CVE-2026-35208
Server-Side HTML Injection in Lichess Streamer Feature
Publication date: 2026-04-06
Last updated on: 2026-04-16
Assigner: GitHub, Inc.
Description
Description
CVSS Scores
EPSS Scores
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Meta Information
Affected Vendors & Products
| Vendor | Product | Version / Range |
|---|---|---|
| lichess | lila | to 2026-03-31 (exc) |
Helpful Resources
Exploitability
| CWE ID | Description |
|---|---|
| CWE-79 | The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users. |
| CWE-116 | The product prepares a structured message for communication with another component, but encoding or escaping of the data is either missing or done incorrectly. As a result, the intended structure of the message is not preserved. |
Attack-Flow Graph
AI Powered Q&A
Can you explain this vulnerability to me?
This vulnerability exists in lichess.org, an open source chess server. Approved streamers can inject arbitrary HTML into the /streamer page and the homepage's "Live streams" widget by placing markup in their Twitch or YouTube stream title. Although a Content Security Policy (CSP) is in place that blocks inline script execution, the vulnerability is a server-side HTML injection sink, meaning the server renders the injected HTML as-is. To exploit this, an attacker only needs a Lichess account that meets normal streamer requirements and gets approved by a moderator. Once approved and live, the platform title is pulled and rendered directly into the UI without sanitization or extra privileges.
How can this vulnerability impact me? :
This vulnerability allows an approved streamer to inject arbitrary HTML into the Lichess UI, which could lead to various impacts such as UI manipulation, phishing attempts, or misleading content being displayed to users. Although inline scripts are blocked by CSP, the injected HTML could still affect the user experience or potentially be used in combination with other vulnerabilities to escalate impact.
What immediate steps should I take to mitigate this vulnerability?
The vulnerability is fixed with commit 0d5002696ae705e1888bf77de107c73de57bb1b3. Immediate mitigation involves updating the Lichess server to include this fix.
Additionally, restricting or reviewing streamer approvals to ensure only trusted accounts can inject content may reduce risk until the fix is applied.