CVE-2026-128337
Received Received - Intake

Prototype Pollution in ExcelJS Library

Publication date: 2026-11-13

Last updated on: 2026-11-13

Assigner: VulnCheck

Description
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.

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Meta Information

Published
2026-11-13
Last Modified
2026-11-13
Generated
2026-11-13
AI Q&A
2026-11-13
EPSS Evaluated
N/A
NVD
CVE-2026-128337
EUVD
EUVD-2026-125040

Affected Vendors & Products

Showing 1 associated CPE
Vendor Product Version / Range
exceljs exceljs From 0 (inc) to 4.4.0 (inc)

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Exploitability

CWE
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KEV
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CWE ID Description
CWE-1321 The product receives input from an upstream component that specifies attributes that are to be initialized or updated in an object, but it does not properly control modifications of attributes of the object prototype.

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Executive Summary

This is a prototype pollution vulnerability in ExcelJS through version 4.4.0. Attackers can craft an xlsx file with a malicious x14:cfRule id set to __proto__. When loaded with xlsx.load(), it modifies Object.prototype, allowing changes to properties like type, direction, border, and cfvo on every object in the process until a restart occurs.

Detection Guidance

To detect this vulnerability, inspect any xlsx files processed by ExcelJS versions through 4.4.0 for malicious x14:cfRule id values set to __proto__. Check logs for unexpected property modifications in Node.js processes handling Excel files.

Impact Analysis

An attacker could exploit this to alter application behavior by modifying core object properties. This might lead to unexpected crashes, data corruption, or unauthorized access in systems processing the malicious xlsx file. The impact persists until the Node.js process restarts.

Mitigation Strategies

Immediately upgrade ExcelJS to a version beyond 4.4.0. If using exceljs-hardened, update to version 5.0.3 or later. Avoid loading untrusted xlsx files in vulnerable applications until patched.

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