{"id":"CVE-2026-78207","title":"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","summary":"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 __prot…","severity":"critical","cvss":9.4,"cvssVector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:L","cwe":["CWE-1321"],"published":"2026-08-24","updated":"2026-08-29","source":"NVD","sourceUrl":"https://nvd.nist.gov/vuln/detail/CVE-2026-78207","references":[{"url":"https://github.com/exceljs/exceljs","label":"disclosure@vulncheck.com"},{"url":"https://github.com/exceljs/exceljs/blob/v4.4.0/lib/utils/under-dash.js#L155-L181","label":"disclosure@vulncheck.com"},{"url":"https://github.com/mateocallec/exceljs-hardened/security/advisories/GHSA-qwr4-7h29-chpf","label":"disclosure@vulncheck.com"},{"url":"https://www.vulncheck.com/advisories/exceljs-through-prototype-pollution-via-deepmerge-reached-from-note-serialization","label":"disclosure@vulncheck.com"}],"tags":["nvd"],"epss":0.00439,"epssPercentile":0.37494,"ingestedAt":"2026-08-29T21:42:33.198Z","slug":"CVE-2026-78207","body":"## Overview\n\nexceljs 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.\n\n## Remediation\n\nRefer to the linked advisories for vendor-supplied fixes and affected version ranges.","depth":"midnight","depthScore":52,"depthScoreParts":{"impact":51.7,"likelihood":0.1,"exploitation":0,"ransomware":0},"changes":[]}