Unbounded recursion depth in "Protobuf.Decoder" (Hex package "protobuf", versions ">= 0.8.0, < 0.16.1") lets an unauthenticated attacker crash any service that decodes untrusted protobuf messages whose schema contains a self-referential or cyclic message type. A small request body (a few KB to a few MB) that nests an embedded field hundreds of thousands to millions of levels deep forces the BEAM to recurse once per level, exhausting memory and pinning a scheduler. A handful of such requests can take the node offline (a request-amplification denial of service).