{"affected":[{"defaultStatus":"unaffected","ecosystem":"npm","packageName":"exceljs","product":"exceljs","source":"CNA: VulnCheck","vendor":"exceljs","versions":[{"lessThanOrEqual":"4.4.0","status":"affected","version":"0","versionType":"semver"}]}],"cveId":"CVE-2026-78207","cwes":["CWE-1321"],"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.","epss":{"date":"2026-11-08","percentile":0.49116,"score":0.00603},"exploits":[],"kev":null,"metrics":[{"score":9.3,"severity":"critical","source":"CNA: VulnCheck","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","version":"4.0"},{"score":9.4,"severity":"critical","source":"CNA: VulnCheck","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:L","version":"3.1"}],"modifiedAt":1794146943861,"publishedAt":1787531447125,"references":[{"source":"CNA: VulnCheck","tags":["product"],"url":"https://github.com/exceljs/exceljs"},{"source":"CNA: VulnCheck","tags":["third-party-advisory","vendor-advisory"],"url":"https://github.com/mateocallec/exceljs-hardened/security/advisories/GHSA-qwr4-7h29-chpf"},{"source":"CNA: VulnCheck","tags":["technical-description"],"url":"https://github.com/exceljs/exceljs/blob/v4.4.0/lib/utils/under-dash.js#L155-L181"},{"source":"CNA: VulnCheck","tags":["third-party-advisory"],"url":"https://www.vulncheck.com/advisories/exceljs-through-prototype-pollution-via-deepmerge-reached-from-note-serialization"}],"remediation":[],"revision":"9ae75769d70f84387ef1321596cf4571cb71af26df479ef0f99d856ef2329011","sources":[{"modifiedAt":1794146943861,"name":"CNA: VulnCheck","retrievedAt":1794151376428,"url":"https://www.cve.org/CVERecord?id=CVE-2026-78207"},{"modifiedAt":null,"name":"FIRST EPSS","retrievedAt":1794151399554,"url":"https://api.first.org/data/v1/epss?cve=CVE-2026-78207"}],"ssvc":{"assessedAt":1787593528866,"automatable":"yes","exploitation":"none","source":"ADP: CISA-ADP","technicalImpact":"total"},"status":"published","title":"exceljs through 4.4.0 Prototype Pollution via deepMerge Reached From Note Serialization","updatedAt":1794151399554}