Experts found undocumented access feature in Siemens SIMATIC PLCs

Researchers discovered a vulnerability in Siemens SIMATIC S7-1200 programmable logic controller (PLC) that could allow attackers to execute arbitrary code on vulnerable devices.

Researchers discovered an undocumented access feature in Siemens SIMATIC S7-1200 programmable logic controller (PLC) that could be exploited by attackers to execute arbitrary code on affected devices.

The feature was discovered by a team of researchers from the Ruhr-University Bochum in Germany composed of Ali Abbasi, Tobias Scharnowski and Thorsten Holz.

The medium-severity flaw was tracked as CVE-2019-13945 and received a CVSS score of 6.8, the issue is hard to exploit because requires a deep knowledge of the operating system used by the Siemens SIMATIC S7-1200

The Siemens S7 is considered one of the most secure controllers in the industry, it is used in power plants, traffic lights, water pumps, building control, production lines, aviation systems, and many other critical infrastructures. 

The researchers focused their analysis on the firmware integrity verification process implemented in the Siemens SIMATIC S7-1200 PLC.

The mechanism is triggered on boot and leverages the bootloader code that is stored on separate SPI flash memory. The teams of researchers discovered that the hardware undocumented access mode was present in the bootloader code since 2013.

“There is an access mode used during manufacturing of S7-1200 CPUs that allows additional diagnostic functionality. Using this functionality requires physical access to the UART interface during boot process.” reads a security advisory published by Siemens. “Siemens is working on a solution and recommends specific countermeasures until the solution is available “

The access feature was implemented to provide additional diagnostic functionality and it could be accessed by an attacker who has physical access to the device.

The attacker could access the feature by sending a special command via the universal asynchronous receiver-transmitter (UART) interface the boot process, before the PLC firmware is loaded.

The attack could leverage the feature to achieve arbitrary code execution in the boot stage.

The experts have developed a proof-of-concept (PoC) exploit that allows writing data to the flash chip by leveraging the PLC’s firmware update feature. 

The experts reported the flaw to Siemens in March and the company confirmed that it is working on a fix.

The advisory published by Siemens includes the following specific workarounds and mitigations that customers can apply to reduce the risk:

• Ensure physical access protection;
• Apply Defense-in-Depth;

The team of experts will present the results of its research in December at the Black Hat Europe conference in London.

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Pierluigi Paganini

(SecurityAffairs –SIEMENS SIMATIC, PLC)

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