diff --git a/.gitignore b/.gitignore
index b8edfb6..5083c09 100644
--- a/.gitignore
+++ b/.gitignore
@@ -332,3 +332,4 @@ __pycache__/
 *.so
 *bore.zip
 *bore_bin
+.venv/
diff --git a/minichain/state.py b/minichain/state.py
index 13c7c02..60c3cab 100644
--- a/minichain/state.py
+++ b/minichain/state.py
@@ -42,6 +42,14 @@ def get_account(self, address):
             }
         return self.accounts[address]
 
+    def _validate_fee(self, fee):
+        """
+        Shared fee validation used by both verify_transaction_logic and
+        validate_and_apply, so the rule lives in exactly one place.
+        Uses exact type check to reject bool (since bool is a subclass of int).
+        """
+        return type(fee) is int and fee >= 0
+
     def verify_transaction_logic(self, tx):
         if not tx.verify():
             logger.error("Error: Invalid signature for tx from %s...", tx.sender[:8])
@@ -51,9 +59,14 @@ def verify_transaction_logic(self, tx):
             logger.error("Error: Invalid chain_id in tx from %s...", tx.sender[:8])
             return False
 
+        fee = getattr(tx, 'fee', 0)
+        if not self._validate_fee(fee):
+            logger.error("Error: Invalid fee in tx from %s...", tx.sender[:8])
+            return False
+
         sender_acc = self.get_account(tx.sender)
 
-        total_cost = tx.amount + getattr(tx, 'fee', 0)
+        total_cost = tx.amount + fee
         if sender_acc['balance'] < total_cost:
             logger.warning("Invalid tx %s: insufficient balance", tx.tx_id)
             return False
@@ -93,9 +106,16 @@ def validate_and_apply(self, tx):
         semantic validation entry points.
         """
         # Semantic validation: amount must be an integer and non-negative
-        if not isinstance(tx.amount, int) or tx.amount < 0:
+        if type(tx.amount) is not int or tx.amount < 0:
             return None
-        # Further checks can be added here
+
+        fee = getattr(tx, 'fee', 0)
+        if not self._validate_fee(fee):
+            return None
+
+        if type(tx.nonce) is not int or tx.nonce < 0:
+            return None
+
         return self.apply_transaction(tx)
 
     def apply_transaction(self, tx):
@@ -109,7 +129,7 @@ def apply_transaction(self, tx):
         sender = self.accounts[tx.sender]
 
         total_cost = tx.amount + getattr(tx, 'fee', 0)
-        
+
         # Deduct funds and increment nonce
         sender['balance'] -= total_cost
         sender['nonce'] += 1
@@ -174,7 +194,7 @@ def apply_transaction(self, tx):
 
             # Execution & transfers valid: commit state changes atomically
             self.update_contract_storage(tx.receiver, result["storage"])
-            
+
             receiver['balance'] -= total_transferred_out
             for t in transfers:
                 target_acc = self.get_account(t["to"])
@@ -217,4 +237,4 @@ def update_contract_storage_partial(self, address, updates):
     def credit_mining_reward(self, miner_address, reward=None):
         reward = reward if reward is not None else self.DEFAULT_MINING_REWARD
         account = self.get_account(miner_address)
-        account['balance'] += reward
+        account['balance'] += reward
\ No newline at end of file
diff --git a/tests/test_negative_fee.py b/tests/test_negative_fee.py
new file mode 100644
index 0000000..d272bb8
--- /dev/null
+++ b/tests/test_negative_fee.py
@@ -0,0 +1,136 @@
+import unittest
+from nacl.signing import SigningKey
+from nacl.encoding import HexEncoder
+
+from minichain import State, Transaction
+
+
+class TestNegativeFeePrevention(unittest.TestCase):
+    def setUp(self):
+        self.state = State()
+        self.state.chain_id = "minichain-default"
+
+        # Setup Alice with a small balance
+        self.alice_sk = SigningKey.generate()
+        self.alice_pk = self.alice_sk.verify_key.encode(encoder=HexEncoder).decode()
+        self.state.credit_mining_reward(self.alice_pk, 10)
+
+        self.bob_pk = "b" * 64
+
+    def _make_tx(self, amount, fee, nonce=0):
+        tx = Transaction(
+            sender=self.alice_pk,
+            receiver=self.bob_pk,
+            amount=amount,
+            nonce=nonce,
+            fee=fee,
+            chain_id="minichain-default",
+        )
+        tx.sign(self.alice_sk)
+        return tx
+
+    # ------------------------------------------------------------------
+    # Core exploit scenario
+    # ------------------------------------------------------------------
+
+    def test_negative_fee_is_rejected(self):
+        """Negative fee MUST be rejected; balance must not be inflated."""
+        initial_balance = self.state.get_account(self.alice_pk)["balance"]
+
+        tx = self._make_tx(amount=0, fee=-1000)
+        receipt = self.state.validate_and_apply(tx)
+
+        self.assertIsNone(receipt, "validate_and_apply should return None for negative fee")
+        self.assertEqual(
+            self.state.get_account(self.alice_pk)["balance"],
+            initial_balance,
+            "Alice's balance must remain unchanged after a rejected negative-fee tx",
+        )
+
+    def test_negative_fee_does_not_change_nonce(self):
+        """Rejected tx must not increment the sender nonce."""
+        initial_nonce = self.state.get_account(self.alice_pk)["nonce"]
+        tx = self._make_tx(amount=0, fee=-1)
+        self.state.validate_and_apply(tx)
+        self.assertEqual(
+            self.state.get_account(self.alice_pk)["nonce"],
+            initial_nonce,
+            "Nonce must not be incremented for rejected transactions",
+        )
+
+    def test_large_negative_fee_is_rejected(self):
+        """Even a very large negative fee (millions of coins) must be rejected."""
+        tx = self._make_tx(amount=0, fee=-10_000_000)
+        receipt = self.state.validate_and_apply(tx)
+        self.assertIsNone(receipt)
+        self.assertEqual(self.state.get_account(self.alice_pk)["balance"], 10)
+
+    def test_fee_of_zero_is_accepted(self):
+        """Fee of exactly 0 is valid and should be accepted."""
+        tx = self._make_tx(amount=5, fee=0)
+        receipt = self.state.validate_and_apply(tx)
+        self.assertIsNotNone(receipt, "Fee of 0 should be accepted")
+        self.assertEqual(receipt.status, 1)
+
+    def test_positive_fee_is_accepted(self):
+        """A normal positive fee should be accepted and correctly deducted."""
+        tx = self._make_tx(amount=3, fee=2)
+        receipt = self.state.validate_and_apply(tx)
+        self.assertIsNotNone(receipt, "Positive fee should be accepted")
+        self.assertEqual(receipt.status, 1)
+        self.assertEqual(self.state.get_account(self.alice_pk)["balance"], 5)
+
+    def test_float_fee_is_rejected(self):
+        """Float fees (e.g. -0.5, 1.5) must be rejected as non-integer."""
+        for bad_fee in [-0.5, 1.5, 0.1]:
+            with self.subTest(fee=bad_fee):
+                tx = Transaction(
+                    sender=self.alice_pk,
+                    receiver=self.bob_pk,
+                    amount=0,
+                    nonce=0,
+                    fee=bad_fee,
+                    chain_id="minichain-default",
+                )
+                tx.sign(self.alice_sk)
+                receipt = self.state.validate_and_apply(tx)
+                self.assertIsNone(receipt, f"Float fee {bad_fee} should be rejected")
+
+    def test_negative_nonce_is_rejected(self):
+        """A negative nonce must also be rejected."""
+        tx = Transaction(
+            sender=self.alice_pk,
+            receiver=self.bob_pk,
+            amount=0,
+            nonce=-1,
+            fee=0,
+            chain_id="minichain-default",
+        )
+        tx.sign(self.alice_sk)
+        receipt = self.state.validate_and_apply(tx)
+        self.assertIsNone(receipt, "Negative nonce should be rejected")
+
+    def test_bool_fee_is_rejected(self):
+        """bool is a subclass of int in Python; True/False must not pass as fee."""
+        for bad_fee in [True, False]:
+            with self.subTest(fee=bad_fee):
+                tx = self._make_tx(amount=0, fee=bad_fee)
+                receipt = self.state.validate_and_apply(tx)
+                self.assertIsNone(receipt, f"bool fee {bad_fee} should be rejected")
+
+    def test_apply_transaction_directly_rejects_negative_fee(self):
+        """Even when validate_and_apply is bypassed, apply_transaction's
+        underlying verify_transaction_logic must reject negative fees."""
+        initial_balance = self.state.get_account(self.alice_pk)["balance"]
+        tx = self._make_tx(amount=0, fee=-1000)
+        receipt = self.state.apply_transaction(tx)
+        self.assertIsNone(receipt, "apply_transaction must reject negative fee directly")
+        self.assertEqual(
+            self.state.get_account(self.alice_pk)["balance"],
+            initial_balance,
+            "Balance must not inflate when apply_transaction is called directly",
+        )
+
+
+if __name__ == "__main__":
+    unittest.main()
\ No newline at end of file