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)
EPSS: 0.003 (22.2th percentile) — read the EPSS interpretation.
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.
Affected products
- Lichess Lila
- Lichess-org Lila — versions < 0d5002696ae705e1888bf77de107c73de57bb1b3
Weakness classification (CWE)
References
- security-advisories@github.com (x_refsource_CONFIRM, Exploit, Vendor Advisory)
- security-advisories@github.com (Patch, x_refsource_MISC)
- security-advisories@github.com (Exploit, x_refsource_MISC)
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.