HTTP request smuggling in Undertow

Exploitation

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

CVSS

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

EPSS

• CVE-2021-20220: 1.20% estimated exploitation activity in the next 30 days; percentile 68.22 (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. A regression in the fix for CVE-2020-10687 was found. HTTP request smuggling related to CVE-2017-2666 is possible against HTTP/1.x and HTTP/2 due to permitting invalid characters in an HTTP request. This flaw allows an attacker to poison a web-cache, perform an XSS attack, or obtain sensitive information from request other than their own. The highest threat from this vulnerability is to data confidentiality and integrity.

Published by the source

16 Jun 2021, 17:47 UTC

Last modified by the source

1 Feb 2023, 05:05 UTC

Source last checked

26 Oct 2026, 16:06 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-qjwc-v72v-fq6r

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# Patch Next advisory brief: CVE-2021-20220

Advisory: HTTP request smuggling in Undertow
IDs: CVE-2021-20220, GHSA-qjwc-v72v-fq6r
Record: https://patchnext.com/advisories/cve-2021-20220 (revision 1, source checked 26 Oct 2026, 16:06 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 4.8)
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 2.1.6, 2.0.34. A vendor fix does not mean a deployment is fixed.

## Affected components
- Maven io.undertow:undertow-core: affected >= 2.1.0, < 2.1.6; < 2.0.34; fixed in 2.0.34, 2.1.6

## 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-qjwc-v72v-fq6r (GitHub-reviewed advisory GHSA-qjwc-v72v-fq6r lists affected ranges: Maven io.undertow:undertow-core >=2.1.0 =0 <2.0.34)

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