CVE-2026-59876: protobufjs: Text Format string map parsing can mutate returned map object prototype

The protobuf.js text format extension parsed string-keyed map entries using ordinary property assignment. A text-format map entry with key __proto__ could therefore change the prototype of the returned map object instead of creating an own map entry.

This affected the optional Text Format extension. Other protobufjs input paths, including binary decode, fromObject, and ProtoJSON conversion, are not affected.