CVE-2026-124279: Streaming request body limit enforced only after an 8 MB read in elixir-grpc grpc_server

Summary

Allocation of Resources Without Limits or Throttling vulnerability in elixir-grpc grpcserver allows unauthenticated attackers to exhaust server memory via client-streaming or bidirectional-streaming requests.

The streaming read clause of GRPC.Server.Adapters.Cowboy.Handler.info/3 (grpcserver/lib/grpc/server/adapters/cowboy/handler.ex) asks Cowboy for up to 8,000,000 bytes per read and compares the running total with maxbodysize only after the read returns. The gRPC length prefix of a message is not checked before the message is buffered. Each stream can therefore buffer up to one full read beyond the configured limit before it is refused, and many concurrent streams exhaust node memory. Releases before 1.0.6 apply no limit to streaming requests at all.

This issue affects grpcserver: from 1.0.0 before 1.1.0.

Details

1. Large reads

asyncreadbody/2 asks Cowboy for up to 8,000,000 bytes per read on the streaming path, whatever maxbodysize is set to.

2. The limit is checked after the read

trackbodybytes/2 adds each returned chunk to a running total and refuses the stream with RESOURCEEXHAUSTED only once the total exceeds maxbodysize. By then the whole chunk is in memory.

3. No check on the length prefix

The 5-byte gRPC prefix can declare up to 4 GiB. The server does not compare it with any limit before buffering, so a prefix that declares an oversized message does not stop the read.

4. Result

Each client-streaming or bidirectional-streaming call can make the server hold several megabytes more than the configured limit, and the attacker can open many such calls at once. Before 1.0.6 the streaming path had no limit, and GRPC.Server.Adapters.Cowboy.readingstream/1 buffered the whole body.

Impact

An unauthenticated client that opens many streaming calls can make the server buffer more request data than its configured limit allows and crash the node, taking down every service on it.

Configurations

The server must expose at least one client-streaming or bidirectional-streaming RPC method.