CVE-2021-21368Public exploitNetwork attack vector

Decoded-object corruption via __proto__ map assignment

Published Mar 12, 2021 · Updated Aug 3, 2024

Prototype poisoning in msgpack5 affected releases allows remote authenticated users to corrupt decoded values via crafted MessagePack data. The plain-object map decoder assigns an attacker-controlled proto value through object[key], replacing the returned object's prototype with the decoded object or null. Exploitation requires crafted data to reach the decoder and user interaction; the returned value can impersonate another type or throw exceptions, but the global prototype remains unchanged.

CVSS severity6.7
Medium
EPSS probability1.65%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Prototype poisoning in msgpack5 affected releases allows remote authenticated users to corrupt decoded values via crafted MessagePack data. The plain-object map decoder assigns an attacker-controlled proto value through object[key], replacing the returned object's prototype with the decoded object or null. Exploitation requires crafted data to reach the decoder and user interaction; the returned value can impersonate another type or throw exceptions, but the global prototype remains unchanged.

The record

CVE
CVE-2021-21368
Published
Mar 12, 2021
Updated
Aug 3, 2024
Vendor
Matteo Collina
Product
msgpack5
Classifications
CWE-915
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Mar 12, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 3.6.1
  2. Affected versionversion=>= 4.0.0, < 4.5.1
  3. Affected versionversion=>= 5.0.0, < 5.2.1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Matteo Collina · msgpack5version=< 3.6.1; version=>= 4.0.0, < 4.5.1; version=>= 5.0.0, < 5.2.1

Published CVSS scores

6.7GitHubCVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:L/I:H/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-915Improperly Controlled Modification of Dynamically-Determined Object Attributes

Public exploit references

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Technologies

Your stack

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Your stack

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