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CVE-2026-35208

MEDIUM CVSS 4.0: 5.3 EPSS 0.06%
Updated Apr 07, 2026
lichess
Parameter Value
CVSS 5.3 (MEDIUM)
Type CWE-79 (Cross-Site Scripting (XSS)), CWE-116 (Improper Encoding or Escaping of Output)
Vendor lichess
Public PoC No

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.

Attack Parameters

Attack Vector
Network
Can be exploited remotely
Attack Complexity
Low
Easy to exploit
Attack Requirements
None
No additional conditions
Privileges Required
Low
Basic privileges needed
User Interaction
None
No user interaction needed

Impact Assessment

Confidentiality
Low
Partial data leak
Integrity
Low
Partial data modification
Availability
None
No disruption

CVSS Vector v4.0

Weakness Type (CWE)

CWE-79 Cross-Site Scripting (XSS)
CWE-116 Improper Encoding or Escaping of Output

References 3

https://github.com/lichess-org/lila/commit/0d5002696ae705e1888bf77de107c73de57b…
security-advisories@github.com
https://github.com/lichess-org/lila/security/advisories/GHSA-v7gh-939r-pfjq
security-advisories@github.com
https://vimeo.com/1175908262
security-advisories@github.com
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