Prototype Pollution in Exceljs
CVE-2026-78207
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.
Vulnerability class: Prototype Pollution
Published · last modified .
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.
EPSS exploit prediction
EPSS: 0.006 (47.2th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 47th percentile — 47.2% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.004 · newest: 0.006 · change: +0.002
Affected products
Weakness classification (CWE)
References
- github.com/exceljs/exceljs (product)
- github.com/mateocallec/exceljs-hardened/security/advisories/GHSA-qwr4-7h29-chpf (third-party-advisory)
- github.com/exceljs/exceljs/blob/v4.4.0/lib/utils/under-dash.js (technical-description)
- www.vulncheck.com/advisories/exceljs-through-prototype-pollution-via-deepmerge-… (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.