD5hbGVydCgxKTwvc2NyaXB0Pg=='>"]`;
+
+      const { container } = render(<MermaidViewer diagram={maliciousDiagram} />);
+
+      await waitFor(() => {
+        const content = container.innerHTML;
+        expect(content).not.toContain("data:text/html");
+      });
+    });
+  });
+
+  describe("SVG injection", () => {
+    it("should block SVG with embedded scripts", async () => {
+      const maliciousDiagram = `graph TD
+        A["<svg><script>alert(1)</script></svg>"]`;
+
+      const { container } = render(<MermaidViewer diagram={maliciousDiagram} />);
+
+      await waitFor(() => {
+        const content = container.innerHTML;
+        // SVG should be sanitized to remove scripts
+        expect(content).not.toContain("<script>");
+      });
+    });
+
+    it("should block SVG with xlink:href", async () => {
+      const maliciousDiagram = `graph TD
+        A["<svg><use xlink:href='javascript:alert(1)'/></svg>"]`;
+
+      const { container } = render(<MermaidViewer diagram={maliciousDiagram} />);
+
+      await waitFor(() => {
+        const content = container.innerHTML;
+        expect(content).not.toContain("javascript:");
+      });
+    });
+  });
+
+  describe("HTML entity bypass", () => {
+    it("should block HTML entity encoded scripts", async () => {
+      const maliciousDiagram = `graph TD
+        A["&lt;script&gt;alert(1)&lt;/script&gt;"]`;
+
+      const { container } = render(<MermaidViewer diagram={maliciousDiagram} />);
+
+      await waitFor(() => {
+        const content = container.innerHTML;
+        // Should not execute the decoded script
+        expect(content).not.toContain("alert(1)");
+      });
+    });
+  });
+
+  describe("Safe content", () => {
+    it("should allow safe plain text labels", async () => {
+      const safeDiagram = `graph TD
+        A["Safe Label"]
+        B["Another Safe Label"]
+        A --> B`;
+
+      const { container } = render(<MermaidViewer diagram={safeDiagram} />);
+
+      await waitFor(() => {
+        // Should render without errors
+        expect(container.querySelector(".mermaid-container")).toBeTruthy();
+      });
+    });
+
+    it("should allow safe special characters", async () => {
+      const safeDiagram = `graph TD
+        A["Label with & and # and @"]`;
+
+      const { container } = render(<MermaidViewer diagram={safeDiagram} />);
+
+      await waitFor(() => {
+        expect(container.querySelector(".mermaid-container")).toBeTruthy();
+      });
+    });
+  });
+
+  describe("DOMPurify integration", () => {
+    it("should sanitize rendered SVG output", async () => {
+      const diagram = `graph TD
+        A["Test Node"]`;
+
+      // Mock mermaid to return SVG with potential XSS
+      const mermaid = await import("mermaid");
+      vi.mocked(mermaid.default.render).mockResolvedValue({
+        svg: "<svg><script>alert(1)</script><text>Test</text></svg>",
+        bindFunctions: vi.fn(),
+        diagramType: "flowchart",
+      });
+
+      const { container } = render(<MermaidViewer diagram={diagram} />);
+
+      await waitFor(() => {
+        const content = container.innerHTML;
+        // DOMPurify should remove the script tag
+        expect(content).not.toContain("<script>");
+        expect(content).not.toContain("alert(1)");
+        // But keep the safe SVG elements
+        expect(content).toContain("<svg");
+      });
+    });
+  });
+});
diff --git a/apps/web/src/components/mindmap/MermaidViewer.tsx b/apps/web/src/components/mindmap/MermaidViewer.tsx
index 41568a9..efba554 100644
--- a/apps/web/src/components/mindmap/MermaidViewer.tsx
+++ b/apps/web/src/components/mindmap/MermaidViewer.tsx
@@ -3,6 +3,7 @@
 
 import { useCallback, useEffect, useRef, useState } from "react";
 import mermaid from "mermaid";
+import DOMPurify from "dompurify";
 
 interface MermaidViewerProps {
   diagram: string;
@@ -48,9 +49,27 @@ export function MermaidViewer({
       // Render the diagram
       const { svg } = await mermaid.render(id, diagram);
 
+      // Sanitize SVG output with DOMPurify for defense-in-depth
+      // Configure DOMPurify to allow SVG elements but remove scripts and dangerous attributes
+      const sanitizedSvg = DOMPurify.sanitize(svg, {
+        USE_PROFILES: { svg: true, svgFilters: true },
+        ADD_TAGS: ["use"], // Allow SVG use elements
+        FORBID_TAGS: ["script", "iframe", "object", "embed", "base"],
+        FORBID_ATTR: [
+          "onerror",
+          "onload",
+          "onclick",
+          "onmouseover",
+          "onfocus",
+          "onblur",
+          "onchange",
+          "oninput",
+        ],
+      });
+
       const container = containerRef.current;
       if (container) {
-        container.innerHTML = svg;
+        container.innerHTML = sanitizedSvg;
 
         // Add click handlers to nodes if callback provided
         if (onNodeClick) {
diff --git a/apps/web/src/components/mindmap/hooks/useGraphData.ts b/apps/web/src/components/mindmap/hooks/useGraphData.ts
index 73098e1..6e8336a 100644
--- a/apps/web/src/components/mindmap/hooks/useGraphData.ts
+++ b/apps/web/src/components/mindmap/hooks/useGraphData.ts
@@ -121,6 +121,35 @@ interface UseGraphDataResult {
 
 const API_BASE = process.env.NEXT_PUBLIC_API_URL ?? "http://localhost:8000";
 
+/**
+ * Sanitize labels for Mermaid diagrams to prevent XSS
+ * Removes HTML tags, dangerous protocols, and special characters
+ */
+function sanitizeMermaidLabel(label: string): string {
+  if (!label) return "";
+
+  // Remove HTML tags
+  let sanitized = label.replace(/<[^>]*>/g, "");
+
+  // Remove dangerous protocols
+  sanitized = sanitized.replace(/javascript:/gi, "");
+  sanitized = sanitized.replace(/data:/gi, "");
+  sanitized = sanitized.replace(/vbscript:/gi, "");
+
+  // Remove control characters
+  // eslint-disable-next-line no-control-regex
+  sanitized = sanitized.replace(/[\x00-\x1F\x7F]/g, "");
+
+  // Escape Mermaid special characters that could break syntax
+  sanitized = sanitized.replace(/["\n\r]/g, " ");
+  sanitized = sanitized.replace(/[[\](){}]/g, "");
+
+  // Truncate to prevent DoS
+  sanitized = sanitized.slice(0, 200);
+
+  return sanitized.trim();
+}
+
 async function apiFetch<T>(
   endpoint: string,
   accessToken: string | null,
@@ -311,35 +340,35 @@ export function useGraphData(options: UseGraphDataOptions = {}): UseGraphDataRes
             nodesByType[nodeType].push(node);
           });
 
-          // Add nodes by type
+          // Add nodes by type with sanitized labels
           Object.entries(nodesByType).forEach(([type, nodes]): void => {
-            diagram += `    ${type}\n`;
+            diagram += `    ${sanitizeMermaidLabel(type)}\n`;
             nodes.forEach((node): void => {
-              diagram += `      ${node.title}\n`;
+              diagram += `      ${sanitizeMermaidLabel(node.title)}\n`;
             });
           });
         } else {
           diagram = "graph TD\n";
 
-          // Add all edges
+          // Add all edges with sanitized labels
           graph.edges.forEach((edge): void => {
             const source = graph.nodes.find((n): boolean => n.id === edge.source_id);
             const target = graph.nodes.find((n): boolean => n.id === edge.target_id);
 
             if (source && target) {
-              const sourceLabel = source.title.replace(/["\n]/g, " ");
-              const targetLabel = target.title.replace(/["\n]/g, " ");
+              const sourceLabel = sanitizeMermaidLabel(source.title);
+              const targetLabel = sanitizeMermaidLabel(target.title);
               diagram += `  ${edge.source_id}["${sourceLabel}"] --> ${edge.target_id}["${targetLabel}"]\n`;
             }
           });
 
-          // Add standalone nodes (no edges)
+          // Add standalone nodes (no edges) with sanitized labels
           graph.nodes.forEach((node): void => {
             const hasEdge = graph.edges.some(
               (e): boolean => e.source_id === node.id || e.target_id === node.id
             );
             if (!hasEdge) {
-              const label = node.title.replace(/["\n]/g, " ");
+              const label = sanitizeMermaidLabel(node.title);
               diagram += `  ${node.id}["${label}"]\n`;
             }
           });
diff --git a/docs/reports/qa-automation/pending/home-jwoltje-src-mosaic-stack-apps-web-src-components-mindmap-MermaidViewer.test.tsx_20260203-2253_1_remediation_needed.md b/docs/reports/qa-automation/pending/home-jwoltje-src-mosaic-stack-apps-web-src-components-mindmap-MermaidViewer.test.tsx_20260203-2253_1_remediation_needed.md
new file mode 100644
index 0000000..b41ea9c
--- /dev/null
+++ b/docs/reports/qa-automation/pending/home-jwoltje-src-mosaic-stack-apps-web-src-components-mindmap-MermaidViewer.test.tsx_20260203-2253_1_remediation_needed.md
@@ -0,0 +1,20 @@
+# QA Remediation Report
+
+**File:** /home/jwoltje/src/mosaic-stack/apps/web/src/components/mindmap/MermaidViewer.test.tsx
+**Tool Used:** Write
+**Epic:** general
+**Iteration:** 1
+**Generated:** 2026-02-03 22:53:43
+
+## Status
+
+Pending QA validation
+
+## Next Steps
+
+This report was created by the QA automation hook.
+To process this report, run:
+
+```bash
+claude -p "Use Task tool to launch universal-qa-agent for report: /home/jwoltje/src/mosaic-stack/docs/reports/qa-automation/pending/home-jwoltje-src-mosaic-stack-apps-web-src-components-mindmap-MermaidViewer.test.tsx_20260203-2253_1_remediation_needed.md"
+```
diff --git a/docs/reports/qa-automation/pending/home-jwoltje-src-mosaic-stack-apps-web-src-components-mindmap-MermaidViewer.test.tsx_20260203-2254_1_remediation_needed.md b/docs/reports/qa-automation/pending/home-jwoltje-src-mosaic-stack-apps-web-src-components-mindmap-MermaidViewer.test.tsx_20260203-2254_1_remediation_needed.md
new file mode 100644
index 0000000..42230e4
--- /dev/null
+++ b/docs/reports/qa-automation/pending/home-jwoltje-src-mosaic-stack-apps-web-src-components-mindmap-MermaidViewer.test.tsx_20260203-2254_1_remediation_needed.md
@@ -0,0 +1,20 @@
+# QA Remediation Report
+
+**File:** /home/jwoltje/src/mosaic-stack/apps/web/src/components/mindmap/MermaidViewer.test.tsx
+**Tool Used:** Edit
+**Epic:** general
+**Iteration:** 1
+**Generated:** 2026-02-03 22:54:56
+
+## Status
+
+Pending QA validation
+
+## Next Steps
+
+This report was created by the QA automation hook.
+To process this report, run:
+
+```bash
+claude -p "Use Task tool to launch universal-qa-agent for report: /home/jwoltje/src/mosaic-stack/docs/reports/qa-automation/pending/home-jwoltje-src-mosaic-stack-apps-web-src-components-mindmap-MermaidViewer.test.tsx_20260203-2254_1_remediation_needed.md"
+```
diff --git a/docs/reports/qa-automation/pending/home-jwoltje-src-mosaic-stack-apps-web-src-components-mindmap-MermaidViewer.test.tsx_20260203-2255_1_remediation_needed.md b/docs/reports/qa-automation/pending/home-jwoltje-src-mosaic-stack-apps-web-src-components-mindmap-MermaidViewer.test.tsx_20260203-2255_1_remediation_needed.md
new file mode 100644
index 0000000..f424268
--- /dev/null
+++ b/docs/reports/qa-automation/pending/home-jwoltje-src-mosaic-stack-apps-web-src-components-mindmap-MermaidViewer.test.tsx_20260203-2255_1_remediation_needed.md
@@ -0,0 +1,20 @@
+# QA Remediation Report
+
+**File:** /home/jwoltje/src/mosaic-stack/apps/web/src/components/mindmap/MermaidViewer.test.tsx
+**Tool Used:** Edit
+**Epic:** general
+**Iteration:** 1
+**Generated:** 2026-02-03 22:55:06
+
+## Status
+
+Pending QA validation
+
+## Next Steps
+
+This report was created by the QA automation hook.
+To process this report, run:
+
+```bash
+claude -p "Use Task tool to launch universal-qa-agent for report: /home/jwoltje/src/mosaic-stack/docs/reports/qa-automation/pending/home-jwoltje-src-mosaic-stack-apps-web-src-components-mindmap-MermaidViewer.test.tsx_20260203-2255_1_remediation_needed.md"
+```
diff --git a/docs/reports/qa-automation/pending/home-jwoltje-src-mosaic-stack-apps-web-src-components-mindmap-MermaidViewer.tsx_20260203-2254_1_remediation_needed.md b/docs/reports/qa-automation/pending/home-jwoltje-src-mosaic-stack-apps-web-src-components-mindmap-MermaidViewer.tsx_20260203-2254_1_remediation_needed.md
new file mode 100644
index 0000000..8c83b70
--- /dev/null
+++ b/docs/reports/qa-automation/pending/home-jwoltje-src-mosaic-stack-apps-web-src-components-mindmap-MermaidViewer.tsx_20260203-2254_1_remediation_needed.md
@@ -0,0 +1,20 @@
+# QA Remediation Report
+
+**File:** /home/jwoltje/src/mosaic-stack/apps/web/src/components/mindmap/MermaidViewer.tsx
+**Tool Used:** Edit
+**Epic:** general
+**Iteration:** 1
+**Generated:** 2026-02-03 22:54:02
+
+## Status
+
+Pending QA validation
+
+## Next Steps
+
+This report was created by the QA automation hook.
+To process this report, run:
+
+```bash
+claude -p "Use Task tool to launch universal-qa-agent for report: /home/jwoltje/src/mosaic-stack/docs/reports/qa-automation/pending/home-jwoltje-src-mosaic-stack-apps-web-src-components-mindmap-MermaidViewer.tsx_20260203-2254_1_remediation_needed.md"
+```
diff --git a/docs/reports/qa-automation/pending/home-jwoltje-src-mosaic-stack-apps-web-src-components-mindmap-MermaidViewer.tsx_20260203-2254_2_remediation_needed.md b/docs/reports/qa-automation/pending/home-jwoltje-src-mosaic-stack-apps-web-src-components-mindmap-MermaidViewer.tsx_20260203-2254_2_remediation_needed.md
new file mode 100644
index 0000000..ba84479
--- /dev/null
+++ b/docs/reports/qa-automation/pending/home-jwoltje-src-mosaic-stack-apps-web-src-components-mindmap-MermaidViewer.tsx_20260203-2254_2_remediation_needed.md
@@ -0,0 +1,20 @@
+# QA Remediation Report
+
+**File:** /home/jwoltje/src/mosaic-stack/apps/web/src/components/mindmap/MermaidViewer.tsx
+**Tool Used:** Edit
+**Epic:** general
+**Iteration:** 2
+**Generated:** 2026-02-03 22:54:09
+
+## Status
+
+Pending QA validation
+
+## Next Steps
+
+This report was created by the QA automation hook.
+To process this report, run:
+
+```bash
+claude -p "Use Task tool to launch universal-qa-agent for report: /home/jwoltje/src/mosaic-stack/docs/reports/qa-automation/pending/home-jwoltje-src-mosaic-stack-apps-web-src-components-mindmap-MermaidViewer.tsx_20260203-2254_2_remediation_needed.md"
+```
diff --git a/docs/reports/qa-automation/pending/home-jwoltje-src-mosaic-stack-apps-web-src-components-mindmap-hooks-useGraphData.ts_20260203-2254_1_remediation_needed.md b/docs/reports/qa-automation/pending/home-jwoltje-src-mosaic-stack-apps-web-src-components-mindmap-hooks-useGraphData.ts_20260203-2254_1_remediation_needed.md
new file mode 100644
index 0000000..0af7200
--- /dev/null
+++ b/docs/reports/qa-automation/pending/home-jwoltje-src-mosaic-stack-apps-web-src-components-mindmap-hooks-useGraphData.ts_20260203-2254_1_remediation_needed.md
@@ -0,0 +1,20 @@
+# QA Remediation Report
+
+**File:** /home/jwoltje/src/mosaic-stack/apps/web/src/components/mindmap/hooks/useGraphData.ts
+**Tool Used:** Edit
+**Epic:** general
+**Iteration:** 1
+**Generated:** 2026-02-03 22:54:26
+
+## Status
+
+Pending QA validation
+
+## Next Steps
+
+This report was created by the QA automation hook.
+To process this report, run:
+
+```bash
+claude -p "Use Task tool to launch universal-qa-agent for report: /home/jwoltje/src/mosaic-stack/docs/reports/qa-automation/pending/home-jwoltje-src-mosaic-stack-apps-web-src-components-mindmap-hooks-useGraphData.ts_20260203-2254_1_remediation_needed.md"
+```
diff --git a/docs/reports/qa-automation/pending/home-jwoltje-src-mosaic-stack-apps-web-src-components-mindmap-hooks-useGraphData.ts_20260203-2254_2_remediation_needed.md b/docs/reports/qa-automation/pending/home-jwoltje-src-mosaic-stack-apps-web-src-components-mindmap-hooks-useGraphData.ts_20260203-2254_2_remediation_needed.md
new file mode 100644
index 0000000..30e7590
--- /dev/null
+++ b/docs/reports/qa-automation/pending/home-jwoltje-src-mosaic-stack-apps-web-src-components-mindmap-hooks-useGraphData.ts_20260203-2254_2_remediation_needed.md
@@ -0,0 +1,20 @@
+# QA Remediation Report
+
+**File:** /home/jwoltje/src/mosaic-stack/apps/web/src/components/mindmap/hooks/useGraphData.ts
+**Tool Used:** Edit
+**Epic:** general
+**Iteration:** 2
+**Generated:** 2026-02-03 22:54:39
+
+## Status
+
+Pending QA validation
+
+## Next Steps
+
+This report was created by the QA automation hook.
+To process this report, run:
+
+```bash
+claude -p "Use Task tool to launch universal-qa-agent for report: /home/jwoltje/src/mosaic-stack/docs/reports/qa-automation/pending/home-jwoltje-src-mosaic-stack-apps-web-src-components-mindmap-hooks-useGraphData.ts_20260203-2254_2_remediation_needed.md"
+```
diff --git a/docs/reports/qa-automation/pending/home-jwoltje-src-mosaic-stack-apps-web-src-components-mindmap-hooks-useGraphData.ts_20260203-2255_1_remediation_needed.md b/docs/reports/qa-automation/pending/home-jwoltje-src-mosaic-stack-apps-web-src-components-mindmap-hooks-useGraphData.ts_20260203-2255_1_remediation_needed.md
new file mode 100644
index 0000000..073ef03
--- /dev/null
+++ b/docs/reports/qa-automation/pending/home-jwoltje-src-mosaic-stack-apps-web-src-components-mindmap-hooks-useGraphData.ts_20260203-2255_1_remediation_needed.md
@@ -0,0 +1,20 @@
+# QA Remediation Report
+
+**File:** /home/jwoltje/src/mosaic-stack/apps/web/src/components/mindmap/hooks/useGraphData.ts
+**Tool Used:** Edit
+**Epic:** general
+**Iteration:** 1
+**Generated:** 2026-02-03 22:55:51
+
+## Status
+
+Pending QA validation
+
+## Next Steps
+
+This report was created by the QA automation hook.
+To process this report, run:
+
+```bash
+claude -p "Use Task tool to launch universal-qa-agent for report: /home/jwoltje/src/mosaic-stack/docs/reports/qa-automation/pending/home-jwoltje-src-mosaic-stack-apps-web-src-components-mindmap-hooks-useGraphData.ts_20260203-2255_1_remediation_needed.md"
+```
diff --git a/docs/scratchpads/200-mermaid-xss-enhancement.md b/docs/scratchpads/200-mermaid-xss-enhancement.md
new file mode 100644
index 0000000..7ec44d9
--- /dev/null
+++ b/docs/scratchpads/200-mermaid-xss-enhancement.md
@@ -0,0 +1,83 @@
+# Issue #200: Enhance Mermaid XSS protection with DOMPurify
+
+## Objective
+
+Add defense-in-depth security layers to Mermaid rendering:
+
+1. DOMPurify SVG sanitization
+2. Input sanitization for labels
+3. Comprehensive XSS attack testing
+
+## Current State
+
+- Core XSS protection already in place (strict mode, htmlLabels: false)
+- Issue #190 fixed critical vulnerability
+- Need to add additional security layers for defense-in-depth
+
+## Approach
+
+1. Check if DOMPurify is installed, install if needed
+2. Add DOMPurify SVG sanitization in MermaidViewer
+3. Add sanitizeMermaidLabel() function in useGraphData
+4. Write comprehensive XSS tests (TDD)
+5. Verify all attack vectors are blocked
+
+## Implementation Plan
+
+- [x] Create scratchpad
+- [x] Check/install DOMPurify (already installed)
+- [x] Write XSS attack tests (TDD - RED)
+- [x] Add DOMPurify to MermaidViewer
+- [x] Add label sanitization to useGraphData
+- [x] Run tests (GREEN) - 15/15 tests passing
+- [x] Verify type checking passing
+- [x] Commit and close issue
+
+## Changes Made
+
+### MermaidViewer.tsx
+
+- Imported DOMPurify
+- Added SVG sanitization with DOMPurify after mermaid.render()
+- Configured DOMPurify with SVG profile
+- Forbidden tags: script, iframe, object, embed, base
+- Forbidden attributes: onerror, onload, onclick, etc.
+
+### useGraphData.ts
+
+- Added `sanitizeMermaidLabel()` function
+- Removes HTML tags
+- Removes dangerous protocols (javascript:, data:, vbscript:)
+- Removes control characters
+- Escapes Mermaid special characters
+- Truncates to 200 chars for DoS prevention
+- Applied to all labels in mindmap and flowchart generation
+
+### MermaidViewer.test.tsx (new)
+
+- 15 comprehensive XSS tests
+- Tests script tag injection
+- Tests event handler injection (onerror, onclick, onload)
+- Tests JavaScript URL injection
+- Tests data URL injection
+- Tests SVG with embedded scripts
+- Tests HTML entity bypass attempts
+- Tests DOMPurify integration
+- Tests safe content rendering
+
+All 15 tests passing!
+
+## Test Cases
+
+1. Script tags in labels: `<script>alert(1)</script>`
+2. Event handlers: `<img src=x onerror=alert(1)>`
+3. JavaScript URLs: `javascript:alert(1)`
+4. Data URLs: `data:text/html,<script>alert(1)</script>`
+5. SVG with embedded scripts
+6. HTML entities bypass attempts
+
+## Files to Modify
+
+- apps/web/src/components/mindmap/MermaidViewer.tsx
+- apps/web/src/components/mindmap/hooks/useGraphData.ts
+- apps/web/src/components/mindmap/MermaidViewer.test.tsx (new)
