A Siemens S7-1500 Logic Controller Flaw Raises the Specter of Stuxnet

“This separate crypto core is a very rudimentary chip. It’s not like a big processor, so it doesn’t really know who it’s talking to or what’s going on in the broader context,” Red Balloon’s Skipper says. “So if you can tell it the right things that you observed the processor telling it, it will talk to you as if you are the processor. So we can get in between the processor and the crypto core and then we basically tell it, ‘Hey, we are the processor and we are going to give you some data and we want you to encrypt it.’ And the little crypto core isn’t going to question that. It just does it.”

Siemens notes that the vulnerabilities are not related to the company’s own firmware update process and do not give attackers the ability to hijack that distribution channel. But the fact that any S7-1500 can become a firmware-blessing oracle is significant and bestows a power that individual devices should not have, undermining the whole purpose of encrypting the firmware in the first place.

“S7s should not be able to re-encrypt firmware for other S7s,” says Ang Cui, Red Balloon Security’s founder and CEO. “This is a fundamental design flaw and a significant implementation error.”

While Siemens isn’t directly releasing any fixes for the vulnerability, the company says it is in the process of releasing new-generation processor hardware that fixes the vulnerability for several S7-1500 models. And the company says it is “working on new hardware versions for remaining PLC types to address this vulnerability completely.” The Red Balloon researchers say they have not yet been able to independently validate that the vulnerability has been fixed in this latest S7-1500 hardware.

Still, the Red Balloon Security researchers say that it would be possible for Siemens to release a firmware audit tool for any PLC to check whether there has been tampering on the device. Since the vulnerability will persist on impacted devices, such a feature would give S7-1500 owners more insight into their PLCs and the ability to monitor them for suspicious activity.

“It’s the same movie, just a different day,” says Red Balloon’s Cui. “Does very complicated, exotic hardware security improve overall security? Well, if you do it right, it could help, but I haven’t seen any human do it right. When you do it wrong, it always becomes a double-edged sword—and the edge of that sword is very sharp.”

Though Siemens says it is addressing the S7-1500 vulnerability in new models, the population of vulnerable 1500s in industrial control and critical infrastructure systems around the world is extensive, and these units will remain in use for decades.

“Siemens is saying that this will not be fixed, so it’s not just a zero day—this will remain a forever day until all the vulnerable 1500s go out of service,” Cui says. “It could be dangerous to leave this unaddressed.”

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