Vulnerability record
CVE-2026-128337
The short answer
CVE-2026-128337 affects exceljs exceljs and is rated high (CVSS 8.3). It is not in CISA's KEV catalog; EPSS puts the chance of exploitation in the next 30 days at 0.45%.
What it is
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.
Weakness: CWE-1321.
Severity
8.3 high
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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
Exploited (CISA KEV)
Not listed
CISA lists confirmed exploitation only
EPSS, next 30 days
0.45%
Higher than 39% of scored CVEs
How it can be exploited
- Attack vector
- Network
- Attack complexity
- High
- Attack requirements
- Present
- Privileges required
- None
- User interaction
- None
- Confidentiality impact
- Low
- Integrity impact
- High
- Availability impact
- Low
Affected products
exceljs. NVD's record lists the exact affected versions.
References
- github.com/exceljs/exceljs
- github.com/exceljs/exceljs/blob/v4.4.0/lib/xlsx/xform/sheet/worksheet-xform.js#L34-L42
- github.com/exceljs/exceljs/blob/v4.4.0/lib/xlsx/xform/sheet/worksheet-xform.js#L55-L74
- github.com/mateocallec/exceljs-hardened/security/advisories/GHSA-86ch-m4gp-r5xf
- www.vulncheck.com/advisories/exceljs-through-4.4.0-prototype-pollution-via-x14-conditional-formatting-rule-id
Questions
Is CVE-2026-128337 being exploited?
It is not in CISA's Known Exploited Vulnerabilities catalog as of 2026-11-13. FIRST's EPSS estimates a 0.45% chance of exploitation in the next 30 days.
How do I fix CVE-2026-128337?
Follow the vendor advisory in the references below. NVD lists the affected versions under its configurations.
From NVD, CISA KEV, FIRST EPSS and OSV.dev, checked . Status on NVD: Awaiting Analysis. The description is NVD's text, quoted as published.
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