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Per draft-ietf-webtrans-http3 section 4.3, a WEBTRANSPORT_STREAM frame is only allowed as the very first frame of a request stream. Receiving it on a stream which is already in use - such as a CONNECT stream - or after another frame must be treated as a connection error of type H3_FRAME_ERROR.
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A
WEBTRANSPORT_STREAMframe (0x41) is accepted on a CONNECT stream without validation, so sevenbytes on that stream turn the control channel into a WebTransport data stream.
h3/connection.pythen fires
WebTransportStreamDataReceivedwithstream_id=0, which is a stream the applicationnever agreed to treat as data. It was found by fuzzing, and the sequence is trivial to send.
The frame is only legal at the start of a stream, where it declares what the stream is. Arriving
after the stream has already carried a frame, it is reinterpreting a stream mid-flight, which the
spec does not allow.
Streams now track
received_frame, and aWEBTRANSPORT_STREAMon a stream that has already receivedone raises
FrameError, a newProtocolErrorcarryingH3_FRAME_ERROR. That is the code the specassigns to a frame that is invalid in its position, so the peer is told what was wrong rather than
seeing a generic failure, and the connection closes instead of the application receiving data on its
control channel.
Tests cover the reported byte sequence and a legitimate WebTransport stream, so the guard cannot pass
by refusing both.
Fixes #638