CVE-2026-128337

Description

ExcelJS through 4.4.0 contains a prototype pollution vulnerability in mergeConditionalFormattings that allows attackers to modify Object.prototype by supplying an x14:cfRule id of __proto__. Attackers can craft an xlsx file that, when loaded with xlsx.load(), sets properties such as type, direction, border and cfvo on every object in the process until it restarts.

Severity scores

Each score is shown as its source gave it. They are not averaged or merged.

• 8.3CVSS 4.0, Highfrom the CNA (VulnCheck)CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:L/VI:H/VA:L/SC:N/SI:N/SA:N
• 7.0CVSS 3.1, Highfrom the CNA (VulnCheck)CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:H/A:L

Historical context and remediation

This reference retains the original CVE publication date. A recent record update does not mean a new attack occurred.

No separately sourced impact summary is available on this page yet. Consult the description, version data and dated exploitation evidence.

No explicit remediation statement is available on this page yet. Consult the original record and vendor references before choosing a fix.

Version statuses below come from the CNA. An “unaffected” version is not necessarily a currently supported release or proof of a patch date. EPSS is a dated risk estimate, not a count of historical attacks.

Read the original CVE record.

Affected products

Timeline

1. 13 Nov 2026 Published

Weakness

• CWE-1321

References

• github.comproduct
• GitHub Security Advisory (GHSA-86ch-m4gp-r5xf)third-party-advisory
• github.comtechnical-description
• github.comtechnical-description
• VulnCheck Advisory: ExcelJS through 4.4.0 Prototype Pollution via x14 Conditional Formatting Rule IDthird-party-advisory