ssh: fix malformed exec payload panic (#5428)

pkg/ssh/server_test.go

@@ -0,0 +1,71 @@
+// Copyright 2026 The frp Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ssh
+
+import (
+	"encoding/binary"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	cryptossh "golang.org/x/crypto/ssh"
+)
+
+func TestParseExecPayload(t *testing.T) {
+	payload := cryptossh.Marshal(&execPayload{Command: "tcp --remote_port 6000"})
+
+	got, ok := parseExecPayload(payload)
+
+	require.True(t, ok)
+	require.Equal(t, "tcp --remote_port 6000", got)
+}
+
+func TestParseExecPayloadRejectsMalformedPayloads(t *testing.T) {
+	overflowLength := make([]byte, 5)
+	binary.BigEndian.PutUint32(overflowLength[:4], ^uint32(0))
+
+	for _, tc := range []struct {
+		name    string
+		payload []byte
+	}{
+		{
+			name:    "empty",
+			payload: nil,
+		},
+		{
+			name:    "short length prefix",
+			payload: []byte{0, 0, 0},
+		},
+		{
+			name:    "declared length exceeds remaining payload",
+			payload: []byte{0, 0, 0, 2, 'x'},
+		},
+		{
+			name:    "overflow length",
+			payload: overflowLength,
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			var (
+				got string
+				ok  bool
+			)
+			require.NotPanics(t, func() {
+				got, ok = parseExecPayload(tc.payload)
+			})
+			require.False(t, ok)
+			require.Empty(t, got)
+		})
+	}
+}