gRPC Erlang package vulnerable to Remote Code Execution with attacker-controlled gRPC payloads

Summary

GRPC.Codec.Erlpack.decode/2 calls :erlang.binary_to_term/1 directly on the raw gRPC message body without the :safe option. Any unauthenticated peer that can reach a gRPC endpoint with Content-Type: application/grpc+erlpack can crash the entire BEAM node via atom table exhaustion or, if a decoded fun term flows into a call site that invokes it, achieve remote code execution inside the server process.

Details

Root cause — lib/grpc/codec/erlpack.ex implements decode/2 as a bare :erlang.binary_to_term(binary) call with no :safe flag, no size limit, and no type validation. This has two independent exploitation paths:

1. DoS via atom exhaustion — BEAM atoms are never garbage-collected and the global atom table is bounded (~1,048,576 entries). A crafted payload encoding large numbers of fresh atoms saturates the table and crashes the entire VM, taking down all applications on the node.

2. RCE via fun materialization — Without :safe, binary_to_term/1 reconstructs fun and external-fun terms from wire data. If the decoded value reaches any call site that applies it (e.g. Enum.map, Task.async, direct invocation), attacker-controlled code executes inside the server process.

Configuration requirement: GRPC.Codec.Erlpack is not registered by default and must be explicitly added to the server's codecs option.

PoC

1. Start a gRPC server with codecs: [GRPC.Codec.Erlpack].
2. Open an HTTP/2 connection to the server.
3. Send a gRPC-framed POST to any RPC path with Content-Type: application/grpc+erlpack and a body of :erlang.term_to_binary(fn -> <malicious_code> end).
4. The server's decode/2 materializes the fun; any downstream call site that invokes the decoded value executes the attacker's code.
5. For DoS only: send payloads encoding fresh atoms in a loop until the atom table is exhausted and the VM crashes.

Impact

Affects grpc ≥ 0.4.0. Any server that explicitly registers GRPC.Codec.Erlpack is vulnerable to unauthenticated node-level DoS and potentially RCE.

References

• Introduction commit: https://github.com/elixir-grpc/grpc/commit/25bcc569fe2cc4478531a6c546c923205fc751c9
• Patch commit: https://github.com/elixir-grpc/grpc/commit/272a97a5ea1b46af1819f14a831fcf35fc91f992