Vulnerability · Published Aug 24, 2026 · Assigned by VulnCheck
CVE-2026-78207exceljs through 4.4.0 Prototype Pollution via deepMerge Reached From Note Serialization
Critical 9.4
In plain English
A critical-severity prototype pollution in exceljs from exceljs. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.6% (higher than 49% of all known vulnerabilities).
Severity (CVSS 3.1)9.4Critical
Exploit odds (30 days)0.6%top 51.0% of all CVEs
Exploited in the wildNo reportsCISA SSVC: none
Attack toolingNoneno public tooling known
Official 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.
Affected products (as reported)
Weakness: CWE-1321 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:L
References
- https://github.com/exceljs/exceljs product
- GitHub Security Advisory (GHSA-qwr4-7h29-chpf) third-party-advisory
- https://github.com/exceljs/exceljs/blob/v4.4.0/lib/utils/under-dash.js#L155-L181 technical-description
- VulnCheck Advisory: exceljs through 4.4.0 Prototype Pollution via deepMerge Reached From Note Serialization third-party-advisory