Chainlit before version 2.12.0 accepts a client‑supplied socket.io sessionId without proper validation, which allows an attacker to craft a carefully constructed sessionId value that contains path traversal sequences. The malformed sessionId can cause the service to resolve filesystem paths outside the intended upload directory. The effect is that the attacker can recursively delete arbitrary directories that are writable by the service process. This vulnerability enables a remote attacker to compromise the integrity and availability of the filesystem hosting the Chainlit deployment. The weakness is a classic input validation flaw, identified as CWE‑22.

Affects the Chainlit application for all releases up through 2.12.0. The vendor, Chainlit, has not issued a separate fix for older minor versions, so any deployment running version 2.12.0 or earlier is at risk. Upgrading to a newer release beyond 2.12.0 removes the flaw.

With a CVSS score of 8.8 the vulnerability is considered high severity. EPSS data is not provided, but because the exploit requires only an unauthenticated HTTP request with a crafted sessionId, the likelihood of exploitation is significant in exposed environments. The vulnerability is not listed in the CISA KEV catalog, however its high impact and lack of authentication requirements make it attractive to threat actors. Exploitation can proceed by simply sending a malicious sessionId, so no credential or privileged access is required.