CVE-2026-85695: Missing Authentication for Critical Function in lm-sys FastChat

CVE-2026-85695 describes an authentication bypass vulnerability in the /register_worker endpoint of lm-sys FastChat. This vulnerability permits unauthenticated attackers to register arbitrary worker addresses, which can be exploited to perform server-side request forgery (SSRF). By registering malicious workers under the names of victim models, attackers can intercept user prompts, images, and responses, as well as probe internal network ports within the worker mesh. The vulnerability affects FastChat versions >=0 and <=0.2.36 and has been assigned a CVSS 4.0 score of 9.3, indicating critical severity.

The vulnerability allows unauthenticated attackers to bypass authentication controls and register arbitrary workers. This can lead to interception of sensitive user data such as prompts, images, and responses, and enables attackers to perform SSRF attacks to probe internal network infrastructure. The impact is critical due to the potential for data interception and network reconnaissance.

Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. Until an official fix is available, restrict access to the /register_worker endpoint and monitor for unauthorized registrations. Implement network-level controls to limit SSRF impact and consider disabling or restricting worker registration functionality if feasible.