---
id: "CVE-2026-128337"
page: "https://infrastack.ai/live/CVE-2026-128337"
kind: "vulnerability"
aliases: ["CVE-2026-128337"]
title: "ExcelJS: crafted xlsx file pollutes Object.prototype when loaded"
severity: "high"
cvss: 8.3
epss: null
exploited: false
status: "open"
category: "package"
products: []
ecosystems: []
tags: ["exceljs", "node.js", "npm", "package"]
not_affected_if: ["no-untrusted-xlsx-input"]
published_at: "2026-11-13T10:18:13+00:00"
first_seen_at: "2026-11-13T10:47:34+00:00"
updated_at: "2026-11-13T10:51:03+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-128337", "first_seen_at": "2026-11-13T10:47:34+00:00"}]
exploit_sources: []
---

# ExcelJS: crafted xlsx file pollutes Object.prototype when loaded

Link for people: https://infrastack.ai/live/CVE-2026-128337 (this .md is for agents)

ExcelJS through 4.4.0 has a prototype pollution flaw in mergeConditionalFormattings: an x14:cfRule id of __proto__ in an xlsx file lets the file write properties onto Object.prototype. An attacker who gets a team's app to load the crafted file can alter behavior of every object in that process until it restarts.

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

## How to check

- Check the installed version with `npm ls exceljs` or the `exceljs` entry in `package-lock.json`.
- Search the codebase for calls to `xlsx.load(` on files that users or external systems can supply.

## Fix

- ExcelJS: unknown (the sources give no fixed version)

## Timeline

- 2026-11-13T10:47:34+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-128337

## Documents

- [advisory] Vendor advisory: github.com https://github.com/mateocallec/exceljs-hardened/security/advisories/GHSA-86ch-m4gp-r5xf
- [advisory] Third-party advisory: www.vulncheck.com https://www.vulncheck.com/advisories/exceljs-through-4.4.0-prototype-pollution-via-x14-conditional-formatting-rule-id

## From the sources

ExcelJS through 4.4.0 contains a prototype pollution vulnerability in mergeConditionalFormattings that allows attackers to modify Object.prototype by supplying an x14:cfRule id of __proto__. Attackers can craft an xlsx file that, when loaded with xlsx.load(), sets properties such as type, direction, border and cfvo on every object in the process until it restarts.

---

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