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CVE-2026-128336

ExcelJS through 4.4.0 Prototype Pollution via __proto__ Defined Name

  • Medium 6.3

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.

Key facts

Published
13 Nov 2026
Last changed
13 Nov 2026
Assigned by
VulnCheck
EPSS
0.474% likely to be exploited in the next 30 days, higher than 41.567% of CVEs (scores for 15 Nov 2026)
SSVC (CISA)
Exploitation: none; Automatable: no; Technical impact: partial

Severity scores

Each score is shown as its source gave it. They are not averaged or merged.

  • 6.3CVSS 4.0, Mediumfrom the CNA (VulnCheck)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
  • 4.8CVSS 3.1, Mediumfrom the CNA (VulnCheck)CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:L

Historical context and remediation

This reference retains the original CVE publication date. A recent record update does not mean a new attack occurred.

No separately sourced impact summary is available on this page yet. Consult the description, version data and dated exploitation evidence.

No explicit remediation statement is available on this page yet. Consult the original record and vendor references before choosing a fix.

Version statuses below come from the CNA. An “unaffected” version is not necessarily a currently supported release or proof of a patch date. EPSS is a dated risk estimate, not a count of historical attacks.

Read the original CVE record.

Affected products

VendorProductVersions
exceljsexceljs
  • Default status: unaffected
  • 0 up to and including 4.4.0 (affected)

Timeline

  1. 13 Nov 2026 Published

Weakness

References