From 508603586c2cb5587dd32e61baf8ab55b56734dd Mon Sep 17 00:00:00 2001
From: MotherFraggr <32443585+MotherFraggr@users.noreply.github.com>
Date: Thu, 23 Apr 2026 15:02:14 -0400
Subject: [PATCH 1/3] Implemented fix proposed by Kevalz95, Issue #564, in
 modbus_server.py

---
 conpot/protocols/modbus/modbus_server.py | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/conpot/protocols/modbus/modbus_server.py b/conpot/protocols/modbus/modbus_server.py
index 5400b105..9edb73f7 100644
--- a/conpot/protocols/modbus/modbus_server.py
+++ b/conpot/protocols/modbus/modbus_server.py
@@ -134,10 +134,21 @@ def handle(self, sock, address):
                     session.add_event({"type": "CONNECTION_TERMINATED"})
                     break
                 _, _, length = struct.unpack(">HHH", request[:6])
+                previous_request = None
+                stall_counter = 0
+                #Implemented fix proposed by Kevalz95, Issue #564
+                #Adds stall counter to prevent infinite loop on malformed requests
                 while len(request) < (length + 6):
                     try:
+                        if previous_request and len(previous_request) == len(request):
+                            stall_counter += 1
+                            if stall_counter >= 1000:
+                                logger.info("Modbus client provided data {} but invalid.".format(session.id))
+                                session.add_event({"type": "CONNECTION_TERMINATED"})
+                                break
                         new_byte = sock.recv(1)
                         request += new_byte
+                        previous_request = request
                     except Exception:
                         break
                 query = modbus_tcp.TcpQuery()

From 452cf4e85e2d61180a67dd2217e117165bdafad9 Mon Sep 17 00:00:00 2001
From: MotherFraggr <32443585+MotherFraggr@users.noreply.github.com>
Date: Tue, 5 May 2026 08:48:20 -0400
Subject: [PATCH 2/3] Reformat modbus_server.py per Black code check

---
 conpot/protocols/modbus/modbus_server.py | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/conpot/protocols/modbus/modbus_server.py b/conpot/protocols/modbus/modbus_server.py
index 9edb73f7..43979cb5 100644
--- a/conpot/protocols/modbus/modbus_server.py
+++ b/conpot/protocols/modbus/modbus_server.py
@@ -136,14 +136,18 @@ def handle(self, sock, address):
                 _, _, length = struct.unpack(">HHH", request[:6])
                 previous_request = None
                 stall_counter = 0
-                #Implemented fix proposed by Kevalz95, Issue #564
-                #Adds stall counter to prevent infinite loop on malformed requests
+                # Implemented fix proposed by Kevalz95, Issue #564
+                # Adds stall counter to prevent infinite loop on malformed requests
                 while len(request) < (length + 6):
                     try:
                         if previous_request and len(previous_request) == len(request):
                             stall_counter += 1
                             if stall_counter >= 1000:
-                                logger.info("Modbus client provided data {} but invalid.".format(session.id))
+                                logger.info(
+                                    "Modbus client provided data {} but invalid.".format(
+                                        session.id
+                                    )
+                                )
                                 session.add_event({"type": "CONNECTION_TERMINATED"})
                                 break
                         new_byte = sock.recv(1)

From 2bfb2d2fe153cbcccdc8be6c2c6e3e68836c2326 Mon Sep 17 00:00:00 2001
From: MotherFraggr <32443585+MotherFraggr@users.noreply.github.com>
Date: Tue, 30 Jun 2026 16:15:36 -0400
Subject: [PATCH 3/3] Reordered length check logic

Resolving "Check request length before and after recv()" PR review issue
---
 conpot/protocols/modbus/modbus_server.py | 21 +++++++++++----------
 1 file changed, 11 insertions(+), 10 deletions(-)

diff --git a/conpot/protocols/modbus/modbus_server.py b/conpot/protocols/modbus/modbus_server.py
index 43979cb5..e0929bc2 100644
--- a/conpot/protocols/modbus/modbus_server.py
+++ b/conpot/protocols/modbus/modbus_server.py
@@ -140,19 +140,20 @@ def handle(self, sock, address):
                 # Adds stall counter to prevent infinite loop on malformed requests
                 while len(request) < (length + 6):
                     try:
+                        # Previous request now assigned before recv() called, length check performed after
+                        previous_request = request
+                        new_byte = sock.recv(1)
+                        request += new_byte
                         if previous_request and len(previous_request) == len(request):
                             stall_counter += 1
-                            if stall_counter >= 1000:
-                                logger.info(
-                                    "Modbus client provided data {} but invalid.".format(
-                                        session.id
-                                    )
+                        if stall_counter >= 1000:
+                            logger.info(
+                                "Modbus client provided data {} but invalid.".format(
+                                    session.id
                                 )
-                                session.add_event({"type": "CONNECTION_TERMINATED"})
-                                break
-                        new_byte = sock.recv(1)
-                        request += new_byte
-                        previous_request = request
+                            )
+                            session.add_event({"type": "CONNECTION_TERMINATED"})
+                            break
                     except Exception:
                         break
                 query = modbus_tcp.TcpQuery()