Direct Memory Access (DMA) is a controller feature that has been available at least since the original IBM PC. It can be used by hackers to compromise your otherwise very heavily protected computer. Fortunately, there are steps you can take to minimize DMA-based attacks.
Although DMA attacks have been possible for decades, they gained notoriety in 2009 when researchers discovered DMA could be used to compromise Microsoft's BitLocker Drive Encryption technology, according to a Princeton University paper. The white-hat researchers were able to recover the private keys in several popular encryption systems, including BitLocker, FileVault, dm-crypt, and TrueCrypt, using off-the-shelf components.