CVE-2026-78207

CriticalCVSS 9.4/10Last modified Oct 14, 2026

Description

exceljs through 4.4.0 contains a prototype pollution vulnerability in the deepMerge helper that fails to reject __proto__, constructor, or prototype keys when merging note objects. Attackers can assign parsed JSON with a malicious __proto__ property to cell notes, modifying Object.prototype and affecting all plain objects created in the process.

Metrics

CVSS 3.1 9.4/10CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:L

Weakness enumeration

CWE-1321

Affected software

Vendor	Product	Versions
exceljs	exceljs	<= 4.4.0

References

Timeline

Published: Aug 24, 2026
Last modified: Oct 14, 2026
NVD status: Deferred

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