undertow Race Condition vulnerability

Exploitation

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

CVSS

5.9 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS

• CVE-2021-3597: 1.16% estimated exploitation activity in the next 30 days; percentile 67.23 (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 flaw was found in undertow. The HTTP2SourceChannel fails to write the final frame under some circumstances, resulting in a denial of service. The highest threat from this vulnerability is availability. This flaw affects Undertow versions prior to 2.0.35.SP1, prior to 2.2.6.SP1, prior to 2.2.7.SP1, prior to 2.0.36.SP1, prior to 2.2.9.Final and prior to 2.0.39.Final.

Published by the source

25 May 2022, 00:00 UTC

Last modified by the source

31 Jan 2023, 05:01 UTC

Source last checked

26 Oct 2026, 16:05 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-mfhv-gwf8-4m88

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# Patch Next advisory brief: CVE-2021-3597

Advisory: undertow Race Condition vulnerability
IDs: CVE-2021-3597, GHSA-mfhv-gwf8-4m88
Record: https://patchnext.com/advisories/cve-2021-3597 (revision 1, source checked 26 Oct 2026, 16:05 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.9)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: io.undertow:undertow-core 2.2.9.Final, 2.0.39.Final. A vendor fix does not mean a deployment is fixed.

## Affected components
- Maven io.undertow:undertow-core: affected >= 2.1.0, <= 2.2.8.Final; <= 2.0.38.Final; fixed in 2.0.39.Final, 2.2.9.Final

## Suggested inspection (read-only)
- Maven io.undertow:undertow-core: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on io.undertow:undertow-core.
- 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://github.com/advisories/GHSA-mfhv-gwf8-4m88 (GitHub-reviewed advisory GHSA-mfhv-gwf8-4m88 lists affected ranges: Maven io.undertow:undertow-core >=2.1.0 =0 <=2.0.38.Final)

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