exceljs through 4.4.0 Prototype Pollution via deepMerge Reached From Note Serialization (CVE-2026-78207) | HOL Guard CVE

Unknown severity · Source severity not reportedCVE-2026-78207

exceljs through 4.4.0 Prototype Pollution via deepMerge Reached From Note Serialization

Answer in brief

CVE-2026-78207 records a Unknown severity vulnerability in exceljs through 4.4.0 Prototype Pollution via deepMerge Reached From Note Serialization. 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.

Published Aug 24, 2026Updated Aug 24, 2026Source checked Aug 24, 2026First seen by HOL Aug 24, 2026Material review Aug 24, 2026

Upstream Advisory

Key facts

Risk: Unknown severitySource severity not reported
Exploitation: Not marked as known exploited
Affected software: 1 mapped package or product
Fix availability: Not reported

Why this deserves its current priority

A CVSS score is not reported in the current record. 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.

Affected scope and exposure questions

The current feed maps exceljs/exceljs (generic). Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
Package	Affected range	Fixed version
exceljs/exceljsgeneric	0	Not reported

Recommended response

Check inventory. Check lockfiles and deployed manifests for exceljs/exceljs.
Review the reported fix. Monitor this advisory for an available fix and review any installs of the affected package.

Evidence timeline and material changes

Published upstream
Aug 24, 2026
Evidence: source:cvelist:source_dates:source-dates:record
Source modified
Aug 24, 2026
Evidence: source:cvelist:source_dates:source-dates:record
First seen by HOL
Aug 24, 2026

Sources and claim methodology

Upstream source description

exceljs-hardened before 5.0.0 contains a prototype pollution vulnerability in the deepMerge helper that fails to reject __proto__, constructor, or prototype keys when merging note objects. Attackers can assign parsed JSON with a malicious __proto__ property to cell notes, modifying Object.prototype and affecting all plain objects created in the process.

Quoted source text, attributed separately from HOL analysis.