CVE-2026-31809 PoC — SiYuan has a SVG Sanitizer Bypass via Whitespace in `javascript:` URI — Unauthenticated XSS

Source

https://github.com/projectdiscovery/nuclei-templates/blob/main/http/cves/2026/CVE-2026-31809.yaml

Associated Vulnerability

Title: SiYuan has a SVG Sanitizer Bypass via Whitespace in `javascript:` URI — Unauthenticated XSS (CVE-2026-31809)
Description:SiYuan is a personal knowledge management system. Prior to 3.5.10, SiYuan's SVG sanitizer (SanitizeSVG) checks href attributes for the javascript: prefix using strings.HasPrefix(). However, inserting ASCII tab (	), newline (
), or carriage return () characters inside the javascript: string bypasses this prefix check. Browsers strip these characters per the WHATWG URL specification before parsing the URL scheme, so the JavaScript still executes. This allows an attacker to inject executable JavaScript into the unauthenticated /api/icon/getDynamicIcon endpoint, creating a reflected XSS. This is a second bypass of the fix for CVE-2026-29183 (fixed in v3.5.9). This vulnerability is fixed in 3.5.10.

Description

SiYuan v3.5.10 contains a reflected XSS caused by improper sanitization of javascript: href attributes allowing ASCII control characters to bypass prefix checks in SVG sanitizer, letting unauthenticated attackers execute JavaScript via /api/icon/getDynamicIcon.

File Snapshot

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