Auth bypass in Lm-Sys Fastchat
CVE-2026-85695
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.
Vulnerability class: Broken Authentication
Published · last modified .
CVSS v3 metric
CVSS v3 base score 9.4 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:L.
EPSS exploit prediction
EPSS: 0.007 (53.1th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 53rd percentile — 53.1% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.004 · newest: 0.007 · change: +0.003
Affected products
Weakness classification (CWE)
References
- github.com/lm-sys/FastChat (product)
- github.com/lm-sys/FastChat/blob/v0.2.36/fastchat/serve/controller.py (technical-description)
- GitHub Issue #3886 (issue-tracking)
- VulnCheck Advisory: FastChat Unauthenticated Worker Registration SSRF and Model Spoofing (third-party-advisory)
Frequently asked questions
- What is CVE-2026-85695?
- CVE-2026-85695 is a critical-severity vulnerability in Lm-Sys Fastchat, classified under Missing Authentication for Critical Function. CVSS score: 9.4/10. Published 2026-09-04.
- How severe is CVE-2026-85695?
- Critical severity. CVSS v3 base score is 9.4 out of 10.