server: hold an unauthenticated connection to a small message

Until a peer authorizes it sends only a public key, a login request, a test delay
and a close reason, none of them large. Nothing said so: a frame header could
declare up to whatever the transport allowed, 1 GiB on TCP and WebRTC, and a
connection holds its place for up to LOGIN_GRACE before it has to authorize. With
MAX_UNAUTHORIZED_CONNS places to fill, that is 64 GiB of header-declared payload
one peer could make us hold - or, on WebSocket, 1 GiB bought outright with a few
hundred bytes of frame headers, because tungstenite reserves a frame's declared
payload as soon as it passes max_frame_size.

The cap goes on in create_tcp_connection, before the identity handshake, so that
read is bounded too, and comes off once the login is settled. It comes off before
connect_port_forward_if_needed rather than beside the rest of authorization: a
multiplexed tunnel narrows the same knob again for its own framing and has to have
the last word.

128 KiB is several times the largest login request anyone sends - a long hostname,
an os_login, an avatar URL, a file-transfer path - and is also the read buffer
tungstenite allocates per WebSocket connection whatever we do, so on that transport
the bound costs nothing beyond a floor already paid. Together with
MAX_UNAUTHORIZED_CONNS it holds every unauthorized connection to 8 MiB. Redis
answered this same shape in CVE-2021-32675 with 16 KiB, tighter because a
per-message bound is the only one it has; here the connection count is the other.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EZ49AbZJYfm8NTp5yDPMab

src/server.rs

@@ -221,6 +221,8 @@ pub async fn create_tcp_connection(
     let Some(unauthorized) = admit_unauthorized(id, addr.ip()) else {
         bail!("too many unauthenticated connections from {}", addr.ip());
     };
+    // Before the handshake, so its read is bounded too; lifted again at authorization.
+    stream.set_max_packet_length(MAX_UNAUTHORIZED_MESSAGE);
     tokio::select! {
         handshake = identity_handshake(&mut stream, secure) => handshake?,
         _ = unauthorized.evicted() => {

src/server/connection.rs

@@ -93,6 +93,13 @@ const MAX_UNAUTHORIZED_CONNS: usize = 64;
 /// of addresses passes it, and the bound above is what holds. Meaningful only while the
 /// address is the controller's own, which punch and relay messages carry today.
 const MAX_UNAUTHORIZED_CONNS_PER_ADDR: usize = 16;
+/// The largest message a connection may send before it authorizes. Until then a peer sends only
+/// a public key, a login request, a test delay and a close reason, none of which carries an
+/// unbounded field - the login request's avatar is a URL. Sized to the read buffer tungstenite
+/// allocates per WebSocket connection regardless, so there the cap costs nothing beyond a floor
+/// already paid; with MAX_UNAUTHORIZED_CONNS it holds them to 8 MiB in all, against the 1 GiB a
+/// single one could make us hold before.
+pub const MAX_UNAUTHORIZED_MESSAGE: usize = 128 * 1024;
 
 /// A place among the unauthorized connections, taken before the identity handshake and given
 /// back on drop: at authorization, or when the connection ends first. The count of live
@@ -1206,6 +1213,9 @@ impl Connection {
             }
         }
         video_service::notify_video_frame_fetched_by_conn_id(id, None);
+        if conn.authorized {
+            password::update_temporary_password();
+        }
         if let Err(err) = conn.try_port_forward_loop(&mut rx_from_cm).await {
             conn.on_close(&err.to_string(), false).await;
             raii::AuthedConnID::check_remove_session(conn.inner.id(), conn.session_key());
@@ -1868,15 +1878,15 @@ impl Connection {
         if let Some(keep_alive) = self.prepare_terminal_login_for_authorization().await {
             return keep_alive;
         }
+        // Lifted here rather than below with the rest of authorization: a multiplexed tunnel
+        // narrows it again for its own framing (`port_forward_mux::cap_packet_size`), so that
+        // call has to come after this one, not before.
+        self.stream.set_max_packet_length(usize::MAX);
         if !self.connect_port_forward_if_needed().await {
             return false;
         }
         self.authorized = true;
         self.unauthorized_id = None;
-        // One-time means gone once it has let a peer in, not once that peer
-        // leaves. This session's later logins come in on the password the
-        // session remembers, so they are not affected.
-        password::update_temporary_password();
         // Releases the budget `check_id_whitelist` charges against this address: only a peer
         // that got this far proved more than a self-reported id.
         self.clear_id_whitelist_failures();