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Implemented fix proposed by Kevalz95, Issue #564, in modbus_server.py #613

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Implemented fix proposed by Kevalz95, Issue #564, in modbus_server.py #613

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Implemented fix proposed by Kevalz95, Issue #564, in modbus_server.py
MotherFraggr Apr 23, 2026
452cf4e
Reformat modbus_server.py per Black code check
MotherFraggr May 5, 2026
2bfb2d2
Reordered length check logic
MotherFraggr Jun 30, 2026
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16 changes: 16 additions & 0 deletions conpot/protocols/modbus/modbus_server.py
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Original file line number Diff line number Diff line change
Expand Up @@ -134,10 +134,26 @@ 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:
# 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):

@glaslos glaslos Jun 7, 2026

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this will be always true on the second iteration.
Check request length before and after recv()

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
except Exception:
break
query = modbus_tcp.TcpQuery()
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