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CVE-2026-128336 ExcelJS through 4.4.0 Prototype Pollution via __proto__ Defined Name

6.3 mediumEPSS 0.4%

Medium (6.3) in exceljs exceljs. EPSS 0.4% estimated probability of exploitation activity in the next 30 days.

On this page
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.

Severity

CVSS 4.0

6.3 medium
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

CVSS 3.1

4.8 medium
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:L
EPSS
0.4% estimated probability of exploitation activity in the next 30 days; 37th percentile. Scored 2026-11-13. A forecast for this CVE across the internet, not for your own systems.
CISA's assessment (SSVC)
Exploitation: none · Automatable: no · Technical impact: partial, assessed 2026-11-13. From CISA's Vulnrichment in the CVE List: whether it is exploited, whether exploiting it can be automated, and how much control it gives.
CWE
CWE-1321 Improperly Controlled Modification of Object Prototype Attributes (Prototype Pollution)
Published
Modified
Assigned by
VulnCheck (CNA)
NVD analysis
Deferred: the NVD does not plan to analyze it
Record from
the NVD, synced

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Affected

Versions

From the CNA's record in the CVE List.

ProductAffected
exceljsthrough 4.4.0

Attack patterns (CAPEC)

  • CAPEC-1 Accessing Functionality Not Properly Constrained by ACLs Likelihood: High · Severity: High
  • CAPEC-77 Manipulating User-Controlled Variables Likelihood: High · Severity: Very High
  • CAPEC-180 Exploiting Incorrectly Configured Access Control Security Levels Likelihood: High · Severity: Medium

The attack patterns MITRE's CAPEC relates to this record's weaknesses, as MITRE names them. Credits.

History

  1. The NVD deferred its analysis

Changes since this site first stored the record. The same for every record: /api/public/changes.

References

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