SiYuan: Unauthenticated Reflected XSS via SVG Injection in /api/icon/getDynamicIcon Endpoint

Summary

An unauthenticated reflected XSS vulnerability exists in the dynamic icon API endpoint:

• GET /api/icon/getDynamicIcon

When type=8, attacker-controlled content is embedded into SVG output without escaping. Because the endpoint is unauthenticated and returns image/svg+xml, a crafted URL can inject executable SVG/HTML event handlers (for example onerror) and run JavaScript in the SiYuan web origin.

This can be chained to perform authenticated API actions and exfiltrate sensitive data when a logged-in user opens the malicious link.

Details

The issue is caused by unsafe output construction and incomplete sanitization:

1. Endpoint is exposed without auth middleware
   - Source: https://github.com/siyuan-note/siyuan/blob/master/kernel/api/router.go#L27-L37
   - GET /api/icon/getDynamicIcon is registered in the unauthenticated section.

2. User input is inserted into SVG via string formatting
   - Source: https://github.com/siyuan-note/siyuan/blob/master/kernel/api/icon.go#L115-L175
   - Source: https://github.com/siyuan-note/siyuan/blob/master/kernel/api/icon.go#L537-L585
   - In generateTypeEightSVG, %s directly injects content into <text>...</text> without XML/HTML escaping.

3. Sanitizer only removes <script> tags
   - Source: https://github.com/siyuan-note/siyuan/blob/master/kernel/util/misc.go#L235-L281
   - RemoveScriptsInSVG removes <script> nodes, but does not remove dangerous attributes (onerror, onload, etc.) or unsafe elements.

As a result, payloads such as </text><image ... onerror=...><text> survive and execute.

PoC

Minimal browser execution PoC

Open this URL in a browser:

GET /api/icon/getDynamicIcon?type=8&content=%3C%2Ftext%3E%3Cimage%20href%3Dx%20onerror%3Dalert(document.domain)%3E%3C%2Fimage%3E%3Ctext%3E

Example full URL:

http://127.0.0.1:6806/api/icon/getDynamicIcon?type=8&content=%3C%2Ftext%3E%3Cimage%20href%3Dx%20onerror%3Dalert(document.domain)%3E%3C%2Fimage%3E%3Ctext%3E

Expected result:

• JavaScript executes (alert(document.domain)), confirming reflected XSS.

Authenticated impact demonstration

If a victim is authenticated in the same browser session, JavaScript running in origin can call privileged APIs and exfiltrate returned data.

Impact

This is a reflected XSS in an unauthenticated endpoint, with realistic account/data compromise impact:

• Arbitrary JavaScript execution in SiYuan web origin.
• Authenticated action abuse via same-origin API calls.
• Sensitive data exposure (notes/config/API responses) from victim context.
• Potential chained server-impact actions depending on victim privileges and deployment mode.