OpenSSH protects in-memory keys against Specter or Meltdown attacks

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The most recent OpenSSH patch makes it more challenging to carry out Spectre, Meltdown, Rowhammer, and RAMbleed attacks.

A new patch for the OpenSSH project stops hackers from abusing hardware flaws that may grant them access to illegal memory areas through the use of unprivileged processes, ultimately leading to the theft of private keys. Other software programs could employ the same technique to safeguard their RAM-stored secrets until fixes for these issues for upcoming SDRAM chip and CPU generations are discovered. After a group of researchers revealed an attack known as RAMBleed, the patch was made available. The latter uses information extracted from memory areas reserved for privileged processes and the kernel to retrieve information from contemporary memory modules. By causing bit jumps inside physical memory cells and retrieving sensitive data via a side channel, RAMBleed uses a software technique called Rowhammer. Using code run with user-level permissions, the researchers’ assault successfully obtained a 2048-bit RSA signing key from an OpenSSH server.

The virtual memory allotted to the kernel is separated from the memory used by user-space programs by modern operating systems. This is so because kernel memory stores private information like passwords and encryption keys. These ought to be protected against non-privileged apps. Any deviation from this fundamental rule creates a serious security vulnerability, giving attackers a wide range of options for gaining access to computer systems’ code execution rights. They avoid malware infections or take advantage of flaws in numerous user space programs. All unprivileged apps running on a computer offer a significantly broader attack surface than the kernel itself.

OpenSSH patch encrypts private keys

Damien Miller, an OpenBSD engineer, published a new OpenSSH patch that “protects against side-channel memory attacks like Spectre, Meltdown, Rowhammer, and Rambleed” and “speculative action and side-channel action attacks against private keys at rest in RAM.” Modern CPUs have a performance-enhancing feature called speculative execution that side-channel exploits like Spectre, Meltdown, and more recently Microarchitectural Data Sampling (MDS) take advantage of. Some of these assaults have the ability to read kernel memory that is protected.

The most recent OpenSSH patch uses another symmetric key generated from 16 KB of random data to encrypt private keys when they are stored in memory and not being used. Damien Miller wrote in his comments to the patch, “We must first attempt to decode the protected private key, although that appears implausible given all of the bit error rates seen in current assaults. “From an implementation standpoint, keys are protected when loaded and automatically and transparently unprotected when used for signatures or when stored or serialized.”

A usable or reproducible patch for other software

The most widely used implementation of the SSH (Secure Shell) protocol is OpenSSH, which is used for automated machine-to-machine interactions as well as remote access to and management of servers and computer systems. The protocol was initially created for OpenBSD, but it is now supported by Windows 10 and is used by default in the majority of modern Linux versions. Other software projects could imitate OpenSSH’s strategy to save their own keys and secrets in memory. However, as the patch notes imply, while using this technique side channel attacks are less likely to succeed, they are still conceivable.

Hackers are continually refining their attack techniques, thus it’s conceivable that future iterations of RAMBleed or Meltdown/Spectre will be able to overcome this mitigation. Future hardware’s fundamental design problems will probably be fixed, but it will take a long time to replace the processors and SDRAM chips currently in use. It’s best to think about how these hardware attacks—and any other attacks—will affect corporate IT over the long term. Anything that software developers can do to at least somewhat reduce these weaknesses and make it more difficult for hackers to operate is really beneficial. Miller continued, “I believe we will be able to resolve this issue within a few years and provide a more secure computer architecture.