AutoNAT v2 can initiate unwanted connections to non-public addresses

Summary

An untrusted AutoNAT v2 client can submit a loopback or private IP address and make the server attempt a connection from its own network position. Peer-ID validation prevents the target from completing the dial-back as the requester, but it does not prevent the initial connection attempt.

Expected behavior

The server should reject explicit non-public IP targets with E_DIAL_REFUSED, as recommended by the current AutoNAT v2 specification.

Actual behavior

After applying the required cross-IP data cost, the server passes the selected Multiaddr directly to DialOpts without checking its address class. The existing dial_back_to_non_libp2p test already exercises /ip6/::1/tcp/1000 and observes the connection failure.

Relevant log output

Add this test to `protocols/autonat/tests/autonatv2.rs`:


#[tokio::test]
async fn private_loopback_request_causes_outbound_dial() {
    let (mut server, mut client) = bootstrap().await;
    let addr: Multiaddr = "/ip6/::1/tcp/1000".parse().unwrap();

    client.behaviour_mut().autonat.on_swarm_event(
        FromSwarm::NewExternalAddrCandidate(NewExternalAddrCandidate { addr: &addr }),
    );

    tokio::select! {
        _ = server.wait(|event| match event {
            SwarmEvent::Dialing { connection_id, .. } => Some(connection_id),
            _ => None,
        }) => {}
        _ = client.loop_on_next() => unreachable!(),
    }
}


Run:


cargo test -p libp2p-autonat --test autonatv2 private_loopback_request_causes_outbound_dial -- --exact


The test passes because submitting the loopback candidate causes a new outbound dial.

Possible Solution

Reject explicit loopback, private, link-local, unspecified, and multicast IP addresses before emitting ToSwarm::Dial. DNS addresses need separate handling because their resolved IP is not available while parsing the request.

Version

Confirmed with libp2p-autonat 0.16.0 at commit 850319b. Earlier versions were not assessed.

Would you like to work on fixing this bug?

Maybe

Start in protocols/autonat/tests/autonatv2.rs and run the named private_loopback_request_causes_outbound_dial test to reproduce the unwanted dial. Then trace AutoNAT v2 request handling through the point where the selected Multiaddr reaches DialOpts. Done means explicit non-public IP targets are refused with E_DIAL_REFUSED while DNS addresses retain their separate handling.