Vulnerability · Published Sep 4, 2026 · Assigned by VulnCheck

CVE-2026-85695FastChat Unauthenticated Worker Registration SSRF and Model Spoofing

Critical 9.4

In plain English

A critical-severity missing authentication in FastChat from lm-sys. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.7% (higher than 54% of all known vulnerabilities).

Severity (CVSS 3.1)9.4Critical

Exploit odds (30 days)0.7%top 45.9% of all CVEs

Exploited in the wildNo reportsCISA SSVC: none

Attack toolingNoneno public tooling known

Official description

FastChat contains an authentication bypass vulnerability in the /register_worker endpoint that allows unauthenticated attackers to register arbitrary worker addresses and perform server-side request forgery. Attackers can register malicious workers under victim model names to intercept user prompts, images, and responses, or probe internal network ports across the worker mesh.

Affected products (as reported)

Vendor	Product	Versions
lm-sys	FastChat	0 – through 0.2.36

Weakness: CWE-306 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:L

References

GitHub Issue #3886 issue-tracking
https://github.com/lm-sys/FastChat product
https://github.com/lm-sys/FastChat/blob/v0.2.36/fastchat/serve/controller.py technical-description
VulnCheck Advisory: FastChat Unauthenticated Worker Registration SSRF and Model Spoofing third-party-advisory