Rack has quadratic complexity in Rack::Utils.select_best_encoding via wildcard Accept-Encoding header

Exploitation

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

CVSS

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

EPSS

• CVE-2026-34230: 0.45% estimated exploitation activity in the next 30 days; percentile 38.91 (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.

Summary Rack::Utils.selectbestencoding processes Accept-Encoding values with quadratic time complexity when the header contains many wildcard () entries. Because this method is used by Rack::Deflater to choose a response encoding, an unauthenticated attacker can send a single request with a crafted Accept-Encoding header and cause disproportionate CPU consumption on the compression middleware path. This results in a denial of service condition for applications using Rack::Deflater. Details Rack::Utils.selectbestencoding expands parsed Accept-Encoding values into a list of candidate encodings. When an entry is , the method computes the set of concrete encodings by subtracting the encodings already present in the request: Because acceptencoding.map(&:first) is evaluated inside the loop, it is recomputed for each wildcard entry. If the request contains N wildcard entries, this produces repeated scans over the full parsed header and causes quadratic behavior. After expansion, the method…

Published by the source

2 Apr 2026, 20:32 UTC

Last modified by the source

13 May 2026, 16:18 UTC

Source last checked

26 Oct 2026, 15:52 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-v569-hp3g-36wr

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# Patch Next advisory brief: CVE-2026-34230

Advisory: Rack has quadratic complexity in Rack::Utils.select_best_encoding via wildcard Accept-Encoding header
IDs: CVE-2026-34230, GHSA-v569-hp3g-36wr
Record: https://patchnext.com/advisories/cve-2026-34230 (revision 1, source checked 26 Oct 2026, 15:52 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: rack 3.2.6, 3.1.21, 2.2.23. A vendor fix does not mean a deployment is fixed.

## Affected components
- RubyGems rack: affected < 2.2.23; >= 3.0.0.beta1, < 3.1.21; >= 3.2.0, < 3.2.6; fixed in 3.1.21, 2.2.23, 3.2.6

## 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-v569-hp3g-36wr (GitHub-reviewed advisory GHSA-v569-hp3g-36wr lists affected ranges: RubyGems rack >=0 =3.0.0.beta1 =3.2.0 <3.2.6)

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