CVE-2026-78207: Exceljs

Critical severity, CVSS 9.4. EPSS: 0.6% chance of exploitation in the next 30 days.

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.

Affected products

• Exceljs Exceljs: up to and including 4.4.0

Published 2026-08-24. Last modified 2026-08-31.