---
id: CVE-2026-123567
title: Sveltejs devalue before 5.6.4 Prototype Injection via parse and unflatten
summary: >-
  Sveltejs devalue before 5.6.4 contains a prototype injection vulnerability
  that allows attackers to inject __proto__ own properties into objects returned
  by devalue.parse and devalue.unflatten. Attackers can supply crafted
  serialized inp…
severity: low
cvss: 3.7
cvssVector: 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N'
cvssSource: cna
cwe:
  - CWE-1321
vendor: sveltejs
product: devalue
affected:
  - devalue < 5.6.4
published: '2026-11-08'
updated: '2026-11-08'
sourceUpdated: '2026-11-08T01:38:05.840Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-123567'
references:
  - url: >-
      https://github.com/sveltejs/devalue/security/advisories/GHSA-mwv9-gp5h-frr4
    label: GitHub Security Advisory (GHSA-mwv9-gp5h-frr4)
  - url: >-
      https://www.vulncheck.com/advisories/sveltejs-devalue-before-5.6.4-prototype-injection-via-parse-and-unflatten
    label: >-
      VulnCheck Advisory: Sveltejs devalue before 5.6.4 Prototype Injection via
      parse and unflatten
tags:
  - cve.org
ingestedAt: '2026-11-08T02:43:18.659Z'
---

## Overview

Sveltejs devalue before 5.6.4 contains a prototype injection vulnerability that allows attackers to inject __proto__ own properties into objects returned by devalue.parse and devalue.unflatten. Attackers can supply crafted serialized input that, when downstream code merges results via Object.assign, alters the target object's prototype.

## Affected

- `devalue < 5.6.4`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.