CVE-2026-128337
ExcelJS through 4.4.0 Prototype Pollution via x14 Conditional Formatting Rule ID
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.
Fixes and workarounds
No remediation reported in collected sources.
Missing remediation information is not evidence that no patch exists.
Affected products and versions
| Product | Versions | Authority |
|---|---|---|
| exceljs exceljs exceljs
npm |
≥ 0; ≤ 4.4.0 (affected, semver)
Other versions: unaffected |
CNA: VulnCheck |
Version ranges are source assertions. Check the vendor advisory and your installed configuration before determining exposure.
CVSS assessments
| Version / score | Vector | Authority |
|---|---|---|
| 4.0 / 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 | CNA: VulnCheck |
| 3.1 / 7.0 high |
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:H/A:L | CNA: VulnCheck |
| 4.0 / 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 | NVD: [email protected] |
| 3.1 / 7.0 high |
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:H/A:L | NVD: [email protected] |
Exploit evidence
No exploit evidence in collected sources. This is not proof of no exploitation.
References
- https://github.com/exceljs/exceljsCNA: VulnCheck · product
- https://github.com/mateocallec/exceljs-hardened/security/advisories/GHSA-86ch-m4gp-r5xfCNA: VulnCheck · third-party-advisory
- https://github.com/exceljs/exceljs/blob/v4.4.0/lib/xlsx/xform/sheet/worksheet-xform.js#L34-L42CNA: VulnCheck · technical-description
- https://github.com/exceljs/exceljs/blob/v4.4.0/lib/xlsx/xform/sheet/worksheet-xform.js#L55-L74CNA: VulnCheck · technical-description
- https://www.vulncheck.com/advisories/exceljs-through-4.4.0-prototype-pollution-via-x14-conditional-formatting-rule-idCNA: VulnCheck · third-party-advisory
- https://github.com/exceljs/exceljs[email protected]
- https://github.com/exceljs/exceljs/blob/v4.4.0/lib/xlsx/xform/sheet/worksheet-xform.js#L34-L42[email protected]
- https://github.com/exceljs/exceljs/blob/v4.4.0/lib/xlsx/xform/sheet/worksheet-xform.js#L55-L74[email protected]
- https://github.com/mateocallec/exceljs-hardened/security/advisories/GHSA-86ch-m4gp-r5xf[email protected]
- https://www.vulncheck.com/advisories/exceljs-through-4.4.0-prototype-pollution-via-x14-conditional-formatting-rule-id[email protected]
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