2026 dont backdate locktime rbf

src/wallet/coincontrol.h

@@ -116,6 +116,8 @@ class CCoinControl
     uint32_t m_version = DEFAULT_WALLET_TX_VERSION;
     //! Locktime
     std::optional<uint32_t> m_locktime;
+    //! Save the previous locktime for replacements
+    std::optional<uint32_t> m_previous_locktime;
     //! Caps weight of resulting tx
     std::optional<int> m_max_tx_weight{std::nullopt};
 

src/wallet/feebumper.cpp

@@ -312,6 +312,10 @@ Result CreateRateBumpTransaction(CWallet& wallet, const Txid& txid, const CCoinC
 
     // We cannot source new unconfirmed inputs(bip125 rule 2)
     new_coin_control.m_min_depth = 1;
+    // If no locktime is set, we save the previous one for anti fee sniping
+    if (!new_coin_control.m_locktime){
+        new_coin_control.m_previous_locktime = wtx.tx->nLockTime;
+    }
 
     auto res = CreateTransaction(wallet, recipients, /*change_pos=*/std::nullopt, new_coin_control, false);
     if (!res) {

src/wallet/spend.cpp

@@ -1016,14 +1016,23 @@ void DiscourageFeeSniping(CMutableTransaction& tx, FastRandomContext& rng_fast,
     // now we ensure code won't be written that makes assumptions about
     // nLockTime that preclude a fix later.
     if (IsCurrentForAntiFeeSniping(chain, block_hash)) {
+        uint32_t previous_locktime = tx.nLockTime;
         tx.nLockTime = block_height;
 
         // Secondly occasionally randomly pick a nLockTime even further back, so
         // that transactions that are delayed after signing for whatever reason,
         // e.g. high-latency mix networks and some CoinJoin implementations, have
         // better privacy.
         if (rng_fast.randrange(10) == 0) {
-            tx.nLockTime = std::max(0, int(tx.nLockTime) - int(rng_fast.randrange(100)));
+            // If a previous lockitme is passed (like in the bump fee case), the
+            // backdating is limited betwen the current height and the previous lockitme
+            if (previous_locktime > 0){
+                // To avoid issues with the randrange, locktime_range cannot be 0
+                int locktime_range = std::max(1, std::min(100, int(block_height - previous_locktime)));
+                tx.nLockTime = std::max(int(previous_locktime), int(tx.nLockTime) - int(rng_fast.randrange(locktime_range)));
+            }else{
+                tx.nLockTime = std::max(0, int(tx.nLockTime) - int(rng_fast.randrange(100)));
+            }
         }
     } else {
         // If our chain is lagging behind, we can't discourage fee sniping nor help
@@ -1317,10 +1326,13 @@ static util::Result<CreatedTransactionResult> CreateTransactionInternal(
             txNew.vin.back().scriptWitness = *scripts.second;
         }
     }
+
     if (coin_control.m_locktime) {
         txNew.nLockTime = coin_control.m_locktime.value();
         // If we have a locktime set, we can't use anti-fee-sniping
         use_anti_fee_sniping = false;
+    } else if (coin_control.m_previous_locktime.has_value()) {
+            txNew.nLockTime = coin_control.m_previous_locktime.value();
     }
     if (use_anti_fee_sniping) {
         DiscourageFeeSniping(txNew, rng_fast, wallet.chain(), wallet.GetLastBlockHash(), wallet.GetLastBlockHeight());

test/functional/wallet_bumpfee.py

@@ -30,6 +30,7 @@
     assert_raises_rpc_error,
     get_fee,
     find_vout_for_address,
+    assert_greater_than_or_equal
 )
 from test_framework.wallet import MiniWallet
 
@@ -114,6 +115,7 @@ def run_test(self):
         # Context independent tests
         test_feerate_checks_replaced_outputs(self, rbf_node, peer_node)
         test_bumpfee_with_feerate_ignores_walletincrementalrelayfee(self, rbf_node, peer_node)
+        test_bumpfee_dont_backdate(self, rbf_node)
 
     def test_invalid_parameters(self, rbf_node, peer_node, dest_address):
         self.log.info('Test invalid parameters')
@@ -828,6 +830,37 @@ def test_bumpfee_with_feerate_ignores_walletincrementalrelayfee(self, rbf_node,
     rbf_node.bumpfee(tx["txid"], {"fee_rate": 2.1})
     self.clear_mempool()
 
+def test_bumpfee_dont_backdate(self, rbf_node):
+    self.log.info('Test to check bumpfee do not backdate the locktime')
+
+    rbf_node.createwallet("backdating_rbf")
+    wallet = rbf_node.get_wallet_rpc("backdating_rbf")
+    self.generatetoaddress(rbf_node, 101, wallet.getnewaddress())
+
+    current_height = rbf_node.getblockchaininfo()["blocks"]
+    # The original tx has an older locktime so we can differentiate when the replacement backdates
+    original_locktime = current_height - 50
+    replaced_locktime = current_height
+
+    # Exti the loop when the locktime backdates
+    while current_height == replaced_locktime:
+        # Original transaction with locktime at the current height
+        tx = wallet.send(outputs={wallet.getnewaddress(): 9}, fee_rate=2, locktime=original_locktime)
+
+        # Replacement with higher fee_rate
+        change_addr = get_change_address(tx["txid"], wallet)[0]
+        bumped = wallet.bumpfee(txid=tx["txid"], options={"fee_rate":5, "outputs": [{change_addr: 10}]})
+
+        replaced_locktime = rbf_node.getrawtransaction(bumped["txid"],True)["locktime"]
+
+        # Clear mempool to create another transaction to replace
+        self.clear_mempool()
+
+    # Even when backdating, the replaced tx must always have locktime >= than the original tx
+    assert_greater_than_or_equal(replaced_locktime, original_locktime)
+
+    wallet.unloadwallet()
+
 
 if __name__ == "__main__":
     BumpFeeTest(__file__).main()