Half-Double is a Rowhammer technique in which many memory accesses to a row two positions from a victim, combined with a smaller number of accesses to the row between them, can induce errors in the victim row. Google researchers disclosed the effect in 2021; a 2022 paper demonstrated an end-to-end attack on specific recent Chromebooks, including systems with ECC- and TRR-protected LPDDR4x. That finding shows those protections did not stop the demonstrated attack on the tested setup—not that every computer is vulnerable.
What is the Half-Double Rowhammer attack?
DRAM stores data in rows. Rowhammer is a disturbance effect in which repeatedly activating one row can change bits in other rows. The familiar simplified picture focuses on effects in immediately neighboring rows. Half-Double showed that, under the researchers’ tested conditions, disturbance could reach a row two positions away.
In Google’s 2021 A-B-C illustration, researchers issued a very large number of accesses to row A and roughly dozens to intervening row B, then observed an effect on row C. Google described B’s accesses as having a nonlinear gating effect that appeared to carry the disturbance from A toward C. This is a qualitative explanation, not a universal access threshold. The disclosure attributed the phenomenon to electrical coupling in the silicon substrate. Google’s Half-Double disclosure introduced the technique on May 25, 2021.
How is Half-Double different from ordinary Rowhammer?
The distinction is the access pattern and the distance to the affected row: Half-Double combines many accesses to a distance-two row with a few accesses to the distance-one row between it and the victim. It therefore challenges the assumption that Rowhammer disturbance is confined to immediate neighbors.
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Google distinguished Half-Double from TRRespass, another Rowhammer-related technique. Google characterized Half-Double as an intrinsic property of the silicon substrate, whereas it described TRRespass as exploiting blind spots in manufacturer-dependent defenses. These are different findings; Half-Double does not show that all defenses are bypassed in the same way.
What did the 2022 attack demonstrate?
The USENIX Security 22 paper reported an end-to-end proof of concept on recent Chromebooks using ECC- and TRR-protected LPDDR4x memory. The paper’s abstract says the attack took under 45 minutes on average on those studied systems. That is a study-specific average, not a general estimate for other computers or a guarantee that an attack will succeed on a particular device.
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The demonstration was a multi-part attack involving side channels, BlindHammering, a spraying technique, and a Spectre attack. Half-Double is evidence that disturbance can extend beyond an adjacent row; it does not establish that every DRAM module is susceptible, that every machine can be attacked, or that a general PC user can reproduce the researchers’ result.
Does ECC RAM or TRR stop Half-Double?
Not invariably: the 2022 proof of concept succeeded on the tested Chromebook setup despite ECC and TRR protection. That result does not mean ECC or TRR never help. It means those labels alone are not proof of immunity, and the effectiveness depends on the implementation, memory, system, and attack pattern.
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Target Row Refresh (TRR) refers to approaches that monitor row activations and refresh nearby rows when activity crosses a threshold. Such mechanisms may be implemented in DRAM or in the host CPU. Google’s 2025 update notes that vendor implementations and proprietary mechanisms vary by manufacturer and device model, making a generic “TRR-protected” claim insufficient to characterize a system’s security.
When assessing a mitigation claim, check the specific memory generation and module, whether protection is on-die or host-side, whether activation tracking and targeted refresh are supported, which attack patterns and threat model were tested, and whether the claim comes from a vendor specification or an independent experiment. The cited sources do not provide a universal device-by-device vulnerability list.
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What has changed in Rowhammer mitigations since Half-Double?
Google’s September 15, 2025 ecosystem update says contemporary DDR5 systems discussed there lacked PRAC or other robust mitigations and relied on probabilistic measures such as ECC and enhanced TRR. It describes PRAC as an approved standard intended for upcoming DDR5 and LPDDR6 support. PRAC tracks memory-row activations and alerts the system when activation counts become excessive. This is a dated account of the ecosystem, not a guarantee about every product available in 2026.
The same 2025 update describes separate Phoenix research that bypassed enhanced TRR on tested DDR5 memory and demonstrated privilege escalation on a production-grade desktop with AMD Zen processors and SK Hynix DDR5. Google said it was investigating applicability to other configurations. Phoenix is a later, separate result—not Half-Double—and indicates that Rowhammer remains an active research area.
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Are FPGA Rowhammer tester boards a fix for a home computer?
No. The FPGA boards discussed in the Rowhammer work are research instruments, not consumer protection products. Google and Antmicro describe an open-source FPGA Rowhammer Tester intended to let researchers control DRAM commands and test memory. The early platform account names Digilent Arty boards for DDR3 and Xilinx ZCU104 boards for DDR4, and describes later work on LPDDR4 support. Google’s later account also describes specialist FPGA platforms for DDR5 testing. These tools require suitable hardware compatibility and expertise; buying an FPGA board, generic RAM, or ECC memory is not an established fix for Half-Double on an existing computer.
The original paper is available through the open-access USENIX Security 22 proceedings. Google’s 2021 open-source DDR controller framework article provides background on the tester platform.
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