CVE-2026-78207CWE-1321

CVE-2026-78207

Critical · published August 24, 2026

CVSS v3.1
9.4
EPSS
1%
Percentile
48.2
In the wild
Unconfirmed
What it is

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.

The record
Technical detail
CVSS v3.1
9.4 · CRITICAL
Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:L
CVSS v4.0
9.3 · 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
EPSS
0.00603 · 48.2th percentile
Weakness
CWE-1321 · Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')
Published
2026-08-24T05:16Z
References (4)
https://github.com/exceljs/exceljs · [email protected]https://github.com/exceljs/exceljs/blob/v4.4.0/lib/utils/under-dash.js#L155-L181 · [email protected]https://github.com/mateocallec/exceljs-hardened/security/advisories/GHSA-qwr4-7h29-chpf · [email protected]https://www.vulncheck.com/advisories/exceljs-through-prototype-pollution-via-deepmerge-reached-from-note-serialization · [email protected]
EPSS history
Timeline
  • 24 AUG 00:30Z
    exceljs through 4.4.0 Prototype Pollution via deepMerge Reached From Note Serialization
    cvelistv5