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Vulnerability record · CVE-2026-85695 · published 4 September 2026

CVE-2026-85695: Missing authentication for critical function vulnerability

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.

9.3 CVSS 4.0 Critical EPSS 0.72% · top 48.0% CWE-306 · Missing authentication for critical function Deferred
9.3CVSS 4.0 base score
0.72%EPSS exploitation probability, 30 days
NoNot in CISA KEV
0Affected product versions listed by NVD
4References
10 Sep 2026Last modified by NVD

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.

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:L/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:

References

LinkTags
https://github.com/lm-sys/FastChat
https://github.com/lm-sys/FastChat/blob/v0.2.36/fastchat/serve/controller.py
https://github.com/lm-sys/FastChat/issues/3886
https://www.vulncheck.com/advisories/fastchat-unauthenticated-worker-registration-ssrf-and-model-spoofing

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Source: NIST National Vulnerability Database (record CVE-2026-85695), CISA KEV, FIRST EPSS (scores of 2026-09-26). This page is refreshed as NVD updates the record.