CVE-2021-21368: msgpack5 is a msgpack v5 implementation for node.js and the browser

Q: How do I fix CVE-2021-21368?

msgpack5: upgrade to 3.6.1, 4.5.1, 5.2.1. Exact commands for each OS are on this page.

Q: Is CVE-2021-21368 being exploited?

CVE-2021-21368 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2021-21368 affect?

CVE-2021-21368 affects msgpack5. Severity is medium (CVSS 6.7). msgpack5 is a msgpack v5 implementation for node.js and the browser. In msgpack5 before versions 3.6.1, 4.5.1, and 5.2.1 there is a "Prototype Poisoning" vulnerability. When msgpack5 decodes a map containing a key "__proto__", it assigns th

Q: How urgent is CVE-2021-21368?

Priority 48/100. Public exploit reference

This month

MEDIUM 6.7UPGRADE

CVE-2021-21368 is a medium-severity flaw (CVSS 6.7) affecting msgpack5. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

msgpack5 is a msgpack v5 implementation for node.js and the browser. In msgpack5 before versions 3.6.1, 4.5.1, and 5.2.1 there is a "Prototype Poisoning" vulnerability. When msgpack5 decodes a map containing a key "__proto__", it assigns the decoded value to __proto__. 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 dec

Why this priority

Public exploit reference

A fixed release or distro patch exists.

Risk facts

CVSS score	6.7 (Medium)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	Low
User interaction	Required
EPSS exploit chance	1.6% (top 24%)
CISA exploited list	No
Weakness	CWE-915, CWE-1321
Published	2021-03-12
Last modified	2026-06-17

Publisher (CNA): GitHub, Inc. · Vendor: msgpack5 project

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2021-03-12 17:15 UTCPublished CVE record published
2026-06-17 03:35 UTCUpdated Record last modified

msgpack5 project msgpack5 UPGRADE

Affected: < 3.6.1 · >= 4.0.0 and < 4.5.1 · >= 5.0.0 and < 5.2.1

Fixed in: 3.6.1, 4.5.1, 5.2.1

Safe to downgrade to: a release before 4.0.0

1. Check installed version

npm ls msgpack5

2. UPGRADE to 3.6.1

npm install msgpack5@">=3.6.1"

3. If no update is available yet, DOWNGRADE to a release before 4.0.0

npm view msgpack5 versions npm install msgpack5@"<4.0.0"

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2021-21368?

msgpack5: upgrade to 3.6.1, 4.5.1, 5.2.1. Exact commands for each OS are on this page.

Is CVE-2021-21368 being exploited?

CVE-2021-21368 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2021-21368 affect?

CVE-2021-21368 affects msgpack5. Severity is medium (CVSS 6.7). msgpack5 is a msgpack v5 implementation for node.js and the browser. In msgpack5 before versions 3.6.1, 4.5.1, and 5.2.1 there is a "Prototype Poisoning" vulnerability. When msgpack5 decodes a map containing a key "__proto__", it assigns th

How urgent is CVE-2021-21368?

Priority 48/100. Public exploit reference

Software pages

Node.js vulnerabilities

References

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