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High, CVSS 8.7amended

Rack: Multipart parsing allows DoS via unbounded scan buffer growth

CVE-2026-125111GHSA-5v64-555j-wjxm

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (rack 3.2.8, 3.1.23, 2.2.25), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

  • RubyGems rack

    Affected
    < 2.2.25>= 3.0.0, < 3.1.23>= 3.2.0, < 3.2.8
    Fixed in
    3.1.23, 2.2.25, 3.2.8

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS 4.0
8.7 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N · score source
EPSS
  • CVE-2026-125111: 0.37% estimated exploitation activity in the next 30 days; percentile 30.73 (FIRST EPSS, 2026-11-10). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

Summary `Rack::Multipart::Parser` reads the request body in chunks of `rack.multipart.buffer_size` (1 MiB by default) into an internal scan buffer. Parsed bytes are only discarded from that buffer when a chunk ends inside a part body. A request whose parts are sized so that every chunk ends inside, or immediately after, a part's headers is never compacted, so the buffer grows to the size of the request. This results in a denial of service condition through memory exhaustion for applications that parse multipart requests.

Details The parser appends each chunk read from the input to a `StringScanner` and advances through boundaries, headers, and bodies. In the body state, when no closing boundary is found, the consumed prefix is removed before more data is read. In the header state, and in the body state when no body bytes have been consumed yet, the parser requests more data without removing anything. Bytes skipped between parts are not counted against any limit. An attacker can…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 2 · amended ·

    Amended from osv record GHSA-5v64-555j-wjxm

  2. Revision 1 · created ·

    Published from ghsa record GHSA-5v64-555j-wjxm

Agent brief

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# Patch Next advisory brief: CVE-2026-125111

Advisory: Rack: Multipart parsing allows DoS via unbounded scan buffer growth
IDs: CVE-2026-125111, GHSA-5v64-555j-wjxm
Record: https://patchnext.com/advisories/cve-2026-125111 (revision 2, source checked 11 Nov 2026, 07:35 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.7)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: rack 3.2.8, 3.1.23, 2.2.25. A vendor fix does not mean a deployment is fixed.

## Affected components
- RubyGems rack: affected < 2.2.25; >= 3.0.0, < 3.1.23; >= 3.2.0, < 3.2.8; fixed in 3.1.23, 2.2.25, 3.2.8

## 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-5v64-555j-wjxm (GitHub-reviewed advisory GHSA-5v64-555j-wjxm lists affected ranges: RubyGems rack >=0 =3.0.0 =3.2.0 <3.2.8)
- Verified: https://osv.dev/vulnerability/GHSA-5v64-555j-wjxm (OSV record GHSA-5v64-555j-wjxm lists affected ranges: RubyGems rack >=0 =3.0.0 =3.2.0 <3.2.8 (fixed))

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