---
id: "CVE-2026-123567"
page: "https://infrastack.ai/live/CVE-2026-123567"
kind: "vulnerability"
aliases: ["CVE-2026-123567"]
title: "devalue: crafted input injects __proto__ into parsed objects"
severity: "medium"
cvss: 6.3
epss: null
exploited: false
status: "open"
category: "package"
products: []
ecosystems: []
tags: ["devalue", "node.js", "package", "svelte", "sveltekit"]
not_affected_if: ["no-object-assign-merge", "trusted-input-only"]
published_at: "2026-11-08T02:16:49+00:00"
first_seen_at: "2026-11-08T02:21:12+00:00"
updated_at: "2026-11-08T02:25:06+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-123567", "first_seen_at": "2026-11-08T02:21:12+00:00"}]
exploit_sources: []
---

# devalue: crafted input injects __proto__ into parsed objects

Link for people: https://infrastack.ai/live/CVE-2026-123567 (this .md is for agents)

devalue, a serialization library used by SvelteKit, lets crafted serialized input add __proto__ own properties to objects returned by devalue.parse and devalue.unflatten. When downstream code merges those results with Object.assign, the target object's prototype can be altered.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Check the installed devalue version with `npm ls devalue` in each project.
- Look for devalue in package-lock.json, pnpm-lock.yaml or yarn.lock.
- Check whether application code calls devalue.parse or devalue.unflatten on untrusted input and merges the result with Object.assign.

## Fix

- devalue: move to `5.6.4` or later.

## Timeline

- 2026-11-08T02:21:12+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-123567

## Documents

- [advisory] Vendor advisory: github.com https://github.com/sveltejs/devalue/security/advisories/GHSA-mwv9-gp5h-frr4
- [advisory] Third-party advisory: www.vulncheck.com https://www.vulncheck.com/advisories/sveltejs-devalue-before-5.6.4-prototype-injection-via-parse-and-unflatten

## From the sources

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.

---

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