WebSocket 1-byte continuation frames bypass `max_fragmented_message_size` in Bandit

Allocation of Resources Without Limits or Throttling vulnerability in the WebSocket fragment reassembly of mtrudel bandit allows an unauthenticated remote attacker to exhaust server memory via a fragmented message made of many 1-byte continuation frames.

Bandit.WebSocket.Connection checks max_fragmented_message_size against the payload bytes received, but keeps every non-final continuation frame as a new list cell around its own binary, and nothing limits the number of fragments. A 1-byte fragment costs 7 bytes on the wire but about 150 to 170 bytes of server memory, so one connection can hold about 150 times the configured limit, roughly 1.2 to 1.4 GB at the default 8 MB. Ping frames between fragments keep the connection alive past the idle timeout.

This issue affects bandit: from 1.11.0 before 1.14.0.

1. Byte-only limit. In Bandit.WebSocket.Connection (lib/bandit/websocket/connection.ex), the fragment limit is checked against fragment_size, which only adds up payload bytes (IO.iodata_length(frame.data)).

2. Per-fragment accumulation. Each non-final continuation frame is stored as a new cons cell around its own binary, [connection.fragment_frame.data | frame.data]. Only zero-length fragments are kept out, so 1-byte fragments are accepted. Each costs 7 bytes on the wire but keeps about 150 to 170 bytes in memory. Against 1.12.5, 1M fragments kept about 169 MB and 3M kept about 450 MB, growing linearly, and about 56 MB of traffic reaches the 8 MB payload cap.

3. Kept alive. Ping frames sent between fragments are handled by handle_control_frame without touching the fragment state, so they keep the connection alive past the idle timeout.

1. Run Bandit with default settings and a WebSock that accepts every connection.
2. Complete the WebSocket upgrade and send a non-final text frame with a 1-byte payload.
3. Send 500,000 masked non-final continuation frames with 1-byte payloads, 3.5 MB in all.
4. Send a Ping. The server answers with a Pong, and BEAM memory has grown by about 82 MB while the message is still far below the 8 MB cap.

Lower websocket_options: [max_fragmented_message_size: ...] to the largest fragmented message the application needs, for example 65_536. Memory per connection stays at about 150 times this limit, so a lower limit lowers the ceiling.