Answer in brief
CVE-2026-128337 records a High severity (CVSS 8.3) vulnerability in ExcelJS through 4.4.0 Prototype Pollution via x14 Conditional Formatting Rule ID. The current sources do not mark it as known exploited. The current feed maps exceljs/exceljs (generic). Check affected ranges and fixed versions before updating.
Analysis pending evidence review
HOL Guard separates source facts from reviewed analysis. See the methodology.
CVSS is 8.3. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.
Analysis status
Analysis pending evidence review
Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.
The current feed maps exceljs/exceljs (generic). Check affected ranges and fixed versions before updating.
| Package | Affected range | Fixed version |
|---|---|---|
| exceljs/exceljsgeneric | >=0 <=4.4.0 | Not reported |
Published upstream
Nov 13, 2026
Evidence: source:cvelist:source_dates:source-dates:recordSource modified
Nov 13, 2026
Evidence: source:cvelist:source_dates:source-dates:recordFirst seen by HOL
Nov 13, 2026
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.
Quoted source text, attributed separately from HOL analysis.