---
id: CVE-2026-78207
title: >-
  exceljs through 4.4.0 contains a prototype pollution vulnerability in the
  deepMerge helper that fails to reject __proto__, constructor, or prototype
  keys when merging note objects
summary: >-
  exceljs through 4.4.0 contains a prototype pollution vulnerability in the
  deepMerge helper that fails to reject __proto__, constructor, or prototype
  keys when merging note objects. Attackers can assign parsed JSON with a
  malicious __prot…
severity: critical
cvss: 9.4
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:L'
cwe:
  - CWE-1321
published: '2026-08-24'
updated: '2026-08-29'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-78207'
references:
  - url: 'https://github.com/exceljs/exceljs'
    label: disclosure@vulncheck.com
  - url: >-
      https://github.com/exceljs/exceljs/blob/v4.4.0/lib/utils/under-dash.js#L155-L181
    label: disclosure@vulncheck.com
  - url: >-
      https://github.com/mateocallec/exceljs-hardened/security/advisories/GHSA-qwr4-7h29-chpf
    label: disclosure@vulncheck.com
  - url: >-
      https://www.vulncheck.com/advisories/exceljs-through-prototype-pollution-via-deepmerge-reached-from-note-serialization
    label: disclosure@vulncheck.com
tags:
  - nvd
epss: 0.00603
epssPercentile: 0.46533
ingestedAt: '2026-08-29T21:42:33.198Z'
---

## Overview

exceljs through 4.4.0 contains a prototype pollution vulnerability in the deepMerge helper that fails to reject __proto__, constructor, or prototype keys when merging note objects. Attackers can assign parsed JSON with a malicious __proto__ property to cell notes, modifying Object.prototype and affecting all plain objects created in the process.

## Remediation

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