ExcelJS up to 4.4.0 xl/workbook.xml workbook.xlsx.load prototype pollution

Summaryinfo

A vulnerability was found in ExcelJS up to 4.4.0. It has been classified as critical. This issue affects the function workbook.xlsx.load of the file xl/workbook.xml. Performing a manipulation results in prototype pollution. This vulnerability is identified as CVE-2026-128336. The attack can be initiated remotely. There is not any exploit available.

Detailsinfo

A vulnerability was found in ExcelJS up to 4.4.0. It has been classified as critical. This affects the function workbook.xlsx.load of the file xl/workbook.xml. The manipulation with an unknown input leads to a prototype pollution vulnerability. CWE is classifying the issue as 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. This is going to have an impact on confidentiality, integrity, and availability. The summary by CVE is:

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.

The advisory is shared at github.com. This vulnerability is uniquely identified as CVE-2026-128336 since 11/13/2026. The exploitability is told to be easy. It is possible to initiate the attack remotely. Technical details are known, but no exploit is available. The price for an exploit might be around USD $0-$5k at the moment (estimation calculated on 11/13/2026). MITRE ATT&CK project uses the attack technique T1059 for this issue.

By approaching the search of inurl:xl/workbook.xml it is possible to find vulnerable targets with Google Hacking.

There is no information about possible countermeasures known. It may be suggested to replace the affected object with an alternative product.

Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

Productinfo

Name

• ExcelJS

Version

• 4.0
• 4.1
• 4.2
• 4.3
• 4.4.0

CPE 2.3info

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CPE 2.2info

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Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

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