CVE-2026-78207
Critical
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.
SeverityCritical
CVSS9.4
Exploit probability—
ProductExceljs
PublishedAug 24, 2026
First seenAug 24, 2026
Weakness typeCWE-1321Prototype pollution
How to fix
Sources list no fixed version or patch link for this vulnerability.
Affected products
| Product | Affected versions | Fixed in |
|---|---|---|
| exceljs exceljs | v ≤ 4.4.0 | — |
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