Mesop, a Python‑based UI framework, is affected by a path traversal flaw in versions 1.2.2 and earlier. An attacker can supply an untrusted state_token through the UI stream payload, causing the FileStateSessionBackend to resolve arbitrary file paths on the host file system. This can result in application denial of service, as the server may crash when attempting to read non‑msgpack configuration files, or enable the attacker to write or delete files outside the application’s intended directory hierarchy.

The affected product is Mesop developed by mesop‑dev. Versions 1.2.2 and older are vulnerable. The issue is fixed in release 1.2.3. Systems that rely on FileStateSessionBackend for session state and are hosting the application are at risk.

The CVSS score of 10 indicates that this is a critical vulnerability. The EPSS score of less than 1 percent suggests that exploitation incidents are currently rare, and the vulnerability is not listed in the CISA KEV catalog, so no known large‑scale attacks have been reported. Nonetheless, the flaw is reachable via the publicly exposed UI stream, so it can be exploited remotely by an attacker without needing privileged access. Attackers would need to craft a malicious state_token and deliver it through the UI; upon processing, the backend will resolve and potentially overwrite or delete arbitrary files on the host, which can compromise system integrity or lead to denial of service.