core: check the length of NotValidBefore attr while decoding

DecodeBinary throws panic otherwise.

pkg/core/transaction/attribute_test.go

@@ -31,6 +31,7 @@ func TestAttribute_EncodeBinary(t *testing.T) {
 		testserdes.EncodeDecodeBinary(t, attr, new(Attribute))
 	})
 	t.Run("NotValidBefore", func(t *testing.T) {
+		t.Run("positive", func(t *testing.T) {
 			attr := &Attribute{
 				Type: NotValidBeforeT,
 				Value: &NotValidBefore{
@@ -39,6 +40,17 @@ func TestAttribute_EncodeBinary(t *testing.T) {
 			}
 			testserdes.EncodeDecodeBinary(t, attr, new(Attribute))
 		})
+		t.Run("bad format: too long", func(t *testing.T) {
+			bw := io.NewBufBinWriter()
+			bw.WriteVarBytes([]byte{1, 2, 3, 4, 5})
+			require.Error(t, testserdes.DecodeBinary(bw.Bytes(), new(NotValidBefore)))
+		})
+		t.Run("bad format: too short", func(t *testing.T) {
+			bw := io.NewBufBinWriter()
+			bw.WriteVarBytes([]byte{1, 2, 3})
+			require.Error(t, testserdes.DecodeBinary(bw.Bytes(), new(NotValidBefore)))
+		})
+	})
 	t.Run("Reserved", func(t *testing.T) {
 		getReservedAttribute := func(t AttrType) *Attribute {
 			return &Attribute{

pkg/core/transaction/not_valid_before.go

@@ -2,6 +2,7 @@ package transaction
 
 import (
 	"encoding/binary"
+	"fmt"
 
 	"github.com/nspcc-dev/neo-go/pkg/io"
 )
@@ -14,6 +15,13 @@ type NotValidBefore struct {
 // DecodeBinary implements io.Serializable interface.
 func (n *NotValidBefore) DecodeBinary(br *io.BinReader) {
 	bytes := br.ReadVarBytes(4)
+	if br.Err != nil {
+		return
+	}
+	if len(bytes) != 4 {
+		br.Err = fmt.Errorf("expected 4 bytes, got %d", len(bytes))
+		return
+	}
 	n.Height = binary.LittleEndian.Uint32(bytes)
 }