MessagePack::Buffer#clear Use-After-Free that Enables Cross-Buffer Disclosure

Exploitation

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

CVSS 4.0

2.1 · CVSS:4.0/AV:L/AC:L/AT:P/PR:N/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N · score source

EPSS

• CVE-2026-54522: 0.23% estimated exploitation activity in the next 30 days; percentile 13.48 (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 MessagePack::Buffer#clear shifts out every chunk and returns its 4 KiB rmem page to the shared pool, but does not reset the buffer's rmem cursor (rmemlast, rmemend, rmemowner). The next write sees "unused rmem space" left over from the freed page and hands back a slice of memory that has already been returned to the pool. A second MessagePack::Buffer then re-acquires that same page, so reading the cleared-and-rewritten buffer discloses the second buffer's bytes — a same-process use-after-free with cross-buffer information disclosure (and the symmetric write-corruption). Details - msgpackbufferclear() → msgpackbuffershiftchunk() (ext/msgpack/buffer.c:151, :128) destroys chunks (msgpackbufferchunkdestroy, :58, returns the page via msgpackrmemfree) but resets only tailbufferend/readbuffer, leaving rmemlast/rmemend/rmemowner pointing into the freed page. - Next Buffer#write → msgpackbufferchunkmalloc() reuse branch (:363) returns b-rmemlast, a pointer into the already-freed page.…

Published by the source

30 Jul 2026, 16:33 UTC

Last modified by the source

10 Sep 2026, 03:51 UTC

Source last checked

26 Oct 2026, 15:52 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-4mrv-5p47-p938

2. Revision 2 · amended · 25 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-4mrv-5p47-p938

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# Patch Next advisory brief: CVE-2026-54522

Advisory: MessagePack::Buffer#clear Use-After-Free that Enables Cross-Buffer Disclosure
IDs: CVE-2026-54522, GHSA-4mrv-5p47-p938
Record: https://patchnext.com/advisories/cve-2026-54522 (revision 3, 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: Low (CVSS 2.1)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: msgpack 1.8.2. A vendor fix does not mean a deployment is fixed.

## Affected components
- RubyGems msgpack: affected <= 1.8.1; fixed in 1.8.2

## Suggested inspection (read-only)
- RubyGems msgpack: 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 msgpack.
- 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://osv.dev/vulnerability/GHSA-4mrv-5p47-p938 (OSV record GHSA-4mrv-5p47-p938 lists affected ranges: RubyGems msgpack >=0 <1.8.2 (fixed))
- Verified: https://github.com/advisories/GHSA-4mrv-5p47-p938 (GitHub-reviewed advisory GHSA-4mrv-5p47-p938 lists affected ranges: RubyGems msgpack >=0 <=1.8.1)

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