lm-sys FastChat up to 0.2.36 Register Worker Endpoint server-side request forgery

Summaryinfo

A vulnerability labeled as critical has been found in lm-sys FastChat up to 0.2.36. The impacted element is an unknown function of the component Register Worker Endpoint. The manipulation results in server-side request forgery. This vulnerability is identified as CVE-2026-85695. The attack can be executed remotely. There is not any exploit available.

Detailsinfo

A vulnerability, which was classified as critical, was found in lm-sys FastChat up to 0.2.36. Affected is an unknown part of the component Register Worker Endpoint. The manipulation with an unknown input leads to a server-side request forgery vulnerability. CWE is classifying the issue as CWE-918. The web server receives a URL or similar request from an upstream component and retrieves the contents of this URL, but it does not sufficiently ensure that the request is being sent to the expected destination. This is going to have an impact on confidentiality, integrity, and availability. CVE summarizes:

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.

The advisory is shared for download at github.com. This vulnerability is traded as CVE-2026-85695 since 09/04/2026. The exploitability is told to be easy. It is possible to launch the attack remotely. The exploitation doesn't require any form of authentication. There are neither technical details nor an exploit publicly available. The current price for an exploit might be approx. USD $0-$5k (estimation calculated on 09/04/2026).

There is no information about possible countermeasures known. It may be suggested to replace the affected object with an alternative product.

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Productinfo

Type

• Chat Software

Vendor

• lm-sys

Name

• FastChat

Version

• 0.2.0
• 0.2.1
• 0.2.2
• 0.2.3
• 0.2.4
• 0.2.5
• 0.2.6
• 0.2.7
• 0.2.8
• 0.2.9
• 0.2.10
• 0.2.11
• 0.2.12
• 0.2.13
• 0.2.14
• 0.2.15
• 0.2.16
• 0.2.17
• 0.2.18
• 0.2.19
• 0.2.20
• 0.2.21
• 0.2.22
• 0.2.23
• 0.2.24
• 0.2.25
• 0.2.26
• 0.2.27
• 0.2.28
• 0.2.29
• 0.2.30
• 0.2.31
• 0.2.32
• 0.2.33
• 0.2.34
• 0.2.35
• 0.2.36

Website

• Product: https://github.com/lm-sys/FastChat/

CPE 2.3info

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CPE 2.2info

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