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GHSA-4q55-j62x-fr9h: devalue: Malformed null-prototype object keys bypass __proto__ rejection via property-key coercion

October 1, 2026

This is another instance of https://github.com/sveltejs/devalue/security/advisories/GHSA-mwv9-gp5h-frr4, where some payloads could cause parse to create objects with a __proto__ own property. This on its own is not enough to cause prototype pollution, and indeed this is actually how JSON.parse works, but we decided to be a little more defensive here and not allow the creation of objects with __proto__ own-properties. It is very unlikely for this to cause any issues.

References

  • github.com/advisories/GHSA-4q55-j62x-fr9h
  • github.com/sveltejs/devalue/commit/9ec513072e31f45e60d44dad6f0dba9c92bb8675
  • github.com/sveltejs/devalue/releases/tag/v5.9.3
  • github.com/sveltejs/devalue/security/advisories/GHSA-4q55-j62x-fr9h

Code Behaviors & Features

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Affected versions

All versions before 5.9.3

Fixed versions

  • 5.9.3

Solution

Upgrade to version 5.9.3 or above.

Impact 3.7 LOW

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N

Learn more about CVSS

Weakness

  • CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')

Source file

npm/devalue/GHSA-4q55-j62x-fr9h.yml

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