# CVE-2026-78207: exceljs prototype pollution in deepMerge helper

Source: https://junglewise.ai/threats/cve-2026-78207-exceljs-prototype-pollution-in-deepmerge-helper  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-08-24. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

CVE-2026-78207 · Severity: critical · CVSS 9.4 · Published 2026-08-24

exceljs is a popular Node.js library for reading and writing Excel files. A prototype pollution vulnerability in its deepMerge helper allows attackers to inject malicious code by crafting specially-formed cell note objects, potentially compromising all objects created within the application and leading to remote code execution or data manipulation.

## Quick answers

**Is CVE-2026-78207 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-78207 being exploited in the wild.

**How severe is CVE-2026-78207?** Critical severity, with a CVSS score of 9.4, published 24 August 2026.

**Which products and versions are affected?** exceljs exceljs through 4.4.0.

**Is there a fix for CVE-2026-78207?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-78207 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-78207?** Check whether anything you run is named in the advisory. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

exceljs through version 4.4.0 contains a prototype pollution vulnerability in the deepMerge helper function, which fails to sanitize __proto__, constructor, or prototype keys when merging note objects. An attacker can supply parsed JSON with a malicious __proto__ property assigned to cell notes, which modifies Object.prototype and affects all plain objects subsequently created in the process. The vulnerability is reachable via any code that parses untrusted Excel files or JSON input and applies merging operations on note fields. This can lead to arbitrary property injection in the prototype chain, enabling remote code execution or application-level denial of service depending on how the affected properties are used downstream.

## Affected products

- exceljs exceljs through 4.4.0

## Timeline

- 2026-08-24: disclosed

## References

- [https://www.npmjs.com/package/exceljs](https://www.npmjs.com/package/exceljs)
- [https://github.com/exceljs/exceljs](https://github.com/exceljs/exceljs)