CVE-2026-59876: Protobufjs Project Protobufjs

Medium severity, CVSS 4.8. EPSS: 0.3% chance of exploitation in the next 30 days.

protobufjs compiles protobuf definitions into JavaScript (JS) functions. From 8.2.0 until 8.6.5, the protobufjs Text Format extension parsed string-keyed map entries using ordinary property assignment, allowing a map entry with key __proto__ to change the prototype of the returned map object instead of creating an own map entry in protobufjs/ext/textformat. This issue is fixed in version 8.6.5.

Affected products

• Protobufjs Project Protobufjs: from 8.2.0, before 8.6.5 (fixed in 8.6.5)

Published 2026-07-08. Last modified 2026-07-13.