fix: punish invalid dstx messages

src/net_processing.cpp

@@ -3583,7 +3583,7 @@ std::pair<bool /*ret*/, bool /*do_return*/> static ValidateDSTX(CDeterministicMN
 
     if (!dmn) {
         LogPrint(BCLog::COINJOIN, "DSTX -- Can't find masternode %s to verify %s\n", dstx.masternodeOutpoint.ToStringShort(), hashTx.ToString());
-        return {false, true};
+        return {true, true};
     }
 
     if (!mn_metaman.IsValidForMixingTxes(dmn->proTxHash)) {
@@ -4666,12 +4666,15 @@ void PeerManagerImpl::ProcessMessage(
 
         // Process custom logic, no matter if tx will be accepted to mempool later or not
         if (nInvType == MSG_DSTX) {
-           // Validate DSTX and return bRet if we need to return from here
-           uint256 hashTx = tx.GetHash();
-           const auto& [bRet, bDoReturn] = ValidateDSTX(*m_dmnman, m_dstxman, m_chainman, m_mn_metaman, m_mempool, dstx, hashTx);
-           if (bDoReturn) {
-               return;
-           }
+            // Validate DSTX and return bRet if we need to return from here
+            uint256 hashTx = tx.GetHash();
+            const auto& [bRet, bDoReturn] = ValidateDSTX(*m_dmnman, m_dstxman, m_chainman, m_mn_metaman, m_mempool, dstx, hashTx);
+            if (bDoReturn) {
+                if (!bRet) {
+                    Misbehaving(pfrom.GetId(), 10, "invalid dstx");
+                }
+                return;
+            }
         }
 
         LOCK(cs_main);

test/functional/p2p_dstx.py

@@ -0,0 +1,71 @@
+#!/usr/bin/env python3
+# Copyright (c) 2026 The Dash Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+"""Test P2P CoinJoin broadcast transaction handling."""
+
+import time
+
+from test_framework.messages import (
+    CCoinJoinBroadcastTx,
+    COIN,
+    COutPoint,
+    CTransaction,
+    CTxIn,
+    CTxOut,
+    msg_dstx,
+)
+from test_framework.p2p import P2PInterface
+from test_framework.script import (
+    CScript,
+    OP_CHECKSIG,
+    OP_DUP,
+    OP_EQUALVERIFY,
+    OP_HASH160,
+)
+from test_framework.test_framework import BitcoinTestFramework
+
+
+class P2PDSTXTest(BitcoinTestFramework):
+    def set_test_params(self):
+        self.num_nodes = 1
+        self.setup_clean_chain = True
+        self.extra_args = [["-debug=net"]]
+
+    def make_dstx(self):
+        tx = CTransaction()
+        tx.vin = [CTxIn(COutPoint(hash=i + 1, n=0)) for i in range(3)]
+        p2pkh = CScript([
+            OP_DUP,
+            OP_HASH160,
+            b"\x01" * 20,
+            OP_EQUALVERIFY,
+            OP_CHECKSIG,
+        ])
+        tx.vout = [CTxOut(nValue=COIN // 1000 + 1, scriptPubKey=p2pkh) for _ in tx.vin]
+        return CCoinJoinBroadcastTx(
+            tx=tx,
+            m_protxHash=1,
+            vchSig=b"\x01" * 96,
+            sigTime=int(time.time()),
+        )
+
+    def run_test(self):
+        self.log.info("Leave IBD so unsolicited DSTX is processed")
+        self.generate(self.nodes[0], 1)
+
+        peer = self.nodes[0].add_p2p_connection(P2PInterface())
+
+        self.log.info("Unknown masternode DSTX is ignored without peer misbehavior")
+        with self.nodes[0].assert_debug_log([], unexpected_msgs=["Misbehaving", "invalid dstx"]):
+            peer.send_and_ping(msg_dstx(self.make_dstx()))
+
+        self.log.info("Structurally invalid DSTX is treated as peer misbehavior")
+        invalid_dstx = self.make_dstx()
+        invalid_dstx.tx.vout.pop()
+        with self.nodes[0].assert_debug_log(["Misbehaving", "invalid dstx"]):
+            peer.send_and_ping(msg_dstx(invalid_dstx))
+
+
+if __name__ == '__main__':
+    P2PDSTXTest().main()

test/functional/test_framework/messages.py

@@ -1835,6 +1835,53 @@ def __repr__(self):
         return "msg_tx(tx=%s)" % (repr(self.tx))
 
 
+class CCoinJoinBroadcastTx:
+    __slots__ = ("tx", "m_protxHash", "vchSig", "sigTime")
+
+    def __init__(self, tx=None, m_protxHash=0, vchSig=b"", sigTime=0):
+        self.tx = tx or CTransaction()
+        self.m_protxHash = m_protxHash
+        self.vchSig = vchSig
+        self.sigTime = sigTime
+
+    def deserialize(self, f):
+        self.tx = CTransaction()
+        self.tx.deserialize(f)
+        self.m_protxHash = deser_uint256(f)
+        self.vchSig = deser_string(f)
+        self.sigTime = struct.unpack("<q", f.read(8))[0]
+
+    def serialize(self):
+        r = b""
+        r += self.tx.serialize()
+        r += ser_uint256(self.m_protxHash)
+        r += ser_string(self.vchSig)
+        r += struct.pack("<q", self.sigTime)
+        return r
+
+    def __repr__(self):
+        return "CCoinJoinBroadcastTx(tx=%s m_protxHash=%064x)" \
+               % (repr(self.tx), self.m_protxHash)
+
+
+class msg_dstx:
+    __slots__ = ("dstx",)
+    msgtype = b"dstx"
+
+    def __init__(self, dstx=None):
+        self.dstx = dstx or CCoinJoinBroadcastTx()
+
+    def deserialize(self, f):
+        self.dstx = CCoinJoinBroadcastTx()
+        self.dstx.deserialize(f)
+
+    def serialize(self):
+        return self.dstx.serialize()
+
+    def __repr__(self):
+        return "msg_dstx(dstx=%s)" % (repr(self.dstx))
+
+
 class msg_block:
     __slots__ = ("block",)
     msgtype = b"block"

test/functional/test_runner.py

@@ -258,6 +258,7 @@
     'p2p_nobloomfilter_messages.py',
     'p2p_filter.py',
     'p2p_blocksonly.py',
+    'p2p_dstx.py',
     'rpc_setban.py --v1transport',
     'rpc_setban.py --v2transport',
     'mining_prioritisetransaction.py',