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Vulnerability record · CVE-2026-78207 · published 24 August 2026

CVE-2026-78207: Prototype pollution vulnerability

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.

9.3 CVSS 4.0 Critical EPSS 0.60% · top 53.3% CWE-1321 · Prototype pollution Deferred
9.3CVSS 4.0 base score
0.60%EPSS exploitation probability, 30 days
NoNot in CISA KEV
0Affected product versions listed by NVD
4References
31 Aug 2026Last modified by NVD

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.

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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:

References

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

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Related vulnerabilities

Same products first, then exploited flaws of the same weakness class.

8.6CVE-2026-34621Adobe Acrobat Reader prototype pollution leads to code executionAdobe Acrobat and Reader (versions 24.001.30356, 26.001.21367 and earlier) are affected by prototype pollution (CWE-1321) that can result in arbitrar…KEVEPSS 2.2%analysed

Source: NIST National Vulnerability Database (record CVE-2026-78207), CISA KEV, FIRST EPSS (scores of 2026-09-27). This page is refreshed as NVD updates the record.