white paper on industrial automation security in fieldbus and

20When the bus system starts, masters whichperform cyclical data exchange initialize theslaves which are assigned to them. Thisinitialization consists of a parameterization and aconfiguration step, in which the PROFIBUS DPparameters as well as possible vendor- ordevice-specific parameters are set in the slave.Additionally, the device number is verifiedagainst that which the master expects, to ensurethat the correct type of device exists at thecorrect address. In the configuration step, thelength and structure of the periodicallytransmitted I/O data is set, as defined in themaster setup. Both the parameterization andconfiguration procedures can be accepted orrejected by the slave. Each slave keeps track ofwhich master address configured them for dataexchange.To avoid the conflicting situation in whichdifferent masters try to command the same slave,it is possible to lock slaves to a single class 1master (the master which performs theinitialization procedure described above). It is,however, also possible that a slave is not locked,which would mean that a slave could be claimedby different masters. This is nevertheless morean issue of network management, i.e. ensuringthat slaves are locked to their primary master.to read the Identification & Maintenance (I&M)information from bus devices. The I&M is astructure of device identification informationwhich at a minimum includes the I&M0information, but optionally also other I&Mfields (see the table below) This kind ofinformation about a device (which can beidentified using its slave number in the bus) canreveal what the device is doing, the device typeand give clues as to how its behavior could becompromised. The I&M information may bechangeable by the owner or operator of a device,so in case this information is not write-protected,an intruder may attempt to change theinformation, e.g. change the text describingfunction, task, location or installation date sothat identification of the device istampered.Table 3. The I&M information inPROFIBUS and PROFINET devices.The PROFIBUS DP-V1 extension specifies anacyclic communication which can be used asneeded to e.g. read or write variables orparameters of a device, if it implements somedevice profile such as PROFIdrive or encoderprofile. A master class 2 can perform read and/orwrite operations targeted at a PROFIBUS DPslave device independent of the slave’srelationship to its primary master. This presentsrisk in case the master class 2 can modifyparameters which affect the operation of thedevice.If a device gains access to the bus, and is able toperform DP-V1 functionality, then it can attempt