Rowhammer is a vulnerability in commodity dynamic random access memory (DRAM) chips that allows an attacker to exploit devices with DRAM memory by repeatedly accessing (hammering) a row of memory until it causes bit flips and transistors in adjacent rows of memory reverse their binary state: ones turn into zeros and vice versa.
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The flaw, first reported in the paper "Flipping Bits in Memory Without Accessing Them: An Experimental Study of DRAM Disturbance Errors," detailed how, as DRAM processes continues to scale to smaller sizes, it becomes more difficult to prevent individual memory cells from interacting with neighboring cells.
The Rowhammer flaw allows memory manipulation to be used by malicious actors to extract data such as passwords from vulnerable systems. The flaw has been detected in DDR3 and DDR4 DRAM chips and, when combined with other attacks, can be used to access the contents of memory on systems using vulnerable chips. The Google Project Zero team published details of its proof of concept code for exploits of Rowhammer on x86-64 Linux machines, but they wrote that the exploit was likely not specific to Linux systems.
Rowhammer accomplishes this manipulation by forcing the repeated reading and recharging of a row of capacitors in a DRAM chip. The repeated reading and recharging of a row happens when an attacker uses the machine code instruction Cache Line Flush (CLFLUSH) to clear the cache, as shown in the 2014 research paper from Carnegie Mellon University and Intel Labs researchers. Caching limits prevent bit flipping from occurring normally, so the repeated CLFLUSH overloads the system. When the bit flipping happens too often and in capacitor rows too close together, neighboring capacitors begin to interact electrically, and this opens up the opportunity to exploit the Rowhammer flaw.
Researchers at Intel became aware of Rowhammer in 2012 and filed patent applications that were publically disclosed, but the vulnerability didn't garner much attention until 2014 when the research paper was published. DRAM chips are an important part of most electronic devices, including those that are essential to computers. As such, DRAM vulnerabilities like Rowhammer cannot be fixed with basic security software or operating system (OS) updates. Rowhammer continues to be used in new attacks.