Possible Denial of Service Vulnerability in Rack's header parsing

Exploitation

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

CVSS

not scored

EPSS

• CVE-2023-27539: 1.06% estimated exploitation activity in the next 30 days; percentile 64.54 (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.

There is a denial of service vulnerability in the header parsing component of Rack. This vulnerability has been assigned the CVE identifier CVE-2023-27539. Versions Affected: = 2.0.0 Not affected: None. Fixed Versions: 2.2.6.4, 3.0.6.1 Impact Carefully crafted input can cause header parsing in Rack to take an unexpected amount of time, possibly resulting in a denial of service attack vector. Any applications that parse headers using Rack (virtually all Rails applications) are impacted. Workarounds Setting Regexp.timeout in Ruby 3.2 is a possible workaround.

Published by the source

15 Mar 2023, 21:36 UTC

Last modified by the source

9 Jan 2025, 15:36 UTC

Source last checked

26 Oct 2026, 15:48 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-c6qg-cjj8-47qp

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# Patch Next advisory brief: CVE-2023-27539

Advisory: Possible Denial of Service Vulnerability in Rack's header parsing
IDs: CVE-2023-27539, GHSA-c6qg-cjj8-47qp
Record: https://patchnext.com/advisories/cve-2023-27539 (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: Unrated
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: rack 3.0.6.1, 2.2.6.4. A vendor fix does not mean a deployment is fixed.

## Affected components
- RubyGems rack: affected >= 2.0.0, < 2.2.6.4; >= 3.0.0, < 3.0.6.1; fixed in 2.2.6.4, 3.0.6.1

## Suggested inspection (read-only)
- RubyGems rack: 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 rack.
- 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-c6qg-cjj8-47qp (GitHub-reviewed advisory GHSA-c6qg-cjj8-47qp lists affected ranges: RubyGems rack >=2.0.0 =3.0.0 <3.0.6.1)

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