Prototype Pollution in Exceljs
CVE-2026-78207
exceljs-hardened before 5.0.0 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.
Vulnerability class: Prototype Pollution
CVSS v3 metric
CVSS v3 base score 9.4 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:L.
Affected products
Weakness classification (CWE)
References
- disclosure@vulncheck.com (product)
- disclosure@vulncheck.com (third-party-advisory)
- disclosure@vulncheck.com (technical-description)
- disclosure@vulncheck.com (third-party-advisory)
Frequently asked questions
- What is CVE-2026-78207?
- CVE-2026-78207 is a critical-severity vulnerability in Exceljs, classified under Improperly Controlled Modification of Object Prototype Attributes (Prototype Pollution). CVSS score: 9.4/10. Published 2026-08-24.
- How severe is CVE-2026-78207?
- Critical severity. CVSS v3 base score is 9.4 out of 10.