Chainlit users are at risk of having their authenticated sessions hijacked because the system accepts any valid session identifier during WebSocket session restoration without verifying ownership. This flaw allows an attacker who can obtain a sessionId to assume a victim’s permissions, including invoking privileged tools and accessing data that should be restricted to the authenticated user.

The vulnerability affects Chainlit versions prior to 2.10.1. Any deployment running the open-source package Chainlit in that version range is susceptible, regardless of the operating system or hosting environment.

With a CVSS score of 8.8, the flaw is considered severe. The EPSS score of <1% indicates a low but non-zero likelihood of exploitation, and the vulnerability has not been listed in CISA’s KEV catalog. An attacker can exploit the flaw remotely by connecting to the WebSocket endpoint and supplying a stolen or guessed sessionId; no additional authentication or privileged infrastructure is required to succeed.