HTTP Response Splitting (Early Hints) in Puma

Exploitation

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

CVSS

6.5 · CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:L

EPSS

• CVE-2020-5249: 1.57% estimated exploitation activity in the next 30 days; percentile 75.40 (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.

Impact If an application using Puma allows untrusted input in an early-hints header, an attacker can use a carriage return character to end the header and inject malicious content, such as additional headers or an entirely new response body. This vulnerability is known as HTTP Response Splitting. While not an attack in itself, response splitting is a vector for several other attacks, such as cross-site scripting (XSS). This is related to CVE-2020-5247, which fixed this vulnerability but only for regular responses. Patches This has been fixed in 4.3.3 and 3.12.4. Workarounds Users can not allow untrusted/user input in the Early Hints response header. For more information If you have any questions or comments about this advisory: Open an issue in puma Email us a project maintainer. Email addresses are listed in our Code of Conduct.

Published by the source

3 Mar 2020, 23:33 UTC

Last modified by the source

9 Nov 2023, 05:00 UTC

Source last checked

26 Oct 2026, 15:48 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-33vf-4xgg-9r58

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# Patch Next advisory brief: CVE-2020-5249

Advisory: HTTP Response Splitting (Early Hints) in Puma
IDs: CVE-2020-5249, GHSA-33vf-4xgg-9r58
Record: https://patchnext.com/advisories/cve-2020-5249 (revision 1, source checked 26 Oct 2026, 15:48 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: puma 4.3.3, 3.12.4. A vendor fix does not mean a deployment is fixed.

## Affected components
- RubyGems puma: affected < 3.12.4; >= 4.0.0, < 4.3.3; fixed in 3.12.4, 4.3.3

## Suggested inspection (read-only)
- RubyGems puma: 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 puma.
- 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-33vf-4xgg-9r58 (GitHub-reviewed advisory GHSA-33vf-4xgg-9r58 lists affected ranges: RubyGems puma >=0 =4.0.0 <4.3.3)

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