# CVE-2026-78207: exceljs through 4.4.0 Prototype Pollution via deepMerge Reached From Note Serialization

Status: published. Published: 2026-08-24. Updated: 2026-11-08.

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.

## Remediation

No remediation reported in collected sources

## Affected products and versions

- exceljs / exceljs (exceljs): ≥ 0; ≤ 4.4.0 (affected, semver). Source: CNA: VulnCheck

## Severity and prioritization

- CVSS 4.0: 9.3 (critical); 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; source: CNA: VulnCheck

- CVSS 3.1: 9.4 (critical); CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:L; source: CNA: VulnCheck

- CISA KEV: No listing in collected sources

- EPSS: 0.60% as of 2026-11-08; percentile 49.12%

- SSVC: exploitation none; automatable yes; technical impact total; source ADP: CISA-ADP

- Weaknesses: CWE-1321

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://github.com/exceljs/exceljs (CNA: VulnCheck; product)

- https://github.com/mateocallec/exceljs-hardened/security/advisories/GHSA-qwr4-7h29-chpf (CNA: VulnCheck; third-party-advisory, vendor-advisory)

- https://github.com/exceljs/exceljs/blob/v4.4.0/lib/utils/under-dash.js#L155-L181 (CNA: VulnCheck; technical-description)

- https://www.vulncheck.com/advisories/exceljs-through-prototype-pollution-via-deepmerge-reached-from-note-serialization (CNA: VulnCheck; third-party-advisory)

## Provenance

- CNA: VulnCheck: https://www.cve.org/CVERecord?id=CVE-2026-78207; retrieved 2026-11-08; source updated 2026-11-08

- FIRST EPSS: https://api.first.org/data/v1/epss?cve=CVE-2026-78207; retrieved 2026-11-08; source updated Not reported

Revision: 9ae75769d70f84387ef1321596cf4571cb71af26df479ef0f99d856ef2329011

Canonical: https://y2.dev/cve/CVE-2026-78207

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