CVE-2026-128336

ExcelJS through 4.4.0 Prototype Pollution via __proto__ Defined Name

Published Nov 13, 2026 CVSS 4.0 · 6.3 Medium EPSS < 1% KEV · Not listed
Description
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.

Analysis

No analysis available yet

Remediation

No remediation available yet

Tracking

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Advisories

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History
Type Values Removed Values Added
Description 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.
Title ExcelJS through 4.4.0 Prototype Pollution via __proto__ Defined Name
First Time appeared Exceljs Project
Exceljs Project exceljs
Weaknesses CWE-1321
CPEs cpe:2.3:a:exceljs_project:exceljs:*:*:*:*:*:*:*:*
Vendors & Products Exceljs Project
Exceljs Project exceljs
References
Metrics cvssV3_1

{'score': 4.8, 'vector': 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:L'}

cvssV4_0

{'score': 6.3, 'vector': 'CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:L/VA:L/SC:N/SI:N/SA:N'}

Weaknesses
  • CWE-1321

    Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')

Sources

Status
PUBLISHED
Assigner
VulnCheck
Published
Updated
Nov 13, 2026, 9:33 AM
Reserved
Nov 13, 2026, 1:20 AM
Status
Deferred
Published
Nov 13, 2026, 10:18 AM
Modified
Nov 13, 2026, 3:45 PM