FastChat has Denial of Service Through Blocking Event Loop in Model Workers (Incomplete Fix for ff66426)

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

5.5 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/E:P · score source

EPSS

• CVE-2026-6607: 0.62% estimated exploitation activity in the next 30 days; percentile 49.93 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

A security vulnerability has been detected in lm-sys fastchat up to 0.2.36. This issue affects the function apigenerate of the component Worker API Endpoint. The manipulation leads to resource consumption. The attack can be initiated remotely. The exploit has been disclosed publicly and may be used. The identifier of the patch is c9e84b89c91d45191dc24466888de526fa04cf33. It is suggested to install a patch to address this issue. Commit ff66426 patched this issue in apigenerate of basemodelworker.py but missed other entry points.

Published by the source

20 Apr 2026, 06:31 UTC

Last modified by the source

13 Jul 2026, 16:42 UTC

Source last checked

26 Oct 2026, 13:40 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-5h65-jx66-j7p5

2. Revision 2 · amended · 25 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-5h65-jx66-j7p5

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# Patch Next advisory brief: CVE-2026-6607

Advisory: FastChat has Denial of Service Through Blocking Event Loop in Model Workers (Incomplete Fix for ff66426)
IDs: CVE-2026-6607, GHSA-5h65-jx66-j7p5
Record: https://patchnext.com/advisories/cve-2026-6607 (revision 3, source checked 26 Oct 2026, 13:40 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor response not established. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI fschat: affected <= 0.2.26

## Suggested inspection (read-only)
- `python -m pip show fschat`: prints the installed version. Run with the environment that serves production active.

## Supported action
No complete vendor fix is recorded. Follow the primary source's mitigation guidance and check this record again before closing anything.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on fschat.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-5h65-jx66-j7p5 (OSV record GHSA-5h65-jx66-j7p5 lists affected ranges: PyPI fschat >=0 <=0.2.26)
- Verified: https://github.com/advisories/GHSA-5h65-jx66-j7p5 (GitHub-reviewed advisory GHSA-5h65-jx66-j7p5 lists affected ranges: PyPI fschat >=0 <=0.2.26)

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