An updated version of the Cold Boot Attack lets threat actors bypass security mechanisms and access data that remain in memory after a machine shuts down. Modern machines from Apple, Dell, Lenovo, and other major tech firms are affected, researchers report.
The Cold Boot Attack itself is not new. Known since 2008, it lets attackers with physical access to a machine steal its encryption keys, which briefly remain in memory after a hard reboot. Most devices now protect against this with a Trusted Computing Group (TCG) mitigation that overwrites data when the machine is rebooted and protects it from third parties.
But Olle Segerdahl, principal security researcher with F-Secure, along with fellow security consultant Pasi Saarinen, found this mechanism can be broken if they manipulate the firmware. The duo found a way to bypass TCG's protection and exploit a weakness in the computer's firmware to steal encryption keys and other data in a successful cold boot attack.
Several types of data could potentially be at risk, says Segerdahl. "Our primary target was hard drive encryption keys stored in memory," he explains, but attackers could also access passwords, network credentials, and any information on the machine that its user can access.
"In one case, we used the machine to connect to corporate networks," says Segerdahl. The duo detected VPN credentials that let them connect to internal networks and plant a backdoor. They could extract passwords for wireless networks and data that wasn't on the hard drive, but was present in memory when they had the machine; for example, online passwords.
How it Works
Here's how the TCG protection is intended to work: "Modern machines have a mitigation against the [cold boot] attack, where the firmware of the machine tries to detect if the machine has been properly shut down or not," explains Segerdahl. A "flag" is set when the operating system boots up, telling firmware to protect data in memory if the device isn't properly shut down. If it isn't, the OS is supposed to clear that sensitive data.
The researchers broke this mitigation by removing the flag themselves and connecting a small device to the Flash memory chip on the motherboard that stores firmware settings.
"We can override the flag by accessing the memory chip directly," says Segerdahl. Just as with the original cold boot attack, the actor needs physical access to a machine. They also need a tool to rewrite the non-volatile memory chip containing the firmware settings, disable memory overwriting, and enable booting from external devices. Cold boot attacks can be carried out with a special program on a USB stick, according to the researchers.
This manipulation gives them access to data that briefly remains in memory after a computer is shut off. How brief? It partly depends on temperature: the colder a memory card, the longer the information will last. Segerdahl says they had five- to ten seconds in their experiments.
The amount of time an attacker has to perform the operation depends on the machine they steal, he explains. If a threat actor finds a machine in sleep mode, or doesn't have pre-boot authentication, "then the attacker has unlimited time," says Segerdahl. If no password is required to boot the machine, they can try multiple times to gain access.
He and Saarinen advise IT departments to configure machines to either shut down or hibernate – not enter sleep mode – and require users to enter a BitLocker PIN when they power up their machines. An attacker can perform a successful cold boot attack, but encryption keys aren't stored in RAM when a machine hibernates or shuts down, so there's no data to steal.
Apple, Microsoft Respond
F-Secure alerted Apple, Microsoft, and Intel to their findings. Microsoft published updated guidance on BitLocker countermeasures and responded to this research with the following:
"This technique requires physical access," says Jeff Jones, senior director at Microsoft, in a statement to Dark Reading. "To protect sensitive info, at a minimum, we recommend using a device with a discreet Trusted Platform Module (TPM), disabling sleep/hibernation and configuring BitLocker with a Personal Identification Number (PIN)."
Apple says Macs equipped with an Apple T2 chip come with security measures designed to protect devices from attacks like this one, the researchers report. The company also advises users to set a firmware password to secure Macs without a T2 chip.
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