diff --git a/ssh/server.go b/ssh/server.go
index 7f0c236a9a..c2dfe3268c 100644
--- a/ssh/server.go
+++ b/ssh/server.go
@@ -213,6 +213,7 @@ func NewServerConn(c net.Conn, config *ServerConfig) (*ServerConn, <-chan NewCha
 	} else {
 		for _, algo := range fullConf.PublicKeyAuthAlgorithms {
 			if !contains(supportedPubKeyAuthAlgos, algo) {
+				c.Close()
 				return nil, nil, nil, fmt.Errorf("ssh: unsupported public key authentication algorithm %s", algo)
 			}
 		}
@@ -220,6 +221,7 @@ func NewServerConn(c net.Conn, config *ServerConfig) (*ServerConn, <-chan NewCha
 	// Check if the config contains any unsupported key exchanges
 	for _, kex := range fullConf.KeyExchanges {
 		if _, ok := serverForbiddenKexAlgos[kex]; ok {
+			c.Close()
 			return nil, nil, nil, fmt.Errorf("ssh: unsupported key exchange %s for server", kex)
 		}
 	}
diff --git a/ssh/server_test.go b/ssh/server_test.go
index 2145dce06f..a9b2bce0c1 100644
--- a/ssh/server_test.go
+++ b/ssh/server_test.go
@@ -5,7 +5,11 @@
 package ssh
 
 import (
+	"io"
+	"net"
+	"sync/atomic"
 	"testing"
+	"time"
 )
 
 func TestClientAuthRestrictedPublicKeyAlgos(t *testing.T) {
@@ -59,27 +63,78 @@ func TestClientAuthRestrictedPublicKeyAlgos(t *testing.T) {
 }
 
 func TestNewServerConnValidationErrors(t *testing.T) {
-	c1, c2, err := netPipe()
-	if err != nil {
-		t.Fatalf("netPipe: %v", err)
-	}
-	defer c1.Close()
-	defer c2.Close()
-
 	serverConf := &ServerConfig{
 		PublicKeyAuthAlgorithms: []string{CertAlgoRSAv01},
 	}
-	_, _, _, err = NewServerConn(c1, serverConf)
+	c := &markerConn{}
+	_, _, _, err := NewServerConn(c, serverConf)
 	if err == nil {
 		t.Fatal("NewServerConn with invalid public key auth algorithms succeeded")
 	}
+	if !c.isClosed() {
+		t.Fatal("NewServerConn with invalid public key auth algorithms left connection open")
+	}
+	if c.isUsed() {
+		t.Fatal("NewServerConn with invalid public key auth algorithms used connection")
+	}
+
 	serverConf = &ServerConfig{
 		Config: Config{
 			KeyExchanges: []string{kexAlgoDHGEXSHA256},
 		},
 	}
-	_, _, _, err = NewServerConn(c1, serverConf)
+	c = &markerConn{}
+	_, _, _, err = NewServerConn(c, serverConf)
 	if err == nil {
 		t.Fatal("NewServerConn with unsupported key exchange succeeded")
 	}
+	if !c.isClosed() {
+		t.Fatal("NewServerConn with unsupported key exchange left connection open")
+	}
+	if c.isUsed() {
+		t.Fatal("NewServerConn with unsupported key exchange used connection")
+	}
+}
+
+type markerConn struct {
+	closed uint32
+	used   uint32
 }
+
+func (c *markerConn) isClosed() bool {
+	return atomic.LoadUint32(&c.closed) != 0
+}
+
+func (c *markerConn) isUsed() bool {
+	return atomic.LoadUint32(&c.used) != 0
+}
+
+func (c *markerConn) Close() error {
+	atomic.StoreUint32(&c.closed, 1)
+	return nil
+}
+
+func (c *markerConn) Read(b []byte) (n int, err error) {
+	atomic.StoreUint32(&c.used, 1)
+	if atomic.LoadUint32(&c.closed) != 0 {
+		return 0, net.ErrClosed
+	} else {
+		return 0, io.EOF
+	}
+}
+
+func (c *markerConn) Write(b []byte) (n int, err error) {
+	atomic.StoreUint32(&c.used, 1)
+	if atomic.LoadUint32(&c.closed) != 0 {
+		return 0, net.ErrClosed
+	} else {
+		return 0, io.ErrClosedPipe
+	}
+}
+
+func (*markerConn) LocalAddr() net.Addr  { return nil }
+func (*markerConn) RemoteAddr() net.Addr { return nil }
+
+func (*markerConn) SetDeadline(t time.Time) error      { return nil }
+func (*markerConn) SetReadDeadline(t time.Time) error  { return nil }
+func (*markerConn) SetWriteDeadline(t time.Time) error { return nil }