---
id: CVE-2026-85695
title: >-
  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
summary: >-
  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 malici…
severity: critical
cvss: 9.4
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:L'
cwe:
  - CWE-306
published: '2026-09-04'
updated: '2026-09-10'
sourceUpdated: '2026-09-10T15:53:23.707'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-85695'
references:
  - url: 'https://github.com/lm-sys/FastChat'
    label: disclosure@vulncheck.com
  - url: >-
      https://github.com/lm-sys/FastChat/blob/v0.2.36/fastchat/serve/controller.py
    label: disclosure@vulncheck.com
  - url: 'https://github.com/lm-sys/FastChat/issues/3886'
    label: disclosure@vulncheck.com
  - url: >-
      https://www.vulncheck.com/advisories/fastchat-unauthenticated-worker-registration-ssrf-and-model-spoofing
    label: disclosure@vulncheck.com
tags:
  - nvd
epss: 0.0072
epssPercentile: 0.51841
ingestedAt: '2026-09-10T16:57:28.706Z'
---

## Overview

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.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.