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Fencer Vulns
medium 6.7

CVE-2021-21368 json

Prototype poisoning

published 2021-03-12 5 y ago modified 2026-09-10 1 mo ago also GHSA-gmjw-49p4-pcfm , EUVD-2021-0658

cvss
6.7
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
1.6%
EPSS 1.6% chance of exploitation in 30 days, percentile 76%
public exploits
0
none known

Summary

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 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. This "prototype poisoning" is sort of a very limited inversion of a prototype pollution attack. Only the decoded value's prototype is affected, and it can only be set to msgpack5 values (though if the victim makes use of custom codecs, anything could be a msgpack5 value). We have not found a way to escalate this to true prototype pollution (absent other bugs in the consumer's code). This has been fixed in msgpack5 version 3.6.1, 4.5.1, and 5.2.1. See the referenced GitHub Security Advisory for an example and more details.

Patch / fix

  • npm msgpack5 3.6.1, 4.5.1, 5.2.1
  • msgpack5_project msgpack5 3.6.1, 4.5.1, 5.2.1

Exploit prediction

1.65% chance of exploitation in the next 30 days, higher than 76% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

  • github.com https://github.com/mcollina/msgpack5/security/advisories/GHSA-gmjw-49p4-pcfm referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
CNA 3.1 6.7 CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:L/I:H/A:H
NVD 3.1 8.8 CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
NVD 2.0 6.5 AV:N/AC:L/Au:S/C:P/I:P/A:P
GitHub 3.1 6.7 CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:L/I:H/A:H

Affected packages

ecosystem package affected fixed in
npm msgpack5
<3.6.1
>=4.0.0, <4.5.1
>=5.0.0, <5.2.1
3.6.1
4.5.1
5.2.1

Affected products

  • mcollina msgpack5

Classification

Weakness
  • CWE-915 Improperly Controlled Modification of Dynamically-Determined Object Attributes
  • CWE-1321 Improperly Controlled Modification of Object Prototype Attributes

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.