Rack's multipart header parsing allows Denial of Service via escape-heavy quoted parameters

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-2026-34827: 0.49% estimated exploitation activity in the next 30 days; percentile 42.47 (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::Multipart::Parser#handlemimehead parses quoted multipart parameters such as Content-Disposition: form-data; name="..." using repeated String#index searches combined with String#slice! prefix deletion. For escape-heavy quoted values, this causes super-linear processing. An unauthenticated attacker can send a crafted multipart/form-data request containing many parts with long backslash-escaped parameter values to trigger excessive CPU usage during multipart parsing. This results in a denial of service condition in Rack applications that accept multipart form data. Details Rack::Multipart::Parser#handlemimehead parses quoted parameter values by repeatedly: 1. Searching for the next quote or backslash, 2. Copying the preceding substring into a new buffer, and 3. Removing the processed prefix from the original string with slice!. An attacker can exploit this by sending a multipart request with many parts whose name parameters contain long escape-heavy values such as: Under…

Published by the source

2 Apr 2026, 20:30 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-v6x5-cg8r-vv6x

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# Patch Next advisory brief: CVE-2026-34827

Advisory: Rack's multipart header parsing allows Denial of Service via escape-heavy quoted parameters
IDs: CVE-2026-34827, GHSA-v6x5-cg8r-vv6x
Record: https://patchnext.com/advisories/cve-2026-34827 (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. A vendor fix does not mean a deployment is fixed.

## Affected components
- RubyGems rack: affected >= 3.0.0.beta1, < 3.1.21; >= 3.2.0, < 3.2.6; fixed in 3.1.21, 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-v6x5-cg8r-vv6x (GitHub-reviewed advisory GHSA-v6x5-cg8r-vv6x lists affected ranges: RubyGems rack >=3.0.0.beta1 =3.2.0 <3.2.6)

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