Row Hammer is a security exploit that targets certain types of dynamic random-access memory (DRAM) chips. It leverages a vulnerability where repeatedly accessing a row of memory cells can cause bit flips in adjacent rows. This phenomenon occurs due to the high density of memory cells in modern DRAM, which makes them susceptible to electrical interference.

First documented by Google's Project Zero in 2014, Row Hammer was initially considered difficult to exploit. However, subsequent research has demonstrated its potential for various malicious uses. The exploit does not require direct access to the memory, making it a significant concern for cybersecurity professionals.

How does Row Hammer Work?

Row Hammer works by exploiting the physical properties of DRAM. When a row of memory cells is repeatedly accessed, or "hammered," it generates electrical interference. This interference can disturb the charge in adjacent rows, causing bits to flip from 0 to 1 or vice versa. This phenomenon is due to the high density and close proximity of the memory cells in modern DRAM chips.

The mechanism behind Row Hammer involves rapid activations of the same memory rows, which cause voltage fluctuations on the row selection lines. These fluctuations induce higher-than-natural discharge rates in the capacitors of nearby memory rows, known as victim rows. If these cells are not refreshed in time, disturbance errors occur, leading to bit flips.

Tests have shown that a disturbance error may be observed after around 139,000 subsequent memory row accesses with cache flushes. The rate of disturbance errors is influenced by the actual contents of DRAM, with certain bit patterns resulting in higher error rates. More sophisticated row hammer patterns include non-uniform, frequency-based patterns, where each aggressor pair is hammered with different frequency, phase, and amplitude.

What are Examples of Row Hammer Attacks?

Several real-world examples highlight the potential impact of Row Hammer attacks. One notable instance is the "Rowhammer.js" exploit, which demonstrated that Row Hammer could be executed through JavaScript, allowing attackers to manipulate memory from within a web browser. This exploit showcased the feasibility of remote attacks, significantly broadening the attack surface.

Another significant example is the "DRAMMER" attack, which targeted Android devices. By exploiting the Row Hammer vulnerability, DRAMMER was able to gain root access on smartphones, bypassing security measures and gaining full control over the device. These examples underscore the diverse methods and platforms through which Row Hammer can be leveraged, from web browsers to mobile devices.

What are the Potential Risks of Row Hammer?

The potential risks of suffering a Row Hammer vulnerability or attack are significant and multifaceted. Here are some of the key risks:

• Privilege Escalation: Attackers can exploit Row Hammer to gain higher-level access to a system, bypassing standard security measures and executing unauthorized actions.

• Data Corruption: The exploit can cause unintended bit flips in memory, leading to corrupted data and unpredictable system behavior, which can compromise the integrity of stored information.

• System Instability: Disturbance errors induced by Row Hammer can result in random changes in memory values, causing system crashes and instability.

• Unauthorized Access: By breaking isolation between different processes, Row Hammer can allow attackers to access sensitive information that they would not normally have permission to view.

• Increased Vulnerability to Other Attacks: The exploit can defeat various layers of memory protection, making the system more susceptible to a wide range of additional attacks.

How can you Protect Against Row Hammer?.

Protecting against Row Hammer involves a combination of hardware and software strategies. Here are some effective measures:

• Use ECC Memory: Error-Correcting Code (ECC) memory can detect and correct bit flips, reducing the risk of data corruption caused by Row Hammer.

• Increase Refresh Rates: More frequent memory refreshing can help prevent bit flips by ensuring that memory cells are refreshed before they can be affected by repeated accesses.

• Adopt TRR Technology: Target Row Refresh (TRR) is a hardware-based solution that refreshes adjacent rows when a row is accessed repeatedly, mitigating the Row Hammer effect.

• Implement Software-Based Mitigations: Techniques such as memory partitioning and access pattern monitoring can help detect and prevent Row Hammer attacks at the software level.

• Upgrade Hardware: Replacing vulnerable DRAM chips with newer, more resilient models can provide a long-term solution to Row Hammer vulnerabilities.