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js-yaml has prototype pollution in merge (<<)

Moderate severity GitHub Reviewed Published Nov 12, 2025 in nodeca/js-yaml • Updated Nov 17, 2025

Package

npm js-yaml (npm)

Affected versions

>= 4.0.0, < 4.1.1
< 3.14.2

Patched versions

4.1.1
3.14.2

Description

Impact

In js-yaml 4.1.0, 4.0.0, and 3.14.1 and below, it's possible for an attacker to modify the prototype of the result of a parsed yaml document via prototype pollution (__proto__). All users who parse untrusted yaml documents may be impacted.

Patches

Problem is patched in js-yaml 4.1.1 and 3.14.2.

Workarounds

You can protect against this kind of attack on the server by using node --disable-proto=delete or deno (in Deno, pollution protection is on by default).

References

https://cheatsheetseries.owasp.org/cheatsheets/Prototype_Pollution_Prevention_Cheat_Sheet.html

References

@puzrin puzrin published to nodeca/js-yaml Nov 12, 2025
Published by the National Vulnerability Database Nov 13, 2025
Published to the GitHub Advisory Database Nov 14, 2025
Reviewed Nov 14, 2025
Last updated Nov 17, 2025

Severity

Moderate

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
Low
Availability
None

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(8th percentile)

Weaknesses

Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')

The product receives input from an upstream component that specifies attributes that are to be initialized or updated in an object, but it does not properly control modifications of attributes of the object prototype. Learn more on MITRE.

CVE ID

CVE-2025-64718

GHSA ID

GHSA-mh29-5h37-fv8m

Source code

Credits

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