core: check signers of on-chained conflict during new tx verification

During new transaction verification if there's an on-chain conflicting
transaction, we should check the signers of this conflicting transaction.
If the signers intersect with signers of the incoming transaction, then
the conflict is treated as valid and verification for new incoming
transaction should fail. Otherwise, the conflict is treated as the
malicious attack attempt and will not be taken into account;
verification for the new incoming transaction should continue.

This commint implements the scheme described at
https://github.com/neo-project/neo/pull/2818#issuecomment-1632972055,
thanks to @shargon for digging.

Signed-off-by: Anna Shaleva <shaleva.ann@nspcc.ru>

pkg/core/blockchain.go

@@ -45,7 +45,7 @@ import (
 
 // Tuning parameters.
 const (
-	version = "0.2.8"
+	version = "0.2.9"
 
 	defaultInitialGAS                      = 52000000_00000000
 	defaultGCPeriod                        = 10000
@@ -2477,7 +2477,7 @@ func (bc *Blockchain) verifyAndPoolTx(t *transaction.Transaction, pool *mempool.
 		return fmt.Errorf("%w: net fee is %v, need %v", ErrTxSmallNetworkFee, t.NetworkFee, needNetworkFee)
 	}
 	// check that current tx wasn't included in the conflicts attributes of some other transaction which is already in the chain
-	if err := bc.dao.HasTransaction(t.Hash()); err != nil {
+	if err := bc.dao.HasTransaction(t.Hash(), t.Signers); err != nil {
 		switch {
 		case errors.Is(err, dao.ErrAlreadyExists):
 			return fmt.Errorf("blockchain: %w", ErrAlreadyExists)
@@ -2578,7 +2578,9 @@ func (bc *Blockchain) verifyTxAttributes(d *dao.Simple, tx *transaction.Transact
 				return fmt.Errorf("%w: Conflicts attribute was found, but P2PSigExtensions are disabled", ErrInvalidAttribute)
 			}
 			conflicts := tx.Attributes[i].Value.(*transaction.Conflicts)
-			if err := bc.dao.HasTransaction(conflicts.Hash); errors.Is(err, dao.ErrAlreadyExists) {
+			// Only fully-qualified dao.ErrAlreadyExists error bothers us here, thus, we
+			// can safely omit the payer argument to HasTransaction call to improve performance a bit.
+			if err := bc.dao.HasTransaction(conflicts.Hash, nil); errors.Is(err, dao.ErrAlreadyExists) {
 				return fmt.Errorf("%w: conflicting transaction %s is already on chain", ErrInvalidAttribute, conflicts.Hash.StringLE())
 			}
 		case transaction.NotaryAssistedT:
@@ -2611,7 +2613,7 @@ func (bc *Blockchain) IsTxStillRelevant(t *transaction.Transaction, txpool *memp
 		return false
 	}
 	if txpool == nil {
-		if bc.dao.HasTransaction(t.Hash()) != nil {
+		if bc.dao.HasTransaction(t.Hash(), t.Signers) != nil {
 			return false
 		}
 	} else if txpool.HasConflicts(t, bc) {

pkg/core/blockchain_neotest_test.go

@@ -1634,28 +1634,272 @@ func TestBlockchain_VerifyTx(t *testing.T) {
 			})
 			t.Run("enabled", func(t *testing.T) {
 				t.Run("dummy on-chain conflict", func(t *testing.T) {
-					tx := newTestTx(t, h, testScript)
+					t.Run("on-chain conflict signed by malicious party", func(t *testing.T) {
-					require.NoError(t, accs[0].SignTx(netmode.UnitTestNet, tx))
+						tx := newTestTx(t, h, testScript)
-					conflicting := transaction.New([]byte{byte(opcode.RET)}, 1000_0000)
+						require.NoError(t, accs[0].SignTx(netmode.UnitTestNet, tx))
-					conflicting.ValidUntilBlock = bc.BlockHeight() + 1
+						conflicting := transaction.New([]byte{byte(opcode.RET)}, 1000_0000)
-					conflicting.Signers = []transaction.Signer{
+						conflicting.ValidUntilBlock = bc.BlockHeight() + 1
-						{
+						conflicting.Signers = []transaction.Signer{
-							Account: validator.ScriptHash(),
+							{
-							Scopes:  transaction.CalledByEntry,
+								Account: validator.ScriptHash(),
-						},
+								Scopes:  transaction.CalledByEntry,
-					}
-					conflicting.Attributes = []transaction.Attribute{
-						{
-							Type: transaction.ConflictsT,
-							Value: &transaction.Conflicts{
-								Hash: tx.Hash(),
 							},
-						},
+						}
-					}
+						conflicting.Attributes = []transaction.Attribute{
-					conflicting.NetworkFee = 1000_0000
+							{
-					require.NoError(t, validator.SignTx(netmode.UnitTestNet, conflicting))
+								Type: transaction.ConflictsT,
-					e.AddNewBlock(t, conflicting)
+								Value: &transaction.Conflicts{
-					require.ErrorIs(t, bc.VerifyTx(tx), core.ErrHasConflicts)
+									Hash: tx.Hash(),
+								},
+							},
+						}
+						conflicting.NetworkFee = 1000_0000
+						require.NoError(t, validator.SignTx(netmode.UnitTestNet, conflicting))
+						e.AddNewBlock(t, conflicting)
+						// We expect `tx` to pass verification, because on-chained `conflicting` doesn't have
+						// `tx`'s payer in the signers list, thus, `conflicting` should be considered as
+						// malicious conflict.
+						require.NoError(t, bc.VerifyTx(tx))
+					})
+					t.Run("multiple on-chain conflicts signed by malicious parties", func(t *testing.T) {
+						m1 := e.NewAccount(t)
+						m2 := e.NewAccount(t)
+						m3 := e.NewAccount(t)
+						good := e.NewAccount(t)
+
+						// txGood doesn't conflict with anyone and signed by good signer.
+						txGood := newTestTx(t, good.ScriptHash(), testScript)
+						require.NoError(t, good.SignTx(netmode.UnitTestNet, txGood))
+
+						// txM1 conflicts with txGood and signed by two malicious signers.
+						txM1 := newTestTx(t, m1.ScriptHash(), testScript)
+						txM1.Signers = append(txM1.Signers, transaction.Signer{Account: m2.ScriptHash()})
+						txM1.Attributes = []transaction.Attribute{
+							{
+								Type: transaction.ConflictsT,
+								Value: &transaction.Conflicts{
+									Hash: txGood.Hash(),
+								},
+							},
+						}
+						txM1.NetworkFee = 1_000_0000
+						require.NoError(t, m1.SignTx(netmode.UnitTestNet, txM1))
+						require.NoError(t, m2.SignTx(netmode.UnitTestNet, txM1))
+						e.AddNewBlock(t, txM1)
+
+						// txM2 conflicts with txGood and signed by one malicious signer.
+						txM2 := newTestTx(t, m3.ScriptHash(), testScript)
+						txM2.Attributes = []transaction.Attribute{
+							{
+								Type: transaction.ConflictsT,
+								Value: &transaction.Conflicts{
+									Hash: txGood.Hash(),
+								},
+							},
+						}
+						txM2.NetworkFee = 1_000_0000
+						require.NoError(t, m3.SignTx(netmode.UnitTestNet, txM2))
+						e.AddNewBlock(t, txM2)
+
+						// We expect `tx` to pass verification, because on-chained `conflicting` doesn't have
+						// `tx`'s payer in the signers list, thus, `conflicting` should be considered as
+						// malicious conflict.
+						require.NoError(t, bc.VerifyTx(txGood))
+
+						// After that txGood can be added to the chain normally.
+						e.AddNewBlock(t, txGood)
+
+						// And after that ErrAlreadyExist is expected on verification.
+						require.ErrorIs(t, bc.VerifyTx(txGood), core.ErrAlreadyExists)
+					})
+
+					t.Run("multiple on-chain conflicts signed by [valid+malicious] parties", func(t *testing.T) {
+						m1 := e.NewAccount(t)
+						m2 := e.NewAccount(t)
+						m3 := e.NewAccount(t)
+						good := e.NewAccount(t)
+
+						// txGood doesn't conflict with anyone and signed by good signer.
+						txGood := newTestTx(t, good.ScriptHash(), testScript)
+						require.NoError(t, good.SignTx(netmode.UnitTestNet, txGood))
+
+						// txM1 conflicts with txGood and signed by one malicious and one good signers.
+						txM1 := newTestTx(t, m1.ScriptHash(), testScript)
+						txM1.Signers = append(txM1.Signers, transaction.Signer{Account: good.ScriptHash()})
+						txM1.Attributes = []transaction.Attribute{
+							{
+								Type: transaction.ConflictsT,
+								Value: &transaction.Conflicts{
+									Hash: txGood.Hash(),
+								},
+							},
+						}
+						txM1.NetworkFee = 1_000_0000
+						require.NoError(t, m1.SignTx(netmode.UnitTestNet, txM1))
+						require.NoError(t, good.SignTx(netmode.UnitTestNet, txM1))
+						e.AddNewBlock(t, txM1)
+
+						// txM2 conflicts with txGood and signed by two malicious signers.
+						txM2 := newTestTx(t, m2.ScriptHash(), testScript)
+						txM2.Signers = append(txM2.Signers, transaction.Signer{Account: m3.ScriptHash()})
+						txM2.Attributes = []transaction.Attribute{
+							{
+								Type: transaction.ConflictsT,
+								Value: &transaction.Conflicts{
+									Hash: txGood.Hash(),
+								},
+							},
+						}
+						txM2.NetworkFee = 1_000_0000
+						require.NoError(t, m2.SignTx(netmode.UnitTestNet, txM2))
+						require.NoError(t, m3.SignTx(netmode.UnitTestNet, txM2))
+						e.AddNewBlock(t, txM2)
+
+						// We expect `tx` to fail verification, because one of the on-chained `conflicting`
+						// transactions has common signers with `tx`, thus, `conflicting` should be
+						// considered as a valid conflict.
+						require.ErrorIs(t, bc.VerifyTx(txGood), core.ErrHasConflicts)
+					})
+
+					t.Run("multiple on-chain conflicts signed by [malicious+valid] parties", func(t *testing.T) {
+						m1 := e.NewAccount(t)
+						m2 := e.NewAccount(t)
+						m3 := e.NewAccount(t)
+						good := e.NewAccount(t)
+
+						// txGood doesn't conflict with anyone and signed by good signer.
+						txGood := newTestTx(t, good.ScriptHash(), testScript)
+						require.NoError(t, good.SignTx(netmode.UnitTestNet, txGood))
+
+						// txM2 conflicts with txGood and signed by two malicious signers.
+						txM2 := newTestTx(t, m2.ScriptHash(), testScript)
+						txM2.Signers = append(txM2.Signers, transaction.Signer{Account: m3.ScriptHash()})
+						txM2.Attributes = []transaction.Attribute{
+							{
+								Type: transaction.ConflictsT,
+								Value: &transaction.Conflicts{
+									Hash: txGood.Hash(),
+								},
+							},
+						}
+						txM2.NetworkFee = 1_000_0000
+						require.NoError(t, m2.SignTx(netmode.UnitTestNet, txM2))
+						require.NoError(t, m3.SignTx(netmode.UnitTestNet, txM2))
+						e.AddNewBlock(t, txM2)
+
+						// txM1 conflicts with txGood and signed by one malicious and one good signers.
+						txM1 := newTestTx(t, m1.ScriptHash(), testScript)
+						txM1.Signers = append(txM1.Signers, transaction.Signer{Account: good.ScriptHash()})
+						txM1.Attributes = []transaction.Attribute{
+							{
+								Type: transaction.ConflictsT,
+								Value: &transaction.Conflicts{
+									Hash: txGood.Hash(),
+								},
+							},
+						}
+						txM1.NetworkFee = 1_000_0000
+						require.NoError(t, m1.SignTx(netmode.UnitTestNet, txM1))
+						require.NoError(t, good.SignTx(netmode.UnitTestNet, txM1))
+						e.AddNewBlock(t, txM1)
+
+						// We expect `tx` to fail verification, because one of the on-chained `conflicting`
+						// transactions has common signers with `tx`, thus, `conflicting` should be
+						// considered as a valid conflict.
+						require.ErrorIs(t, bc.VerifyTx(txGood), core.ErrHasConflicts)
+					})
+
+					t.Run("multiple on-chain conflicts signed by [valid + malicious + valid] parties", func(t *testing.T) {
+						m1 := e.NewAccount(t)
+						m2 := e.NewAccount(t)
+						m3 := e.NewAccount(t)
+						good := e.NewAccount(t)
+
+						// txGood doesn't conflict with anyone and signed by good signer.
+						txGood := newTestTx(t, good.ScriptHash(), testScript)
+						require.NoError(t, good.SignTx(netmode.UnitTestNet, txGood))
+
+						// txM1 conflicts with txGood and signed by one malicious and one good signers.
+						txM1 := newTestTx(t, m1.ScriptHash(), testScript)
+						txM1.Signers = append(txM1.Signers, transaction.Signer{Account: good.ScriptHash()})
+						txM1.Attributes = []transaction.Attribute{
+							{
+								Type: transaction.ConflictsT,
+								Value: &transaction.Conflicts{
+									Hash: txGood.Hash(),
+								},
+							},
+						}
+						txM1.NetworkFee = 1_000_0000
+						require.NoError(t, m1.SignTx(netmode.UnitTestNet, txM1))
+						require.NoError(t, good.SignTx(netmode.UnitTestNet, txM1))
+						e.AddNewBlock(t, txM1)
+
+						// txM2 conflicts with txGood and signed by two malicious signers.
+						txM2 := newTestTx(t, m2.ScriptHash(), testScript)
+						txM2.Signers = append(txM2.Signers, transaction.Signer{Account: m3.ScriptHash()})
+						txM2.Attributes = []transaction.Attribute{
+							{
+								Type: transaction.ConflictsT,
+								Value: &transaction.Conflicts{
+									Hash: txGood.Hash(),
+								},
+							},
+						}
+						txM2.NetworkFee = 1_000_0000
+						require.NoError(t, m2.SignTx(netmode.UnitTestNet, txM2))
+						require.NoError(t, m3.SignTx(netmode.UnitTestNet, txM2))
+						e.AddNewBlock(t, txM2)
+
+						// txM3 conflicts with txGood and signed by one good and one malicious signers.
+						txM3 := newTestTx(t, good.ScriptHash(), testScript)
+						txM3.Signers = append(txM3.Signers, transaction.Signer{Account: m1.ScriptHash()})
+						txM3.Attributes = []transaction.Attribute{
+							{
+								Type: transaction.ConflictsT,
+								Value: &transaction.Conflicts{
+									Hash: txGood.Hash(),
+								},
+							},
+						}
+						txM3.NetworkFee = 1_000_0000
+						require.NoError(t, good.SignTx(netmode.UnitTestNet, txM3))
+						require.NoError(t, m1.SignTx(netmode.UnitTestNet, txM3))
+						e.AddNewBlock(t, txM3)
+
+						// We expect `tx` to fail verification, because one of the on-chained `conflicting`
+						// transactions has common signers with `tx`, thus, `conflicting` should be
+						// considered as a valid conflict.
+						require.ErrorIs(t, bc.VerifyTx(txGood), core.ErrHasConflicts)
+					})
+
+					t.Run("on-chain conflict signed by single valid sender", func(t *testing.T) {
+						tx := newTestTx(t, h, testScript)
+						tx.Signers = []transaction.Signer{{Account: validator.ScriptHash()}}
+						require.NoError(t, validator.SignTx(netmode.UnitTestNet, tx))
+						conflicting := transaction.New([]byte{byte(opcode.RET)}, 1000_0000)
+						conflicting.ValidUntilBlock = bc.BlockHeight() + 1
+						conflicting.Signers = []transaction.Signer{
+							{
+								Account: validator.ScriptHash(),
+								Scopes:  transaction.CalledByEntry,
+							},
+						}
+						conflicting.Attributes = []transaction.Attribute{
+							{
+								Type: transaction.ConflictsT,
+								Value: &transaction.Conflicts{
+									Hash: tx.Hash(),
+								},
+							},
+						}
+						conflicting.NetworkFee = 1000_0000
+						require.NoError(t, validator.SignTx(netmode.UnitTestNet, conflicting))
+						e.AddNewBlock(t, conflicting)
+						// We expect `tx` to fail verification, because on-chained `conflicting` has
+						// `tx`'s payer as a signer.
+						require.ErrorIs(t, bc.VerifyTx(tx), core.ErrHasConflicts)
+					})
 				})
 				t.Run("attribute on-chain conflict", func(t *testing.T) {
 					tx := neoValidatorsInvoker.Invoke(t, stackitem.NewBool(true), "transfer", neoOwner, neoOwner, 1, nil)

pkg/core/dao/dao.go

@@ -684,8 +684,11 @@ func (dao *Simple) StoreHeaderHashes(hashes []util.Uint256, height uint32) error
 
 // HasTransaction returns nil if the given store does not contain the given
 // Transaction hash. It returns an error in case the transaction is in chain
-// or in the list of conflicting transactions.
+// or in the list of conflicting transactions. If non-zero signers are specified,
-func (dao *Simple) HasTransaction(hash util.Uint256) error {
+// then additional check against the conflicting transaction signers intersection
+// is held. Do not omit signers in case if it's important to check the validity
+// of a supposedly conflicting on-chain transaction.
+func (dao *Simple) HasTransaction(hash util.Uint256, signers []transaction.Signer) error {
 	key := dao.makeExecutableKey(hash)
 	bytes, err := dao.Store.Get(key)
 	if err != nil {
@@ -695,10 +698,33 @@ func (dao *Simple) HasTransaction(hash util.Uint256) error {
 	if len(bytes) < 6 {
 		return nil
 	}
-	if bytes[5] == transaction.DummyVersion {
+	if bytes[5] != transaction.DummyVersion {
+		return ErrAlreadyExists
+	}
+	if len(signers) == 0 {
 		return ErrHasConflicts
 	}
-	return ErrAlreadyExists
+
+	sMap := make(map[util.Uint160]struct{}, len(signers))
+	for _, s := range signers {
+		sMap[s.Account] = struct{}{}
+	}
+	br := io.NewBinReaderFromBuf(bytes[6:])
+	for {
+		var u util.Uint160
+		u.DecodeBinary(br)
+		if br.Err != nil {
+			if errors.Is(br.Err, iocore.EOF) {
+				break
+			}
+			return fmt.Errorf("failed to decode conflict record: %w", err)
+		}
+		if _, ok := sMap[u]; ok {
+			return ErrHasConflicts
+		}
+	}
+
+	return nil
 }
 
 // StoreAsBlock stores given block as DataBlock. It can reuse given buffer for
@@ -805,18 +831,50 @@ func (dao *Simple) StoreAsTransaction(tx *transaction.Transaction, index uint32,
 	}
 	dao.Store.Put(key, buf.Bytes())
 	if dao.Version.P2PSigExtensions {
-		var value []byte
+		var (
-		for _, attr := range tx.GetAttributes(transaction.ConflictsT) {
+			valuePrefix []byte
+			newSigners  []byte
+		)
+		attrs := tx.GetAttributes(transaction.ConflictsT)
+		for _, attr := range attrs {
 			hash := attr.Value.(*transaction.Conflicts).Hash
 			copy(key[1:], hash.BytesBE())
-			if value == nil {
+
-				buf.Reset()
+			old, err := dao.Store.Get(key)
-				buf.WriteB(storage.ExecTransaction)
+			if err != nil && !errors.Is(err, storage.ErrKeyNotFound) {
-				buf.WriteU32LE(index)
+				return fmt.Errorf("failed to retrieve previous conflict record for %s: %w", hash.StringLE(), err)
-				buf.BinWriter.WriteB(transaction.DummyVersion)
+			}
-				value = buf.Bytes()
+			if err == nil {
+				if len(old) <= 6 { // storage.ExecTransaction + U32LE index + transaction.DummyVersion
+					return fmt.Errorf("invalid conflict record format of length %d", len(old))
+				}
+			}
+			buf.Reset()
+			buf.WriteBytes(old)
+			if len(old) == 0 {
+				if len(valuePrefix) != 0 {
+					buf.WriteBytes(valuePrefix)
+				} else {
+					buf.WriteB(storage.ExecTransaction)
+					buf.WriteU32LE(index)
+					buf.WriteB(transaction.DummyVersion)
+				}
+			}
+			newSignersOffset := buf.Len()
+			if len(newSigners) == 0 {
+				for _, s := range tx.Signers {
+					s.Account.EncodeBinary(buf.BinWriter)
+				}
+			} else {
+				buf.WriteBytes(newSigners)
+			}
+			val := buf.Bytes()
+			dao.Store.Put(key, val)
+
+			if len(attrs) > 1 && len(valuePrefix) == 0 {
+				valuePrefix = slice.Copy(val[:6])
+				newSigners = slice.Copy(val[newSignersOffset:])
 			}
-			dao.Store.Put(key, value)
 		}
 	}
 	return nil

pkg/core/dao/dao_test.go

@@ -186,8 +186,8 @@ func TestStoreAsTransaction(t *testing.T) {
 		}
 		err := dao.StoreAsTransaction(tx, 0, aer)
 		require.NoError(t, err)
-		err = dao.HasTransaction(hash)
+		err = dao.HasTransaction(hash, nil)
-		require.NotNil(t, err)
+		require.ErrorIs(t, err, ErrAlreadyExists)
 		gotAppExecResult, err := dao.GetAppExecResults(hash, trigger.All)
 		require.NoError(t, err)
 		require.Equal(t, 1, len(gotAppExecResult))
@@ -197,34 +197,84 @@ func TestStoreAsTransaction(t *testing.T) {
 	t.Run("P2PSigExtensions on", func(t *testing.T) {
 		dao := NewSimple(storage.NewMemoryStore(), false, true)
 		conflictsH := util.Uint256{1, 2, 3}
-		tx := transaction.New([]byte{byte(opcode.PUSH1)}, 1)
+		signer1 := util.Uint160{1, 2, 3}
-		tx.Signers = append(tx.Signers, transaction.Signer{})
+		signer2 := util.Uint160{4, 5, 6}
-		tx.Scripts = append(tx.Scripts, transaction.Witness{})
+		signer3 := util.Uint160{7, 8, 9}
-		tx.Attributes = []transaction.Attribute{
+		signerMalicious := util.Uint160{10, 11, 12}
+		tx1 := transaction.New([]byte{byte(opcode.PUSH1)}, 1)
+		tx1.Signers = append(tx1.Signers, transaction.Signer{Account: signer1}, transaction.Signer{Account: signer2})
+		tx1.Scripts = append(tx1.Scripts, transaction.Witness{}, transaction.Witness{})
+		tx1.Attributes = []transaction.Attribute{
 			{
 				Type:  transaction.ConflictsT,
 				Value: &transaction.Conflicts{Hash: conflictsH},
 			},
 		}
-		hash := tx.Hash()
+		hash1 := tx1.Hash()
-		aer := &state.AppExecResult{
+		tx2 := transaction.New([]byte{byte(opcode.PUSH1)}, 1)
-			Container: hash,
+		tx2.Signers = append(tx2.Signers, transaction.Signer{Account: signer3})
+		tx2.Scripts = append(tx2.Scripts, transaction.Witness{})
+		tx2.Attributes = []transaction.Attribute{
+			{
+				Type:  transaction.ConflictsT,
+				Value: &transaction.Conflicts{Hash: conflictsH},
+			},
+		}
+		hash2 := tx2.Hash()
+		aer1 := &state.AppExecResult{
+			Container: hash1,
 			Execution: state.Execution{
 				Trigger: trigger.Application,
 				Events:  []state.NotificationEvent{},
 				Stack:   []stackitem.Item{},
 			},
 		}
-		err := dao.StoreAsTransaction(tx, 0, aer)
+		err := dao.StoreAsTransaction(tx1, 0, aer1)
 		require.NoError(t, err)
-		err = dao.HasTransaction(hash)
+		aer2 := &state.AppExecResult{
+			Container: hash2,
+			Execution: state.Execution{
+				Trigger: trigger.Application,
+				Events:  []state.NotificationEvent{},
+				Stack:   []stackitem.Item{},
+			},
+		}
+		err = dao.StoreAsTransaction(tx2, 0, aer2)
+		require.NoError(t, err)
+		err = dao.HasTransaction(hash1, nil)
 		require.ErrorIs(t, err, ErrAlreadyExists)
-		err = dao.HasTransaction(conflictsH)
+		err = dao.HasTransaction(hash2, nil)
+		require.ErrorIs(t, err, ErrAlreadyExists)
+
+		// Conflicts: unimportant payer.
+		err = dao.HasTransaction(conflictsH, nil)
 		require.ErrorIs(t, err, ErrHasConflicts)
-		gotAppExecResult, err := dao.GetAppExecResults(hash, trigger.All)
+
+		// Conflicts: payer is important, conflict isn't malicious, test signer #1.
+		err = dao.HasTransaction(conflictsH, []transaction.Signer{{Account: signer1}})
+		require.ErrorIs(t, err, ErrHasConflicts)
+
+		// Conflicts: payer is important, conflict isn't malicious, test signer #2.
+		err = dao.HasTransaction(conflictsH, []transaction.Signer{{Account: signer2}})
+		require.ErrorIs(t, err, ErrHasConflicts)
+
+		// Conflicts: payer is important, conflict isn't malicious, test signer #3.
+		err = dao.HasTransaction(conflictsH, []transaction.Signer{{Account: signer3}})
+		require.ErrorIs(t, err, ErrHasConflicts)
+
+		// Conflicts: payer is important, conflict is malicious.
+		err = dao.HasTransaction(conflictsH, []transaction.Signer{{Account: signerMalicious}})
+		require.NoError(t, err)
+
+		gotAppExecResult, err := dao.GetAppExecResults(hash1, trigger.All)
 		require.NoError(t, err)
 		require.Equal(t, 1, len(gotAppExecResult))
-		require.Equal(t, *aer, gotAppExecResult[0])
+		require.Equal(t, *aer1, gotAppExecResult[0])
+
+		gotAppExecResult, err = dao.GetAppExecResults(hash2, trigger.All)
+		require.NoError(t, err)
+		require.Equal(t, 1, len(gotAppExecResult))
+		require.Equal(t, *aer2, gotAppExecResult[0])
 	})
 }
 

pkg/core/mempool/mem_pool.go

@@ -536,17 +536,31 @@ func (mp *Pool) checkTxConflicts(tx *transaction.Transaction, fee Feer) ([]*tran
 			}
 		}
 		// Step 2: check if mempooled transactions were in `tx`'s attributes.
-		for _, attr := range tx.GetAttributes(transaction.ConflictsT) {
+		conflictsAttrs := tx.GetAttributes(transaction.ConflictsT)
-			hash := attr.Value.(*transaction.Conflicts).Hash
+		if len(conflictsAttrs) != 0 {
-			existingTx, ok := mp.verifiedMap[hash]
+			txSigners := make(map[util.Uint160]struct{}, len(tx.Signers))
-			if !ok {
+			for _, s := range tx.Signers {
-				continue
+				txSigners[s.Account] = struct{}{}
 			}
-			if !tx.HasSigner(existingTx.Signers[mp.payerIndex].Account) {
+			for _, attr := range conflictsAttrs {
-				return nil, fmt.Errorf("%w: not signed by the sender of conflicting transaction %s", ErrConflictsAttribute, existingTx.Hash().StringBE())
+				hash := attr.Value.(*transaction.Conflicts).Hash
+				existingTx, ok := mp.verifiedMap[hash]
+				if !ok {
+					continue
+				}
+				var signerOK bool
+				for _, s := range existingTx.Signers {
+					if _, ok := txSigners[s.Account]; ok {
+						signerOK = true
+						break
+					}
+				}
+				if !signerOK {
+					return nil, fmt.Errorf("%w: not signed by a signer of conflicting transaction %s", ErrConflictsAttribute, existingTx.Hash().StringBE())
+				}
+				conflictingFee += existingTx.NetworkFee
+				conflictsToBeRemoved = append(conflictsToBeRemoved, existingTx)
 			}
-			conflictingFee += existingTx.NetworkFee
-			conflictsToBeRemoved = append(conflictsToBeRemoved, existingTx)
 		}
 		if conflictingFee != 0 && tx.NetworkFee <= conflictingFee {
 			return nil, fmt.Errorf("%w: conflicting transactions have bigger or equal network fee: %d vs %d", ErrConflictsAttribute, tx.NetworkFee, conflictingFee)