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CVE-2026-128337 ExcelJS through 4.4.0 Prototype Pollution via x14 Conditional Formatting Rule ID

8.3 highEPSS 0.4%

High (8.3) in exceljs exceljs. EPSS 0.4% estimated probability of exploitation activity in the next 30 days.

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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

CVSS 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

CVSS 3.1

7.0 high
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:H/A:L
EPSS
0.4% estimated probability of exploitation activity in the next 30 days; 39th percentile. Scored 2026-11-13. A forecast for this CVE across the internet, not for your own systems.
CWE
CWE-1321 Improperly Controlled Modification of Object Prototype Attributes (Prototype Pollution)
Published
Modified
Assigned by
VulnCheck (CNA)
NVD analysis
Awaiting analysis
Record from
the NVD, synced

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Affected

Versions

From the CNA's record in the CVE List.

ProductAffected
exceljsthrough 4.4.0

Attack patterns (CAPEC)

  • CAPEC-1 Accessing Functionality Not Properly Constrained by ACLs Likelihood: High · Severity: High
  • CAPEC-77 Manipulating User-Controlled Variables Likelihood: High · Severity: Very High
  • CAPEC-180 Exploiting Incorrectly Configured Access Control Security Levels Likelihood: High · Severity: Medium

The attack patterns MITRE's CAPEC relates to this record's weaknesses, as MITRE names them. Credits.

References

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