msgpack5: Partial options disable prototype protection

Exploitation

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

CVSS

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

EPSS

• CVE-2026-107301: 0.37% estimated exploitation activity in the next 30 days; percentile 30.53 (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 Passing an empty or partial options object disables the default protoAction: 'error' protection. A map containing a proto key can then replace the prototype of the decoded object, potentially changing inherited properties or causing unexpected behavior in downstream code. Only the decoded object's prototype is affected; this does not modify Object.prototype globally. Patches Options are now merged with secure defaults without modifying the caller's object. Unsupported protoAction values are rejected. Workarounds Explicitly set protoAction: 'error' whenever constructing a msgpack5 instance, and validate decoded values before use.

Published by the source

8 Oct 2026, 17:39 UTC

Last modified by the source

8 Oct 2026, 18:00 UTC

Source last checked

26 Oct 2026, 13:07 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-8hq7-ggx2-cc6m

2. Revision 2 · amended · 24 Oct 2026

   Amended from osv record GHSA-8hq7-ggx2-cc6m

3. Revision 1 · created · 23 Oct 2026

   Published from ghsa record GHSA-8hq7-ggx2-cc6m

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# Patch Next advisory brief: CVE-2026-107301

Advisory: msgpack5: Partial options disable prototype protection
IDs: CVE-2026-107301, GHSA-8hq7-ggx2-cc6m
Record: https://patchnext.com/advisories/cve-2026-107301 (revision 3, source checked 26 Oct 2026, 13:07 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: msgpack5 6.1.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm msgpack5: affected < 6.1.0; fixed in 6.1.0

## Suggested inspection (read-only)
- `npm ls msgpack5 --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## 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 msgpack5.
- 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-8hq7-ggx2-cc6m (GitHub-reviewed advisory GHSA-8hq7-ggx2-cc6m lists affected ranges: npm msgpack5 >=0 <6.1.0)
- Verified: https://osv.dev/vulnerability/GHSA-8hq7-ggx2-cc6m (OSV record GHSA-8hq7-ggx2-cc6m lists affected ranges: npm msgpack5 >=0 <6.1.0 (fixed))

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