CVE-2026-78207

exceljs through 4.4.0 Prototype Pollution via deepMerge Reached From Note Serialization

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.

We have discovered 385 live websites that are affected by CVE-2026-78207.

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Affected Software

Product  ExcelJS
Category Font Scripts
Vulnerable Domains385 live websites (100% of ExcelJS install base)
Vulnerable Versions
  • from 0 through 4.4
Vulnerable Versions Count9 versions ( 100% of all versions)



Details

  • Published - Aug 24, 2026
  • Updated - Aug 24, 2026

Credits

  • Matéo Florian Callec (finder)

Website Distribution by Country

Number of websites using CVE-2026-78207
United States243 websites


China10 websites
France10 websites
Germany9 websites
GB9 websites
Turkey7 websites
Brazil7 websites
Russia6 websites
Australia6 websites
Spain6 websites

Website Distribution by TLD

Number of websites using CVE-2026-78207
.com194 websites
.org51 websites
.net14 websites
.co13 websites
.com.br7 websites
.com.au5 websites
.es5 websites
.ru4 websites
.it4 websites
.co.uk3 websites

Vulnerable Versions

Vulnerable versions are highlighted in red

Websites affected by CVE-2026-78207

Top websites that are affected by CVE-2026-78207. Please click on the "Contact us" link to get more information.
DomainCountryRankContacts
***.***.au Australia*,***
***.*****.co United States***,***
****.*********.gov United States***,***
*************.org United States***,***
*****.*********.net United States***,***
************.com United States***,***
******.************.de Germany*,***,***
***.********.gov United States*,***,***
**********.nl Netherlands*,***,***
***********.com United States*,***,***
See full domain list

FAQ

A total of 385 websites have been identified as vulnerable to CVE-2026-78207, based on global website indexing conducted by WebTechSurvey.
The ExcelJS is affected by the CVE-2026-78207 vulnerability.
ExcelJS versions up to and including 4.4 are vulnerable to CVE-2026-78207.