CVE-2025-66456Public exploitNetwork attack vector

Prototype pollution via unsafe mergeDeep key copying

Published Dec 9, 2025 · Updated Dec 9, 2025

Prototype pollution in Elysia 1.4.0 through 1.4.16 allows remote attackers to alter object prototypes via crafted JSON request bodies. The mergeDeep utility copies attacker-controlled proto, constructor, and prototype keys while combining same-key standalone schema results. Exploitation requires multiple standalone validations and an any-type guard; pairing the polluted state with a separate cookie flaw enables code execution.

CVSS severity9.1
Critical
EPSS probability0.56%
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 Elysia 1.4.0 through 1.4.16 allows remote attackers to alter object prototypes via crafted JSON request bodies. The mergeDeep utility copies attacker-controlled proto, constructor, and prototype keys while combining same-key standalone schema results. Exploitation requires multiple standalone validations and an any-type guard; pairing the polluted state with a separate cookie flaw enables code execution.

The record

CVE
CVE-2025-66456
Published
Dec 9, 2025
Updated
Dec 9, 2025
Vendor
ElysiaJS
Product
Elysia
Classifications
CWE-1321, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Dec 9, 2025CVE publishedPublication date reported by the CVE source.
  2. Dec 9, 2025Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

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

  1. Affected versionversion=>= 1.4.0, < 1.4.17

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

ElysiaJS · Elysiaversion=>= 1.4.0, < 1.4.17

Published CVSS scores

9.1GitHubCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N

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.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

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')
T1190Exploit Public-Facing Application

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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