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Back to active CVEs
Critical · CVSS 9.4CVE-2026-78207

exceljs through 4.4.0 Prototype Pollution via deepMerge Reached From Note SerializationCVE-2026-78207

Answer in brief

CVE-2026-78207 records a Critical severity (CVSS 9.4) 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), 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 31, 2026Source checked Oct 8, 2026First seen by HOL Aug 24, 2026Material review Aug 31, 2026
Upstream Advisory

Key facts

Risk
Critical · CVSS 9.4
Exploitation
Not marked as known exploited
Affected software
2 mapped packages or products
Fix availability
Not reported

Why this deserves its current priority

CVSS is 9.4. 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), exceljs/exceljs (generic). Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
PackageAffected rangeFixed version
exceljs/exceljsgeneric0Not reported
exceljs/exceljsgeneric>=0 <=4.4.0Not reported

Recommended response

  1. 1Check inventory. Check lockfiles and deployed manifests for exceljs/exceljs, exceljs/exceljs.
  2. 2Review the reported fix. Monitor this advisory for an available fix and review any installs of the affected package.

Evidence timeline and material changes

  1. Published upstream

    Aug 24, 2026

    Evidence: source:cvelist:source_dates:source-dates:record

  2. Source modified

    Aug 31, 2026

    Evidence: source:cvelist:source_dates:source-dates:record

  3. First seen by HOL

    Aug 24, 2026

Sources and claim methodology

  • GitHub security advisorygithub.com
  • GitHub security advisorygithub.com
  • GitHub security advisorygithub.com
  • Source referencevulncheck.com
Upstream source description

exceljs through 4.4.0 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.

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Record context

Vulnerability class
Vulnerability
EPSS
Not reported
CWE IDs
CWE-1321
Source
CVE List V5
Source checked
Oct 8, 2026
References
4 linked sources
Open source record