---
id: "CVE-2026-93753"
kind: "vulnerability"
aliases: ["CVE-2026-93753", "GHSA-gcqx-w48m-w2h4"]
title: "deepmerge mergeObject() allows prototype poisoning of returned objects"
severity: "high"
cvss: 8.7
epss: 0.00514
exploited: false
status: "open"
category: "package"
products: [{"name": "deepmerge", "fixed": [], "affected": ">= 4.2.0, <= 4.3.1", "ecosystem": "npm"}]
ecosystems: ["npm"]
tags: ["cwe-1321", "deepmerge", "node", "npm", "package"]
not_affected_if: ["default-config", "trusted-input-only"]
published_at: "2026-09-18T18:18:34+00:00"
first_seen_at: "2026-10-27T21:39:56+00:00"
updated_at: "2026-10-27T21:59:20+00:00"
sources: [{"source": "ghsa", "url": "https://github.com/advisories/GHSA-gcqx-w48m-w2h4", "first_seen_at": "2026-10-27T21:39:56+00:00"}, {"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-93753", "first_seen_at": "2026-10-27T21:59:20+00:00"}]
---

# deepmerge mergeObject() allows prototype poisoning of returned objects

deepmerge through 4.3.1 fails to validate keys in mergeObject(), so attacker-supplied source objects can write properties into the prototype of the returned object. Applications that read properties without own-property checks then inherit attacker-chosen values.

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

## Affected

- npm:deepmerge affected >= 4.2.0, <= 4.3.1 fixed ?

## How to check

- Run `npm ls deepmerge` to find the installed version and whether it is a dependency of something else
- Check the lockfile for `deepmerge` versions between `4.2.0` and `4.3.1`

## Fix

- Upgrade `deepmerge` to a version later than `4.3.1` (fixed version unknown from sources)
- Until upgraded, do not pass untrusted input as source objects to mergeObject()

## Timeline

- 2026-10-27T21:39:56+00:00 ghsa https://github.com/advisories/GHSA-gcqx-w48m-w2h4
- 2026-10-27T21:59:20+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-93753

## From the sources

deepmerge through 4.3.1 contains a prototype poisoning vulnerability in the mergeObject() function that fails to properly validate keys being written to target objects. Attackers can supply malicious source objects in merge operations to inject attacker-controlled properties into the returned object's prototype, causing applications to inherit unintended values when accessing properties without own-property checks.

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_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_