diff --git a/app/src/lib/render/markdown.ts b/app/src/lib/render/markdown.ts
index d36bf64..0f6f2ff 100644
--- a/app/src/lib/render/markdown.ts
+++ b/app/src/lib/render/markdown.ts
@@ -1,4 +1,4 @@
-import { marked } from 'marked';
+import { Marked } from 'marked';
 import DOMPurify from 'dompurify';
 import hljs from 'highlight.js/lib/core';
 import javascript from 'highlight.js/lib/languages/javascript';
@@ -14,28 +14,40 @@ hljs.registerLanguage('bash', bash);
 hljs.registerLanguage('sh', bash);
 hljs.registerLanguage('json', json);
 
-marked.use({
-  breaks: true,
-  gfm: true,
-  renderer: {
-    code({ text, lang }) {
-      const language = lang && hljs.getLanguage(lang) ? lang : undefined;
-      const highlighted = language
-        ? hljs.highlight(text, { language }).value
-        : hljs.highlightAuto(text).value;
-      const cls = language ? ` language-${language}` : '';
-      return `<pre><code class="hljs${cls}">${highlighted}</code></pre>`;
-    },
-  },
+/**
+ * Lokaler Marked-Instance, damit die globale `marked`-Singleton nicht
+ * mutiert wird — andere Module können `marked` unbeeinflusst weiterverwenden.
+ * (Spec §3: lokale Ersetzbarkeit der Engine.)
+ */
+const markedInstance = new Marked({
+	breaks: true,
+	gfm: true,
+	renderer: {
+		code({ text, lang }) {
+			const language = lang && hljs.getLanguage(lang) ? lang : undefined;
+			const highlighted = language
+				? hljs.highlight(text, { language }).value
+				: hljs.highlightAuto(text).value;
+			const cls = language ? ` language-${language}` : '';
+			return `<pre><code class="hljs${cls}">${highlighted}</code></pre>`;
+		}
+	}
 });
 
 /**
  * Rendert einen Markdown-String zu sanitized HTML.
  * Einziger Export des Moduls — so bleibt Austausch der Engine lokal.
+ *
+ * Nur im Browser/jsdom aufrufen: DOMPurify braucht ein DOM. Die SPA
+ * hat SSR global ausgeschaltet (`+layout.ts: ssr = false`), Vitest läuft
+ * in jsdom — beide Szenarien sind abgedeckt. Ein Aufruf in reiner
+ * Node-Umgebung würde hier laut fehlschlagen statt stumm unsicher
+ * durchzulaufen.
  */
 export function renderMarkdown(md: string): string {
-  const raw = marked.parse(md, { async: false }) as string;
-  return DOMPurify.sanitize(raw, {
-    ADD_ATTR: ['target', 'rel'],
-  });
+	if (typeof window === 'undefined') {
+		throw new Error('renderMarkdown: DOM-Kontext erforderlich (Browser oder jsdom).');
+	}
+	const raw = markedInstance.parse(md, { async: false }) as string;
+	return DOMPurify.sanitize(raw);
 }
diff --git a/app/tests/unit/markdown.test.ts b/app/tests/unit/markdown.test.ts
index 3cddab0..5d05f81 100644
--- a/app/tests/unit/markdown.test.ts
+++ b/app/tests/unit/markdown.test.ts
@@ -28,10 +28,11 @@ describe('renderMarkdown', () => {
     expect(html).toContain('<hr>');
   });
 
-  it('rendert fenced code blocks', () => {
+  it('rendert fenced code blocks mit hljs-klasse', () => {
     const html = renderMarkdown('```js\nconst x = 1;\n```');
     expect(html).toContain('<pre>');
     expect(html).toContain('<code');
+    expect(html).toContain('class="hljs');
   });
 
   it('rendert GFM tables', () => {
@@ -46,4 +47,35 @@ describe('renderMarkdown', () => {
     expect(html).toContain('<img');
     expect(html).toContain('src="https://example.com/img.png"');
   });
+
+  // Erweiterte XSS-Matrix — relevant ab Reply-Komponenten (3rd-party Content).
+  it('entfernt onerror-Attribute auf inline-HTML-img', () => {
+    const html = renderMarkdown('<img src="x" onerror="alert(1)">');
+    expect(html.toLowerCase()).not.toContain('onerror');
+  });
+
+  it('entfernt onclick-Attribute auf inline-HTML', () => {
+    const html = renderMarkdown('<a href="#" onclick="alert(1)">x</a>');
+    expect(html.toLowerCase()).not.toContain('onclick');
+  });
+
+  it('entfernt iframe-Tags', () => {
+    const html = renderMarkdown('<iframe src="https://evil.com"></iframe>');
+    expect(html.toLowerCase()).not.toContain('<iframe');
+  });
+
+  it('entfernt data:text/html-URLs in Links', () => {
+    const html = renderMarkdown('[x](data:text/html,<script>alert(1)</script>)');
+    expect(html.toLowerCase()).not.toMatch(/href="data:text\/html/);
+  });
+
+  it('entfernt vbscript:-URLs', () => {
+    const html = renderMarkdown('<a href="vbscript:msgbox(1)">x</a>');
+    expect(html.toLowerCase()).not.toContain('vbscript:');
+  });
+
+  it('entfernt script-Tag innerhalb svg', () => {
+    const html = renderMarkdown('<svg><script>alert(1)</script></svg>');
+    expect(html.toLowerCase()).not.toContain('<script');
+  });
 });