When the function code received in a MODBUS request is not supported by a slave device, the device generates an Exception response with an exception code of 0x01. Frequent Invalid Function Exception responses can indicate attempted reconnaissance of the device.This signature will alert if 3 exception responses are seen in a short period time between the same devices.You are entitled to use this signature only if you have purchased an IPS SCADA Signature License.
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A possible benign trigger is when a MODBUS master is mis-configured or designed to use functionality not supported by a particular device queries the slave for the unsupported function code at a high frequency.
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