See the user report at: https://archives.passt.top/passt-user/ZrxRFvQvzsqHdmfn@zatzit.fritz.box/

Long story short: if a process manages to send a packet to TCP port 0, which shouldn't be allowed (but Linux doesn't seem to drop), we crash.

I don't think it's necessarily security-relevant because the guest/container can anyway kill its own connectivity, and there would be nothing else gained from doing this, but this looks pretty annoying and I would rather fix it anyway.