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.
Scoring
- Severity
- CRITICAL
- CVSS base score
- 9.4
- CVSS vector
- CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:L/SC:N/SI:N/SA:N
- EPSS probability
- 0.44%
- CWE
- CWE-1321
- Published
- 2026-08-24
- Last modified
- 2026-08-29
Affected products
- exceljs exceljs
Weakness type
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