PatchSiren

PatchSiren cyber security CVE debrief

CVE-2026-35208 Lichess CVE debrief

A server-side HTML injection vulnerability exists in Lichess, a free, adless, and open-source chess server. The issue allows approved streamers to inject arbitrary HTML into the /streamer and homepage 'Live streams' widget by placing markup in their Twitch/YouTube stream title. Although CSP is present and blocks inline script execution, the issue remains a server-side HTML injection sink. To trigger this, a Lichess account only needs to satisfy normal streamer requirements and get approved, which means an account older than 2 days with at least 15 games, or a verified/titled account. The vulnerability is fixed with commit 0d5002696ae705e1888bf77de107c73de57bb1b3.

Vendor
Lichess
Product
Lila
CVSS
MEDIUM 5.3
CISA KEV
Not listed in stored evidence
Original CVE published
2026-04-06
Original CVE updated
2026-07-24
Advisory published
2026-04-06
Advisory updated
2026-07-24

Who should care

Users of Lichess, particularly those with streamer profiles, should be aware of this vulnerability and take necessary precautions. Additionally, administrators and moderators of Lichess should prioritize patching and monitoring for potential exploitation. Defenders should review streamer privileges and implement additional security measures to prevent similar vulnerabilities.

Technical summary

The vulnerability exists due to the lack of proper sanitization of user-input data in the streamer title. An approved streamer can inject arbitrary HTML into the /streamer and homepage 'Live streams' widget by placing markup in their Twitch/YouTube stream title. The issue is fixed with commit 0d5002696ae705e1888bf77de107c73de57bb1b3. This fix addresses the server-side HTML injection sink, but defenders should monitor for potential exploitation attempts.

Defensive priority

Medium

Recommended defensive actions

  • Apply the patch with commit 0d5002696ae705e1888bf77de107c73de57bb1b3
  • Monitor for potential exploitation attempts
  • Restrict streamer privileges to trusted users
  • Implement additional security measures to prevent similar vulnerabilities
  • Review compensating controls for exposed systems while remediation is scheduled and verified
  • Check relevant monitoring, detection, and logs for exposed assets that need extra review
  • Track exceptions, retest remediated assets, and close the item only after evidence is documented

Evidence notes

The vulnerability is confirmed by the CVE record and NVD detail. The patch is available in the Lichess repository. Evidence is limited to public CVE and NVD records. Defenders should verify the patch status and review streamer privileges. The issue allows approved streamers to inject arbitrary HTML into the /streamer and homepage 'Live streams' widget by placing markup in their Twitch/YouTube stream title.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-35208 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-35208

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2026-35208 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-35208

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.