CVE-2021-3918Public exploitNetwork attack vector

Object prototype pollution via __proto__ schema defaults

Published Nov 13, 2021 · Updated Jan 17, 2025

Prototype pollution in Kris Zyp json-schema before 0.4.0 allows attackers to alter Object.prototype via a crafted schema. The validate function iterates schema properties and treats proto as an ordinary key when applying defaults or coercion to the instance. Any application that validates attacker-controlled schemas can propagate injected properties to subsequently created objects, with downstream impact determined by their use.

CVSS severity9.8
Critical
EPSS probability3.83%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Prototype pollution in Kris Zyp json-schema before 0.4.0 allows attackers to alter Object.prototype via a crafted schema. The validate function iterates schema properties and treats proto as an ordinary key when applying defaults or coercion to the instance. Any application that validates attacker-controlled schemas can propagate injected properties to subsequently created objects, with downstream impact determined by their use.

The record

CVE
CVE-2021-3918
Published
Nov 13, 2021
Updated
Jan 17, 2025
Vendor
Kris Zyp
Product
json-schema
Classifications
CWE-1321, CWE-915
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Nov 13, 2021CVE publishedPublication date reported by the CVE source.
  2. Jan 17, 2025Record 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=unspecified <=0.3.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Kris Zyp · json-schemaversion=unspecified <=0.3.0

Published CVSS scores

9.8GitHub Advisory DatabaseCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/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
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-1321Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')
CWE-915Improperly Controlled Modification of Dynamically-Determined Object Attributes

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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