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No digital contagion has been fingered in the latest incident, said Terry Johnson, spokesman for the Tennessee Valley Authority, the public power company that runs the Browns Ferry power plant.

"The integrated control system (ICS) network is not connected to the network outside the plant, but it is connected to a very large number of controllers and devices in the plant," Johnson said. "You can end up with a lot of information, and it appears to be more than it could handle."

The device responsible for flooding the network with data appears to be a programmable logic controller (PLC) connected to the plant's Ethernet network, according to an NRC information notice on the incident (PDF). The PLC controlled Unit 3's condensate demineralizer - essentially a water softener for nuclear plants. The flood of data spewed out by the malfunctioning controller caused the variable frequency drive (VFD) controllers for the recirculation pumps to hang.

Such failures are common among PLC and supervisory control and data acquisition (SCADA) systems, because the manufacturers do not test the devices' handling of bad data, said Dale Peterson, CEO of industrial system security firm DigitalBond.

"What is happening in this marketplace is that vendors will build their own (network) stacks to make it cheaper," Peterson said. "And it works, but when (the device) gets anything that it didn't expect, it will gag."

In many cases, a simple vulnerability scan will even cause the devices to crash, Peterson said. During tests in an electrical substation, Nessus running in safe scan mode crashed devices, he said. In some cases, sending out broadcast data on the network will crash several of connected devices, he added.

"If you were to test any control systems that have any more than three or four different network-connected devices, they could be knocked over very easily," Peterson said.

The Browns Ferry nuclear power plant has had its share of difficulties. All three units of the plant were shutdown in 1985 due to performance and management problems, according to the NRC. Unit 2 was restarted in 1991, and Unit 3 started operating again in 1995. On Tuesday, the NRC gave the Tennessee Valley Authority permission to restart Unit 1.

The Committee on Homeland Security gave the NRC until 14 June to respond to its letter.

This article originally appeared in Security Focus.

Copyright © 2007, SecurityFocus

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