CVE-2026-48853: Unauthenticated Code Execution via Erlang Term Deserialization in Elixir gRPC

### 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