ssh: fix deadlock on unexpected global responses

Previously, the mux implementation handled global request responses by
blocking until the response could be sent to the globalResponses channel.
Since this channel has a buffer size of 1, unsolicited responses from a
server (or responses arriving after a timeout) would fill the buffer.
Subsequent unsolicited responses would block handleGlobalPacket, stalling
the entire connection's read loop and causing a denial of service.

This change modifies handleGlobalPacket to use a non-blocking send. If
no goroutine is waiting for a response (or the buffer is full), the
message is dropped. This aligns with OpenSSH behavior, which ignores
unexpected global responses.

Additionally, SendRequest now drains the globalResponses channel after
acquiring the mutex but before sending the request. This ensures that
any stale responses or "spam" buffered just before the lock was acquired
are discarded, preventing race conditions where a legitimate request
might otherwise consume an unrelated response.

This issue was found during a security audit by NCC Group Cryptography
Services, sponsored by Teleport.

Fixes golang/go#79564
Fixes CVE-2026-39830

Change-Id: Ia0c46355203d557eadcd432c10b87c8a044e1089
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/781640
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Neal Patel <nealpatel@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

ssh/mux.go

diff --git a/ssh/mux.go b/ssh/mux.go
index d2d24c6..3bc4afb 100644
--- a/ssh/mux.go
+++ b/ssh/mux.go
@@ -91,9 +91,10 @@
 
 	incomingChannels chan NewChannel
 
-	globalSentMu     sync.Mutex
-	globalResponses  chan interface{}
-	incomingRequests chan *Request
+	globalSentMu      sync.Mutex
+	globalSentPending atomic.Bool
+	globalResponses   chan interface{}
+	incomingRequests  chan *Request
 
 	errCond *sync.Cond
 	err     error
@@ -141,6 +142,24 @@
 	if wantReply {
 		m.globalSentMu.Lock()
 		defer m.globalSentMu.Unlock()
+
+		// Open the gate so that responses arriving while this request is in
+		// flight are allowed to reach globalResponses. Any response arriving
+		// while no request is pending is dropped by handleGlobalPacket.
+		m.globalSentPending.Store(true)
+		defer m.globalSentPending.Store(false)
+
+		// Drain any spurious responses that may have been buffered. This prevents
+		// a previously buffered unexpected response from being consumed instead
+		// of the actual response for this request.
+	drain:
+		for {
+			select {
+			case <-m.globalResponses:
+			default:
+				break drain
+			}
+		}
 	}
 
 	if err := m.sendMessage(globalRequestMsg{
@@ -267,7 +286,16 @@
 			mux:       m,
 		}
 	case *globalRequestSuccessMsg, *globalRequestFailureMsg:
-		m.globalResponses <- msg
+		// Drop responses that arrive when no SendRequest is waiting, to
+		// prevent a malicious peer from staging responses for a future
+		// caller.
+		if !m.globalSentPending.Load() {
+			return nil
+		}
+		select {
+		case m.globalResponses <- msg:
+		default:
+		}
 	default:
 		panic(fmt.Sprintf("not a global message %#v", msg))
 	}

ssh/mux_test.go

diff --git a/ssh/mux_test.go b/ssh/mux_test.go
index 95874fa..a0cae34 100644
--- a/ssh/mux_test.go
+++ b/ssh/mux_test.go
@@ -881,3 +881,257 @@
 		t.Error("transport debug switched on")
 	}
 }
+
+func TestMuxUnexpectedGlobalResponsesDiscarded(t *testing.T) {
+	clientPipe, serverPipe := memPipe()
+	client := newMux(clientPipe)
+	defer serverPipe.Close()
+	defer client.Close()
+
+	done := make(chan error, 1)
+	go func() {
+		// Send multiple unexpected global responses, this should not block the
+		// globalResponses channel.
+		for i := range 5 {
+			err := serverPipe.writePacket(Marshal(globalRequestSuccessMsg{
+				Data: []byte{byte(i)},
+			}))
+			if err != nil {
+				done <- fmt.Errorf("send success msg %d: %w", i, err)
+				return
+			}
+		}
+		for i := range 5 {
+			err := serverPipe.writePacket(Marshal(globalRequestFailureMsg{
+				Data: []byte{byte(i)},
+			}))
+			if err != nil {
+				done <- fmt.Errorf("send failure msg %d: %w", i, err)
+				return
+			}
+		}
+
+		// Now send a global request and wait for the response. This
+		// verifies the mux is still processing packets.
+		err := serverPipe.writePacket(Marshal(globalRequestMsg{
+			Type:      "keepalive@golang.org",
+			WantReply: true,
+			Data:      nil,
+		}))
+		if err != nil {
+			done <- fmt.Errorf("send global request: %w", err)
+			return
+		}
+
+		packet, err := serverPipe.readPacket()
+		if err != nil {
+			done <- fmt.Errorf("read packet: %w", err)
+			return
+		}
+		decoded, err := decode(packet)
+		if err != nil {
+			done <- fmt.Errorf("decode: %w", err)
+			return
+		}
+		switch decoded.(type) {
+		case *globalRequestSuccessMsg, *globalRequestFailureMsg:
+			// Expected response
+		default:
+			done <- fmt.Errorf("unexpected packet type: %T", decoded)
+			return
+		}
+		done <- nil
+	}()
+
+	// Handle the incoming request from the server and reply
+	req, ok := <-client.incomingRequests
+	if !ok {
+		t.Fatal("incomingRequests channel closed unexpectedly")
+	}
+	if req.Type != "keepalive@golang.org" {
+		t.Fatalf("unexpected request type: %s", req.Type)
+	}
+	if err := req.Reply(true, nil); err != nil {
+		t.Fatalf("Reply: %v", err)
+	}
+
+	if err := <-done; err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestMuxConcurrentGlobalRequests(t *testing.T) {
+	clientMux, serverMux := muxPair()
+	defer serverMux.Close()
+	defer clientMux.Close()
+
+	const numRequests = 50
+
+	serverDone := make(chan struct{})
+	go func() {
+		defer close(serverDone)
+		for r := range serverMux.incomingRequests {
+			if r.WantReply {
+				replyData := append([]byte("reply:"), r.Payload...)
+				r.Reply(true, replyData)
+			}
+		}
+	}()
+
+	var clientWg sync.WaitGroup
+	clientWg.Add(numRequests)
+
+	errCh := make(chan error, numRequests)
+
+	for i := range numRequests {
+		go func(id int) {
+			defer clientWg.Done()
+
+			payloadStr := fmt.Sprintf("req-%d", id)
+			payload := []byte(payloadStr)
+
+			// This call blocks until the globalSentMu is acquired.
+			// The mutex ensures that even with many concurrent attempts,
+			// the "drain" and "send" logic happens atomically per request.
+			ok, data, err := clientMux.SendRequest("echo", true, payload)
+			if err != nil {
+				errCh <- fmt.Errorf("req %d error: %v", id, err)
+				return
+			}
+			if !ok {
+				errCh <- fmt.Errorf("req %d failed (want success)", id)
+				return
+			}
+
+			expected := "reply:" + payloadStr
+			if string(data) != expected {
+				errCh <- fmt.Errorf("req %d mismatch: got %q, want %q", id, string(data), expected)
+			}
+		}(i)
+	}
+
+	clientWg.Wait()
+	close(errCh)
+
+	for err := range errCh {
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	clientMux.Close()
+	<-serverDone
+}
+
+func TestMuxGlobalResponseDroppedWhenIdle(t *testing.T) {
+	clientPipe, serverPipe := memPipe()
+	clientMux := newMux(clientPipe)
+	defer serverPipe.Close()
+	defer clientMux.Close()
+
+	errCh := make(chan error, 1)
+	go func() {
+		// Send a spurious response while no SendRequest is pending.
+		if err := serverPipe.writePacket(Marshal(globalRequestSuccessMsg{
+			Data: []byte("spurious"),
+		})); err != nil {
+			errCh <- fmt.Errorf("send spurious: %w", err)
+			return
+		}
+		// Follow with a global request; once the client observes this on
+		// incomingRequests, the mux loop has necessarily processed (and
+		// dropped) the prior spurious response.
+		if err := serverPipe.writePacket(Marshal(globalRequestMsg{
+			Type:      "sync@example.com",
+			WantReply: false,
+		})); err != nil {
+			errCh <- fmt.Errorf("send sync request: %w", err)
+			return
+		}
+		errCh <- nil
+	}()
+
+	if err := <-errCh; err != nil {
+		t.Fatal(err)
+	}
+
+	req, ok := <-clientMux.incomingRequests
+	if !ok {
+		t.Fatal("incomingRequests closed unexpectedly")
+	}
+	if req.Type != "sync@example.com" {
+		t.Fatalf("unexpected sync request type %q", req.Type)
+	}
+
+	// The spurious response preceded the sync request, so by now the mux
+	// loop has processed it. The pending-gate must have caused it to be
+	// dropped rather than buffered.
+	if n := len(clientMux.globalResponses); n != 0 {
+		t.Fatalf("globalResponses buffer should be empty after idle drop, has %d entries", n)
+	}
+}
+
+func TestMuxStaleResponseDrained(t *testing.T) {
+	// Simulate a stale response sitting in globalResponses (e.g. a response
+	// that slipped in through the pending-gate on a prior SendRequest that
+	// exited without consuming it). The drain step in the next SendRequest
+	// must discard it so the caller receives the correct reply.
+	clientMux, serverMux := muxPair()
+	defer serverMux.Close()
+	defer clientMux.Close()
+
+	clientMux.globalResponses <- &globalRequestSuccessMsg{Data: []byte("stale")}
+
+	serverDone := make(chan struct{})
+	go func() {
+		defer close(serverDone)
+		for req := range serverMux.incomingRequests {
+			if req.WantReply {
+				req.Reply(true, append([]byte("reply:"), req.Payload...))
+			}
+		}
+	}()
+
+	ok, data, err := clientMux.SendRequest("test", true, []byte("hello"))
+	if err != nil {
+		t.Fatalf("request failed: %v", err)
+	}
+	if !ok {
+		t.Fatal("expected success response")
+	}
+	if string(data) != "reply:hello" {
+		t.Fatalf("got %q, want %q (drain did not remove stale response)", data, "reply:hello")
+	}
+
+	clientMux.Close()
+	<-serverDone
+}
+
+func TestMuxGlobalResponseAcceptedWhilePending(t *testing.T) {
+	// Positive control: when a SendRequest is actually pending, the
+	// response must be delivered (the gate is open).
+	clientMux, serverMux := muxPair()
+	defer serverMux.Close()
+	defer clientMux.Close()
+
+	serverDone := make(chan struct{})
+	go func() {
+		defer close(serverDone)
+		for req := range serverMux.incomingRequests {
+			if req.WantReply {
+				req.Reply(true, []byte("pong"))
+			}
+		}
+	}()
+
+	ok, data, err := clientMux.SendRequest("ping", true, nil)
+	if err != nil {
+		t.Fatalf("SendRequest: %v", err)
+	}
+	if !ok || string(data) != "pong" {
+		t.Fatalf("unexpected response: ok=%v data=%q", ok, data)
+	}
+
+	clientMux.Close()
+	<-serverDone
+}