mint: Unbounded streams map growth via PUSH_PROMISE without follow-up HEADERS

Summary

Mint's HTTP/2 client accepts PUSH_PROMISE frames from any server it connects to and inserts every promised stream into a per-connection map without consulting max_concurrent_streams. A malicious or compromised HTTP/2 server can flood the client with PUSH_PROMISE frames and withhold the matching response HEADERS, pinning one map entry per frame indefinitely until the client process runs out of memory.

Details

'Elixir.Mint.HTTP2':handle_push_promise/3 in lib/mint/http2.ex dispatches every inbound PUSH_PROMISE frame to 'Elixir.Mint.HTTP2':decode_push_promise_headers_and_add_response/5, which inserts a :reserved_remote entry into conn.streams for the promised ID. The only validation applied is that the promised ID is even and not already present; client_settings.max_concurrent_streams is not consulted at promise time.

The concurrency cap is only checked when the response HEADERS for the promised stream arrive. A server that emits PUSH_PROMISE frames and never sends the matching HEADERS never trips that check, and the existing tally counts only streams in open states, not :reserved_remote entries.

HTTP/2 server push is accepted by default (client_settings.enable_push defaults to true), so no application opt-in is required. A single long-lived HTTP/2 connection to a hostile server lets it pin one conn.streams entry per PUSH_PROMISE frame, with no upper bound.

PoC

1. Stand up a raw TCP HTTP/2 server that completes the handshake and ACKs the client's SETTINGS.
2. Wait for the client's request HEADERS and capture its odd stream ID.
3. Send a flood of PUSH_PROMISE frames (flags = END_HEADERS) associated with the captured stream, each promising a fresh even stream ID and carrying a minimal HPACK-encoded header block.
4. Never send the matching response HEADERS for any of the promised IDs.