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Medium, CVSS 6.7amended

Prototype poisoning

CVE-2021-21368GHSA-gmjw-49p4-pcfm

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (msgpack5 5.2.1, 4.5.1, 3.6.1), 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

  • npm msgpack5

    Affected
    < 3.6.1>= 4.0.0, < 4.5.1>= 5.0.0, < 5.2.1
    Fixed in
    5.2.1, 4.5.1, 3.6.1

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS
6.7 · CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:L/I:H/A:H
EPSS
  • CVE-2021-21368: 1.65% estimated exploitation activity in the next 30 days; percentile 76.53 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

Impact The issue is as follows: when msgpack5 decodes a map containing a key "proto", it assigns the decoded value to proto. As you are no doubt aware, Object.prototype.proto is an accessor property for the receiver's prototype. If the value corresponding to the key proto decodes to an object or null, msgpack5 sets the decoded object's prototype to that value. An attacker who can submit crafted MessagePack data to a service can use this to produce values that appear to be of other types; may have unexpected prototype properties and methods (for example length, numeric properties, and push et al if proto's value decodes to an Array); and/or may throw unexpected exceptions when used (for example if the proto value decodes to a Map or Date). Other unexpected behavior might be produced for other types. There is no effect on the global prototype. An example: This "prototype poisoning" is sort of a very limited inversion of a prototype pollution attack. Only the decoded value's prototype…

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

    Amended from ghsa record GHSA-gmjw-49p4-pcfm

  2. Revision 1 · created ·

    Published from osv record GHSA-gmjw-49p4-pcfm

Agent brief

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# Patch Next advisory brief: CVE-2021-21368

Advisory: Prototype poisoning
IDs: CVE-2021-21368, GHSA-gmjw-49p4-pcfm
Record: https://patchnext.com/advisories/cve-2021-21368 (revision 2, source checked 26 Oct 2026, 12:17 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.7)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: msgpack5 5.2.1, 4.5.1, 3.6.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm msgpack5: affected < 3.6.1; >= 4.0.0, < 4.5.1; >= 5.0.0, < 5.2.1; fixed in 5.2.1, 4.5.1, 3.6.1

## 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://osv.dev/vulnerability/GHSA-gmjw-49p4-pcfm (OSV record GHSA-gmjw-49p4-pcfm lists affected ranges: npm msgpack5 >=0 =4.0.0 =5.0.0 <5.2.1 (fixed))
- Verified: https://github.com/advisories/GHSA-gmjw-49p4-pcfm (GitHub-reviewed advisory GHSA-gmjw-49p4-pcfm lists affected ranges: npm msgpack5 >=0 =4.0.0 =5.0.0 <5.2.1)

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