Prevent web ui SSH connections from leaking (#41501)

The session closing in TerminalHandler.Close did not always close
established ssh sessions if the client closed the web socket
at any point between the new browser tab being opened and the session
being established. In these cases the SSH connection was leaked for
the duration of the Proxy process. To ensure sessions are always
terminated a check to see if the client closed the web socket was
added prior to writing the session metadata and after the shell
is established for the ssh session.

lib/web/terminal.go

@@ -489,6 +489,14 @@ func (t *TerminalHandler) makeClient(ctx context.Context, stream *TerminalStream
 	// to allow future window changes.
 	tc.OnShellCreated = func(s *tracessh.Session, c *tracessh.Client, _ io.ReadWriteCloser) (bool, error) {
 		t.stream.sessionCreated(s)
+
+		// The web session was closed by the client while the ssh connection was being established.
+		// Attempt to close the SSH session instead of proceeding with the window change request.
+		if t.closedByClient.Load() {
+			t.log.Debug("websocket was closed by client, terminating established ssh connection to host")
+			return false, trace.Wrap(s.Close())
+		}
+
 		if err := s.WindowChange(ctx, t.term.H, t.term.W); err != nil {
 			t.log.Error(err)
 		}
@@ -808,9 +816,17 @@ func (t *TerminalHandler) streamTerminal(ctx context.Context, tc *client.Telepor
 		t.stream.writeError(err.Error())
 		return
 	}
-
 	defer nc.Close()
 
+	// If the session was terminated by client while the connection to the host
+	// was being established, then return early before creating the shell. Any terminations
+	// by the client from here on out should either get caught in the OnShellCreated callback
+	// set on the [tc] or in [TerminalHandler.Close].
+	if t.closedByClient.Load() {
+		t.log.Debug("websocket was closed by client, aborting establishing ssh connection to host")
+		return
+	}
+
 	var beforeStart func(io.Writer)
 	if t.participantMode == types.SessionModeratorMode {
 		beforeStart = func(out io.Writer) {
@@ -850,7 +866,7 @@ func (t *TerminalHandler) streamTerminal(ctx context.Context, tc *client.Telepor
 	}
 
 	if err := t.stream.Close(); err != nil {
-		t.log.WithError(err).Error("Unable to send close event to web client.")
+		t.log.WithError(err).Error("Unable to close client web socket.")
 		return
 	}