# CVE-2026-78207

## Summary

- **CVE ID:** CVE-2026-78207
- **Severity:** CRITICAL
- **CVSS Score:** 9.4 (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)
- **CWE:** CWE-1321
- **Published:** Aug 24, 2026
- **Last Modified:** Aug 29, 2026

## Description

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.

## Affected Products

- exceljs — exceljs (0)

## References

- [CNA](https://github.com/exceljs/exceljs)
- [CNA](https://github.com/mateocallec/exceljs-hardened/security/advisories/GHSA-qwr4-7h29-chpf)
- [CNA](https://github.com/exceljs/exceljs/blob/v4.4.0/lib/utils/under-dash.js#L155-L181)
- [CNA](https://www.vulncheck.com/advisories/exceljs-through-prototype-pollution-via-deepmerge-reached-from-note-serialization)

## Exploitation Prediction (EPSS)

- **EPSS Score:** 0.44%
- **EPSS Percentile:** 37.0

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_Exported from OnDuty AI Vulnerability Intelligence on 2026-09-11._