security: block data:image/svg+xml + sandbox HtmlBlock iframes

M-5: safeUrl() blocked javascript:/vbscript:/data:text/html but
allowed data:image/svg+xml, which can execute inline <script>/onload=
when loaded as a document/navigation target despite its "image" MIME
type (defense in depth -- not currently reachable to execution via
this sink, but closing it). Added `data:image/svg+xml` to the existing
DANGEROUS_SCHEME_PREFIXES check, so it's caught after the same
entity-decode/whitespace-strip/lowercase normalization used for the
other blocked schemes (obfuscated variants included). Other
data:image/* types (png/jpeg/gif/webp, ...) remain allowed unchanged.

M-6: HtmlBlock's purifyHtml() allowed <iframe src> through with no
`sandbox` attribute -- a clickjacking/phishing vector even with
DOMPurify already stripping script/on*=. Added a DOMPurify
afterSanitizeAttributes hook, scoped tightly to each purifyHtml() call
(added right before sanitize(), removed in a finally right after) so
it can't leak onto other DOMPurify uses or accumulate duplicates
across repeated calls, that force-sets a restrictive sandbox
(allow-scripts allow-same-origin allow-popups allow-forms -- no
allow-top-navigation) and referrerpolicy=no-referrer on every iframe
that survives sanitization.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

craft/src/components/basic/HtmlBlock.test.ts

@@ -21,3 +21,46 @@ describe('purifyHtml', () => {
     expect(purifyHtml('<form><input name="x"></form>')).not.toContain('<form');
   });
 });
+
+describe('purifyHtml iframe sandboxing (M-6)', () => {
+  test('forces a restrictive sandbox attribute onto every iframe', () => {
+    const out = purifyHtml('<iframe src="https://example.com/"></iframe>');
+    expect(out).toMatch(/<iframe[^>]*\bsandbox="[^"]+"/);
+  });
+
+  test('sandbox value omits allow-top-navigation (no top-level nav escape)', () => {
+    const out = purifyHtml('<iframe src="https://example.com/"></iframe>');
+    const sandbox = out.match(/sandbox="([^"]*)"/)![1];
+    expect(sandbox).not.toMatch(/allow-top-navigation/);
+  });
+
+  test('legitimate embeds (YouTube) still work and get sandboxed too', () => {
+    const out = purifyHtml('<iframe src="https://www.youtube.com/embed/abc" allowfullscreen></iframe>');
+    expect(out).toContain('youtube.com/embed/abc');
+    expect(out).toMatch(/<iframe[^>]*\bsandbox="[^"]+"/);
+  });
+
+  test('adds referrerpolicy=no-referrer to iframes', () => {
+    const out = purifyHtml('<iframe src="https://example.com/"></iframe>');
+    expect(out).toContain('referrerpolicy="no-referrer"');
+  });
+
+  test('script/on* attributes are still stripped alongside the sandboxed iframe', () => {
+    const out = purifyHtml('<iframe src="https://example.com/" onload="alert(1)"></iframe><script>alert(2)</script>');
+    expect(out).not.toContain('onload');
+    expect(out).not.toContain('<script');
+  });
+
+  test('repeated calls do not leak/accumulate the hook (no duplicate sandbox attr, no cross-call state)', () => {
+    purifyHtml('<iframe src="https://a.example/"></iframe>');
+    purifyHtml('<iframe src="https://b.example/"></iframe>');
+    const out = purifyHtml('<iframe src="https://c.example/"></iframe>');
+    const sandboxMatches = out.match(/sandbox="/g) || [];
+    expect(sandboxMatches.length).toBe(1);
+  });
+
+  test('a non-iframe element sanitized alongside an iframe is not touched by the hook', () => {
+    const out = purifyHtml('<p>hi</p><iframe src="https://example.com/"></iframe>');
+    expect(out).toContain('<p>hi</p>');
+  });
+});