rowhammer

Rowhammer is a class of hardware vulnerability in dynamic random-access memory (DRAM) that enables bit flips in memory by repeatedly and rapidly accessing (hammering) certain DRAM rows. It was first demonstrated publicly in 2014 by researchers who showed that electrical interference between neighboring rows could cause unintended changes to bits in adjacent rows, even without direct writes to those rows. The effect, often called the row hammer phenomenon, depends on DRAM design, density, and refresh dynamics, and it can undermine memory isolation.

Mechanism and scope: DRAM stores data as electrical charges in cells arranged in rows and columns. Repeatedly activating a target row can disturb the charge in nearby rows, causing some cells to flip their stored value. This disturbance is a physical fault mechanism rather than a software bug, and its exploitability depends on hardware generation, memory controller behavior, and error-correcting capabilities. While flips are not guaranteed, they can be induced under realistic conditions and used to corrupt data.

Attacks and defenses: Early demonstrations focused on local attackers with direct memory access, but later work showed that attacks could be attempted from within a web browser using JavaScript to trigger bit flips, raising concerns about cross-site or cross-VM impacts. Mitigations include hardware and firmware measures such as Target Row Refresh (TRR) and improved refresh strategies, error-correcting code (ECC) memory, and memory scrubbing. Software and operating system techniques, such as memory isolation and allocation strategies, also help reduce exposure. Rowhammer remains an active area of hardware security research and mitigation.