Some protocols, such as older implementations of Siemens S7Comm or Modbus TCP, do not require authentication handshakes. An attacker can simply request the project file or memory contents without a password. Here, the "key" is simply network access.
For legacy devices (Siemens S7-300, older Mitsubishi FX), holding a specific key combination (e.g., MRES + Stop) resets the CPU to factory. This deletes the program. Only use this if you have a backup. all plc hmi password key
Cons / Risks