Apple Mac EFI before 2015-001, as used in OS X before 10.10.4 and other products, does not properly set refresh rates for DDR3 RAM, which might make it easier for remote attackers to conduct row-hammer attacks, and consequently gain privileges or cause a denial of service (memory corruption), by triggering certain patterns of access to memory locations.
Apple Security Advisory 2015-06-30-3 - Mac EFI Security Update 2015-001 is now available and addresses EFI flash memory modification and memory corruption issues.