Unauthenticated worker denial of service via event-loop blocking
Published Apr 20, 2026 · Updated Apr 20, 2026
Denial of service in LMSYS FastChat 0.2.0 through 0.2.36 allows remote attackers to freeze model workers via unauthenticated HTTP requests. Async worker_generate and worker_get_embeddings handlers call synchronous inference or embedding operations directly, blocking the single asyncio event loop. One slow request stalls concurrent requests and heartbeats, and sustained requests cause controller deregistration and interrupt every model served by that worker.
Summary
What happened
Denial of service in LMSYS FastChat 0.2.0 through 0.2.36 allows remote attackers to freeze model workers via unauthenticated HTTP requests. Async worker_generate and worker_get_embeddings handlers call synchronous inference or embedding operations directly, blocking the single asyncio event loop. One slow request stalls concurrent requests and heartbeats, and sustained requests cause controller deregistration and interrupt every model served by that worker.
The record
- CVE
- CVE-2026-6607
- Published
- Apr 20, 2026
- Updated
- Apr 20, 2026
- Vendor
- LMSYS Org
- Product
- FastChat
- Classifications
- CWE-404, CWE-400, T1499.004
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Apr 20, 2026CVE publishedPublication date reported by the CVE source.
- Apr 20, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
Compare your product and version with the public record. A matching version still requires validation against your environment.
Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=0.2.0
- Affected versionversion=0.2.1
- Affected versionversion=0.2.10
- Affected versionversion=0.2.11
- Affected versionversion=0.2.12
- Affected versionversion=0.2.13
- Affected versionversion=0.2.14
- Affected versionversion=0.2.15
- Affected versionversion=0.2.16
- Affected versionversion=0.2.17
- Affected versionversion=0.2.18
- Affected versionversion=0.2.19
- Affected versionversion=0.2.2
- Affected versionversion=0.2.20
- Affected versionversion=0.2.21
- Affected versionversion=0.2.22
- Affected versionversion=0.2.23
- Affected versionversion=0.2.24
- Affected versionversion=0.2.25
- Affected versionversion=0.2.26
- Affected versionversion=0.2.27
- Affected versionversion=0.2.28
- Affected versionversion=0.2.29
- Affected versionversion=0.2.3
- Affected versionversion=0.2.30
- Affected versionversion=0.2.31
- Affected versionversion=0.2.32
- Affected versionversion=0.2.33
- Affected versionversion=0.2.34
- Affected versionversion=0.2.35
- Affected versionversion=0.2.36
- Affected versionversion=0.2.4
- Affected versionversion=0.2.5
- Affected versionversion=0.2.6
- Affected versionversion=0.2.7
- Affected versionversion=0.2.8
- Affected versionversion=0.2.9
What conditions does exploitation require?
What is affected?
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
- FastChat blocking-event-loop proof of conceptproof of concept · demonstrated
Labels summarize the accepted research assessment. They do not indicate a test against your environment.
Technologies
Your stack
See the directory against your own environment.
Your stack
Check the software in your environment
Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.
Book a demo