[UTXO-BUG] HIGH-2: Unbounded mining_reward — no output cap on minting transactions

When tx_type='mining_reward' and inputs=[], the conservation check is
completely skipped (line 347: 'if inputs and ...' is falsy). Any code
that can call apply_transaction() with tx_type='mining_reward' can
mint arbitrary amounts with no upper bound.

Fix: add MAX_COINBASE_OUTPUT_NRTC = 150 * UNIT (1.5 RTC, matching the
block reward) and reject minting transactions whose output_total
exceeds this cap.

Tests added:
- test_mining_reward_at_cap_allowed (boundary)
- test_mining_reward_over_cap_rejected (demonstrates the bug)

Existing test updated:
- test_mempool_block_candidates: lowered coinbase from 200 to 120 RTC
  to stay within the new cap

All 36 tests pass.

Bounty: #2819 (High, 100 RTC)

Author: ArokyaMatthew

node/test_utxo_db.py

@@ -13,7 +13,7 @@
 
 from utxo_db import (
     UtxoDB, coin_select, compute_box_id, address_to_proposition,
-    proposition_to_address, UNIT, DUST_THRESHOLD,
+    proposition_to_address, UNIT, DUST_THRESHOLD, MAX_COINBASE_OUTPUT_NRTC,
 )
 
 
@@ -317,7 +317,7 @@ def test_mempool_add_and_remove(self):
 
     def test_mempool_block_candidates(self):
         self._apply_coinbase('alice', 100 * UNIT, block_height=1)
-        self._apply_coinbase('alice', 200 * UNIT, block_height=2)
+        self._apply_coinbase('alice', 120 * UNIT, block_height=2)
         boxes = self.db.get_unspent_for_address('alice')
 
         # Add two txs with different fees (outputs + fee <= inputs)
@@ -330,7 +330,7 @@ def test_mempool_block_candidates(self):
         self.db.mempool_add({
             'tx_id': 'high' * 16,
             'inputs': [{'box_id': boxes[1]['box_id']}],
-            'outputs': [{'address': 'bob', 'value_nrtc': 200 * UNIT - 5000}],
+            'outputs': [{'address': 'bob', 'value_nrtc': 120 * UNIT - 5000}],
             'fee_nrtc': 5000,
         })
 
@@ -445,6 +445,27 @@ def test_spending_proof_accepted_without_verification(self):
         self.assertTrue(ok, "UTXO layer should accept any spending_proof "
                             "(verification is endpoint's job)")
 
+    def test_mining_reward_at_cap_allowed(self):
+        """Mining reward exactly at MAX_COINBASE_OUTPUT_NRTC must succeed."""
+        ok = self._apply_coinbase('miner', MAX_COINBASE_OUTPUT_NRTC)
+        self.assertTrue(ok)
+        self.assertEqual(self.db.get_balance('miner'), MAX_COINBASE_OUTPUT_NRTC)
+
+    def test_mining_reward_over_cap_rejected(self):
+        """Mining reward exceeding MAX_COINBASE_OUTPUT_NRTC must be rejected.
+        Without this, any caller that passes tx_type='mining_reward' can
+        mint unlimited funds (bounty #2819 HIGH-2)."""
+        ok = self.db.apply_transaction({
+            'tx_type': 'mining_reward',
+            'inputs': [],
+            'outputs': [{'address': 'attacker',
+                         'value_nrtc': MAX_COINBASE_OUTPUT_NRTC + 1}],
+            'fee_nrtc': 0,
+            'timestamp': int(time.time()),
+        }, block_height=10)
+        self.assertFalse(ok)
+        self.assertEqual(self.db.get_balance('attacker'), 0)
+
     def test_mempool_empty_inputs_rejected_for_transfer(self):
         """Mempool must also reject non-minting txs with empty inputs."""
         tx = {

node/utxo_db.py

@@ -27,6 +27,7 @@
 
 UNIT = 100_000_000          # 1 RTC = 100,000,000 nanoRTC (8 decimals)
 DUST_THRESHOLD = 1_000      # nanoRTC below which change is absorbed into fee
+MAX_COINBASE_OUTPUT_NRTC = 150 * UNIT  # Max minting output per block (1.5 RTC)
 MAX_POOL_SIZE = 10_000
 MAX_TX_AGE_SECONDS = 3_600  # 1 hour mempool expiry
 P2PK_PREFIX = b'\x00\x08'   # Pay-to-Public-Key proposition prefix
@@ -397,6 +398,13 @@ def apply_transaction(self, tx: dict, block_height: int,
                     conn.execute("ROLLBACK")
                     return False
 
+            # Cap minting (coinbase) output to prevent unbounded fund creation.
+            # Without this, any caller that passes tx_type='mining_reward'
+            # can mint arbitrary amounts.
+            if tx_type in MINTING_TX_TYPES and output_total > MAX_COINBASE_OUTPUT_NRTC:
+                conn.execute("ROLLBACK")
+                return False
+
             if fee < 0:
                 conn.execute("ROLLBACK")
                 return False