Today, US CERT released a Vulernability Notice for UEFI firmware:
Vulnerability Note VU#577140
BIOS implementations fail to properly set UEFI write protections after waking from sleep mode
Multiple BIOS implementations fail to properly set write protections after waking from sleep, leading to the possibility of an arbitrary BIOS image reflash.
According to Cornwell, Butterworth, Kovah, and Kallenberg, who reported the issue affecting certain Dell client systems (CVE-2015-2890):
There are a number of chipset mechanisms on Intel x86-based computers that provide protection of the BIOS from arbitrary reflash with attacker-controlled data. One of these is the BIOSLE and BIOSWE pair of bits found in the BIOS_CNTL register in the chipset. When the BIOSLE bit is set, the protection mechanism is enabled. The BIOS_CNTL is reset to its default value after a system reset. By default, the BIOSLE bit of the BIOS_CNTL register is cleared (disabled). The BIOS is responsible for re-enabling it after a reset. When a system goes to sleep and then wakes up, this is considered a reset from the hardware’s point of view.
Therefore, the BIOS_CNTL register must be reconfigured after waking from sleep. In a normal boot, the BIOS_CNTL is properly configured. However, in some instances BIOS makers do not properly re-set BIOS_CNTL bits upon wakeup. Therefore, an attacker is free to reflash the BIOS with an arbitrary image simply by forcing the system to go to sleep and wakes again. This bypasses the enforcement of signed updates or any other vendor mechanisms for protecting the BIOS from an arbitary reflash.
A similar issue affecting Apple systems (CVE-2015-3692) involves the FLOCKDN bit remaining unset after waking from sleep. For more information, refer to Pedro Vilaça’s blog disclosure.
See URL for full Notice.