Memory Remanence Attacks
A memory remanence attack is a side-channel attack that reads the encryption key from memory after restarting a PC. Although a PC’s memory is often considered to be cleared when the PC is restarted, memory chips don’t immediately lose their memory when you disconnect power. Therefore, an attacker who has physical access to the PC’s memory might be able to read data directly from the memory—including the encryption key.
When performing this type of cold boot attack, the attacker accesses the PC’s physical memory and recovers the encryption key within a few seconds or minutes of disconnecting power. This type of attack was demonstrated by researchers at
Princeton University. With the encryption key, the attacker would be able to decrypt the drive and access its files.
To acquire the keys, attackers follow this process:
1. Freeze the PC’s memory. For example, an attacker can freeze the memory to −50°C by spraying it with aerosol air duster spray.
Restart the PC.
2. Instead of restarting Windows, boot to another operating system. Typically, this is done by connecting a bootable flash drive or loading a bootable DVD.
3. The bootable media loads the memory remanence attack tools, which the attacker uses to scan the system memory and locate the encryption keys.
4. The attacker uses the encryption keys to access the drive’s data.
If the attacker is unable to boot the device to another operating system (for example, if bootable flash drives have been disabled or Secure Boot is enabled), the attacker can attempt to physically remove the frozen memory from the device and attach it to a different, possibly identical device. Fortunately, this process has proven extremely unreliable, as evidenced by the Defence Research and Development Canada (DRDC) Valcartier group’s analysis (see
An In-depth Analysis of the Cold Boot Attack). On an increasing portion of modern devices, this type of attack is not even possible, because memory is soldered directly to the motherboard.
Although Princeton’s research proved that this type of attack was possible on devices that have removable memory, device hardware has changed since the research was published in 2008:
* Secure Boot prevents the malicious tools that the Princeton attack depends on from running on the target device.
* Windows systems with BIOS or UEFI can be locked down with a password, and booting to a USB drive can be prevented.
* If booting to USB is required on the device, it can be limited to starting trusted operating systems by using Secure Boot.
* The discharge rates of memory are highly variable among devices,
and many devices have memory that is completely immune to memory remanence attacks.
* Increased density of memory diminishes their remanence properties and reduces the likelihood that the attack can be successfully executed, even when memory is physically removed and placed in an identical system where the system’s configuration may enable booting to the malicious tools.
Because of these factors, this type of attack is rarely possible on modern devices. Even in cases where the risk factors exist on legacy devices, attackers will find the attack unreliable. For detailed info about the practical uses for forensic memory acquisition and the factors that make a computer vulnerable or resistant to memory remanence attacks, read
An In-depth Analysis of the Cold Boot Attack.
The BitLocker pre-boot authentication feature can successfully mitigate memory remanence attacks on most devices, but you can also mitigate such attacks by protecting the system UEFI or BIOS and prevent the PC from booting from external media (such as a USB flash drive or DVD). The latter option is often a better choice, because it provides sufficient protection without inconveniencing users with pre-boot authentication.
