RAMBleed: What you need to know

Yesterday; security researchers disclosed a vulnerability relating to how data is accessed after it is stored within computer memory modules eventually leading to partial data disclosure

This is a low severity (CVSS Base Score: 3.8) but notable vulnerability which cannot be exploited remotely. For organisations and customers; no action is required. It is up to software developers to use trusted execution environments (TEE) e.g. AMD SEV, ARM TrustZone or Intel SGX to protect important data or clear such data from memory after use. Some DDR4 modules are not vulnerable to Rowhammer.

How does this attack take place?
An attacker would first need to compromise your system and persuade you to run an application. Due to the physical effects of creating memory modules which are smaller and smaller the space between memory cells used to store data are subject to electrical interference. This can be exploited by an attacker by reading the data from a memory address of interest over and over again which eventually leads to data corruption causes the binary contents (0 or 1) used to store data to change/”flip” from 0 to 1 or vice versa.

This effect has been seen before in an attack dubbed “Rowhammer” in 2014. That attack can be mitigated by the use of memory modules that use ECC (Error Correction Code). However, this new technique RAMBleed cannot be mitigated by ECC (defined).

What must an attacker do to exploit this vulnerability?
An attacker must first map the memory which contains the data they wish to acquire. They can then work to control data each side in memory of the target data. Accessing this data over and over “hammers” the row with the data within it. If the data is 0, it will flip to 1 and if 1 becomes a zero (0). The attacker can then proceed to repeat this for one column down in the memory segment to obtain the next piece of target data. Researchers were able to obtain 3 to 4 bits (either 0 or 1) per second.

Researchers used this technique to obtain a 2048 bit OpenSSH key from the memory of a server. They did so by first using a technique they named “Frame Feng-Shui” that allows them to place the target data within a physical memory frame (area) of their choice in. The speed was 0.3 bits per second with an accuracy of 82%. By only obtaining some of the data and using a variant of the technique documented within the Heninger-Shacham algorithm they succeeded in obtaining the remainder of the key.

How can an organisation or a consumer/end-user defend against this attack?
Encrypted memory achieved by the use of trusted execution environments (TEEs) e.g. AMD Secure Encrypted Virtualization (SEV), ARM TrustZone or Intel Software Guard Extensions (SGX) will mitigate this attack since the attackers will obtain encrypted rather than ready to use/plain text data.

Alternatively; software developers can clear encryption keys or other sensitive data from memory after using it. Intel recommends it’s guidelines for resisting side-channel and timing side channel attackers:

A lesser known mitigation is the use of DDR4 memory modules that should disrupt the success of the Rowhammer attack. The Maximum Activation Count (MAC) of a memory row is not vulnerable to Rowhammer when the MAC has a value of “unlimited”.

This field exists within the SPD (Serial Presence Detect) technique of accessing memory. From the following page, many but not all of the examined DDR4 modules feature this setting. For example, my 4x 16 GB (64GB) Corsair Dominator Platinum PC4-21300 (CMX64GX4M4A2666C15) modules feature this setting and so appear not to be vulnerable to the Rowhammer technique. You can see this from the first attached screenshot (denoted by the value “Unlimited MAC”):

These screenshots were obtained from the RAMMon application available from PassMark.

Thank you.

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