Is CVE-2026-59876 being exploited?

No public exploit is currently tracked.

How severe is CVE-2026-59876?

CVE-2026-59876 has a CVSS v3 severity of MEDIUM (4.8).

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CVE-2026-59876

Description

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.