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No exploitation reportedAwaiting NVD analysis

CVE-2026-128337

ExcelJS through 4.4.0 Prototype Pollution via x14 Conditional Formatting Rule ID

No exploitation reported by the sources we read.

  • Published2026-11-13
  • WeaknessCWE-1321

Am I affected?

With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.

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.

Timeline

dates in UTC
  1. 2026-11-1309:33CVE publishedCNA

Versions per branch

Products declared by the CNA

As VulnCheck, the CNA that assigned this CVE, declared them in the CVE record; versions as written. NVD lists no product for this CVE yet: this is the only product data so far.

  • exceljsother versions: not affected
    Affected
    • ≤ 4.4.0no fix statedsemver

Source: the CNA record (CVE Program), © The MITRE Corporation,CVE Terms of UseCVE recordupdated 2026-11-13

Neither OSV.dev nor the vendor advisories we read list affected versions for CVE-2026-128337 yet.

Full record on NVD

Fix

No fix published by the sources we read yet. NVD record

Detection

Nuclei template
none so far
ProjectDiscovery
Exploit-DB
no entry
Exploit-DB
Metasploit modules
none listed
Metasploit Framework

Sources

  • CNACVSS 4.08.32026-11-13
  • CNACVSS 3.172026-11-13
  • NVDCVSS 3.17read 2026-11-13
  • CNAweakness (CWE)CWE-13212026-11-13
  • NVDweakness (CWE)CWE-1321read 2026-11-13
  • GitHub Advisoriesadvisory—

Identifiers

Technical details

CVSS v3 Vector
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:H/A:L
CVSS 4.0 vector
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
Published
2026-11-13
Last Modified
2026-11-13
NVD status
Awaiting Analysis
GitHub Advisory
GHSA-2vgm-9qpx-xwqx

Frequently asked questions

What is CVE-2026-128337?

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.

Is CVE-2026-128337 actively exploited?

Active exploitation of CVE-2026-128337 has not been confirmed.

What is the CVSS score for CVE-2026-128337?

CVE-2026-128337 has a CVSS v3 base score of 7 (HIGH severity), with vector string CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:H/A:L.

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