MessagePack for Python: Out-of-bounds read / crash on Unpacker reuse after a caught error

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-2026-57585: 0.49% estimated exploitation activity in the next 30 days; percentile 42.04 (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.

Impact If the Unpacker is used repeatedly after an error occurs, the process may crash with a SEGV. If the Unpacker is used repeatedly to unpack untrusted input from external sources, it may be vulnerable to a DoS attack. Patches v1.2.1 Workarounds Users should create a new Unpacker instead of reusing the same Unpacker after an error occurs. Applying the above patch can prevent SEGV, but reusing the Streaming Unpacker after it has encountered an error will not yield correct data. If an error occurs during Streaming Unpacking, the Stream and Streaming Unpacker should be discarded. Therefore, this is not just a workaround but the correct solution. The above patch only prevents crashes from incorrect usage.

Published by the source

19 Jun 2026, 21:42 UTC

Last modified by the source

10 Sep 2026, 03:51 UTC

Source last checked

26 Oct 2026, 13:50 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-6v7p-g79w-8964

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-6v7p-g79w-8964

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# Patch Next advisory brief: CVE-2026-57585

Advisory: MessagePack for Python: Out-of-bounds read / crash on Unpacker reuse after a caught error
IDs: CVE-2026-57585, GHSA-6v7p-g79w-8964
Record: https://patchnext.com/advisories/cve-2026-57585 (revision 2, source checked 26 Oct 2026, 13:50 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: msgpack 1.2.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI msgpack: affected <= 1.2.0; fixed in 1.2.1

## Suggested inspection (read-only)
- `python -m pip show msgpack`: prints the installed version. Run with the environment that serves production active.

## 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-6v7p-g79w-8964 (OSV record GHSA-6v7p-g79w-8964 lists affected ranges: PyPI msgpack >=0 <1.2.1 (fixed))
- Verified: https://github.com/advisories/GHSA-6v7p-g79w-8964 (GitHub-reviewed advisory GHSA-6v7p-g79w-8964 lists affected ranges: PyPI msgpack >=0 <=1.2.0)

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