Denial of service in quinn-proto when using `Endpoint::retry()`

Summary

As of quinn-proto 0.11, it is possible for a server to accept(), retry(), refuse(), or ignore() an Incoming connection. However, calling retry() on an unvalidated connection exposes the server to a likely panic in the following situations:

• Calling refuse or ignore on the resulting validated connection, if a duplicate initial packet is received
  • This issue can go undetected until a server's refuse()/ignore() code path is exercised, such as to stop a denial of service attack.
• Accepting when the initial packet for the resulting validated connection fails to decrypt or exhausts connection IDs, if a similar initial packet that successfully decrypts and doesn't exhaust connection IDs is received.
  • This issue can go undetected if clients are well-behaved.

The former situation was observed in a real application, while the latter is only theoretical.

Details

Location of panic: https://github.com/quinn-rs/quinn/blob/bb02a12a8435a7732a1d762783eeacbb7e50418e/quinn-proto/src/endpoint.rs#L213

Impact

Denial of service for internet-facing server