diff --git a/src/app/utils/sanitize.test.ts b/src/app/utils/sanitize.test.ts
new file mode 100644
index 000000000..170d3ff8f
--- /dev/null
+++ b/src/app/utils/sanitize.test.ts
@@ -0,0 +1,52 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { sanitizeCustomHtml, sanitizeText } from './sanitize';
+
+test('sanitizeText escapes HTML metacharacters', () => {
+  assert.equal(sanitizeText('<script>'), '&lt;script&gt;');
+  assert.equal(sanitizeText('a & b'), 'a &amp; b');
+  assert.equal(sanitizeText(`"'`), '&quot;&#39;');
+  assert.equal(sanitizeText('plain text'), 'plain text');
+});
+
+test('sanitizeCustomHtml removes <script> content', () => {
+  const out = sanitizeCustomHtml('<script>alert(1)</script>hello');
+  assert.ok(!out.includes('alert'));
+  assert.ok(out.includes('hello'));
+});
+
+test('sanitizeCustomHtml strips event-handler attributes', () => {
+  const out = sanitizeCustomHtml('<b onclick="evil()">hi</b>');
+  assert.ok(!out.includes('onclick'));
+  assert.ok(out.includes('hi'));
+});
+
+test('sanitizeCustomHtml drops disallowed tags, keeps allowed ones', () => {
+  assert.ok(!sanitizeCustomHtml('<iframe src="x"></iframe>').includes('iframe'));
+  assert.ok(sanitizeCustomHtml('<strong>bold</strong>').includes('<strong>'));
+});
+
+test('sanitizeCustomHtml neutralizes javascript: links', () => {
+  const out = sanitizeCustomHtml('<a href="javascript:alert(1)">x</a>');
+  // eslint-disable-next-line no-script-url -- asserting the scheme was stripped
+  assert.ok(!out.includes('javascript:'));
+});
+
+test('sanitizeCustomHtml hardens anchors (noreferrer/noopener/_blank)', () => {
+  const out = sanitizeCustomHtml('<a href="https://example.com">x</a>');
+  assert.ok(out.includes('rel="noreferrer noopener"'));
+  assert.ok(out.includes('target="_blank"'));
+});
+
+test('sanitizeCustomHtml converts a non-mxc <img> into a link', () => {
+  const out = sanitizeCustomHtml('<img src="https://evil.example/x.png" alt="pic">');
+  assert.ok(!out.includes('<img'));
+  assert.ok(out.includes('href="https://evil.example/x.png"'));
+});
+
+test('N100: <pre> class is restricted to the language-* whitelist', () => {
+  assert.ok(sanitizeCustomHtml('<pre class="language-js">x</pre>').includes('language-js'));
+  assert.ok(
+    !sanitizeCustomHtml('<pre class="evil-site-class">x</pre>').includes('evil-site-class'),
+  );
+});