CVE-2026-78207 [CRITICAL] – exceljs through 4.4.0 contains a prototype pollution vulnerability in the deepMerge helper

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.