XSS in B3log Siyuan
CVE-2026-31809
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.
Vulnerability class: XSS (Cross-Site Scripting)
Published · last modified .
CVSS v3 metric
CVSS v3 base score 6.1 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N.
EPSS exploit prediction
EPSS: 0.006 (44.9th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 45th percentile — 44.9% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.005 · newest: 0.006 · change: +0.001
Affected products
- B3log Siyuan
- Siyuan-Note Siyuan — versions < 3.5.10
Weakness classification (CWE)
References
- github.com/siyuan-note/siyuan/security/advisories/GHSA-pmc9-f5qr-2pcr (Exploit, Vendor Advisory)
Frequently asked questions
- What is CVE-2026-31809?
- CVE-2026-31809 is a medium-severity vulnerability in B3log Siyuan, classified under Cross-site Scripting. CVSS score: 6.1/10. Published 2026-03-10.
- How severe is CVE-2026-31809?
- Medium severity. CVSS v3 base score is 6.1 out of 10.