XSS in Lichess Lila
CVE-2026-35208
lichess.org is the forever free, adless and open source chess server. Any approved streamer can inject arbitrary HTML into /streamer and the homepage “Live streams” widget by placing markup in their Twitch/YouTube stream title. CSP is present and blocks inline script execution, but the issue is still a server-side HTML injection sink. To trigger this, a Lichess account only needs to satisfy the normal streamer requirements and get approved. Per Streamer.canApply, that means an account older than 2 days with at least 15 games, or a verified/titled account. After moderator approval, once the streamer goes live, Lichess pulls the platform title and renders it into the UI as-is. No extra privileges are needed beyond a normal approved streamer profile. This vulnerability is fixed with commit 0d5002696ae705e1888bf77de107c73de57bb1b3.
Vulnerability class: XSS (Cross-Site Scripting)
Published · last modified .
CVSS v3 metric
CVSS v3 base score 5.4 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N.
EPSS exploit prediction
EPSS: 0.004 (26.6th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 27th percentile — 26.6% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.003 · newest: 0.004 · change: +0.001
Affected products
- Lichess Lila
- Lichess-Org Lila — versions < 0d5002696ae705e1888bf77de107c73de57bb1b3
Weakness classification (CWE)
References
- github.com/lichess-org/lila/security/advisories/GHSA-v7gh-939r-pfjq (Exploit, Vendor Advisory)
- github.com/lichess-org/lila/commit/0d5002696ae705e1888bf77de107c73de57bb1b3 (Patch)
- vimeo.com/1175908262 (Exploit)
Frequently asked questions
- What is CVE-2026-35208?
- CVE-2026-35208 is a medium-severity vulnerability in Lichess Lila, classified under Cross-site Scripting. CVSS score: 5.4/10. Published 2026-04-06.
- How severe is CVE-2026-35208?
- Medium severity. CVSS v3 base score is 5.4 out of 10.