---
id: "CVE-2026-128336"
page: "https://infrastack.ai/live/CVE-2026-128336"
kind: "vulnerability"
aliases: ["CVE-2026-128336"]
title: "ExcelJS: crafted spreadsheet pollutes JavaScript object prototype during load"
severity: "medium"
cvss: 6.3
epss: null
exploited: false
status: "open"
category: "package"
products: []
ecosystems: []
tags: ["exceljs", "node", "nodejs", "package"]
not_affected_if: ["no-untrusted-xlsx", "write-only-usage"]
published_at: "2026-11-13T10:18:13+00:00"
first_seen_at: "2026-11-13T10:47:34+00:00"
updated_at: "2026-11-13T10:55:09+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-128336", "first_seen_at": "2026-11-13T10:47:34+00:00"}]
exploit_sources: []
---

# ExcelJS: crafted spreadsheet pollutes JavaScript object prototype during load

Link for people: https://infrastack.ai/live/CVE-2026-128336 (this .md is for agents)

ExcelJS through 4.4.0 lets a crafted xlsx file with a __proto__ defined name add numeric keys to Object.prototype when the workbook is loaded. The pollution applies process-wide, so an attacker can alter behavior of the running Node.js application.

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

## How to check

- Find the ExcelJS version in the project: `npm ls exceljs`
- Check whether the installed version is `4.4.0` or earlier
- Look for calls to `workbook.xlsx.load()` that process uploaded or external xlsx files

## Fix

- ExcelJS: unknown (sources do not name a fixed version)

## Timeline

- 2026-11-13T10:47:34+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-128336

## Documents

- [advisory] Vendor advisory: github.com https://github.com/mateocallec/exceljs-hardened/security/advisories/GHSA-pwgq-2cgg-6ph9
- [advisory] Third-party advisory: www.vulncheck.com https://www.vulncheck.com/advisories/exceljs-through-4.4.0-prototype-pollution-via-proto-defined-name

## From the sources

ExcelJS through 4.4.0 contains a prototype pollution vulnerability that allows attackers to add numeric keys to Object.prototype through a crafted defined name referencing a __proto__ sheet. Attackers can supply an xlsx whose xl/workbook.xml definedName range targets __proto__, causing CellMatrix to write empty arrays onto Object.prototype process-wide when workbook.xlsx.load() runs.

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_Free to read, with fair use of about 60 pages a minute. For higher limits or API access: sales@infrastack.ai_