'BoundHook' Technique Enables Attacker Persistence on Windows Systems

Security researchers at CyberArk have developed a technique showing how attackers can exploit a feature in the Memory Protection Extension (MPX) technology on modern Intel chips to steal data from Windows 10 systems and to remain completely undetected on them.

CyberArk's new BoundHook technique is similar to the GhostHook method that the company revealed earlier this year in that it is a post-exploitation technique. In other words, for BoundHook to work, an attacker would need to already have privileged access on a Windows 10 system.

Microsoft itself, for that reason, has refused to categorize the issue as a vulnerability that merits a security patch. "The technique described in this marketing report does not represent a security vulnerability and requires a machine to already be compromised to potentially work," the company said in a statement. "We encourage customers to always keep their systems updated for the best protection."

Intel's MPX technology, introduced with the chipmaker's Skylake line in 2015, is designed to protect applications against buffer overflows, out-of-bounds access, and other memory errors and attacks. Applications running on Windows 10 systems use the feature as protection against buffer overflow attacks.

CyberArk's BoundHook technique uses a boundary check instruction in MPX to hook processes on a system, and to essentially change its behavior. "The BoundHook technique allows you to run your own code inside foreign processes and change its normal behavior, without leaving any traces inside these foreign processes," says Doron Naim, senior security researcher at CyberArk.

Hooking is about changing the behavior of certain functions in the operating system or application software on a system, he says. As one example, he points to the key input function. "If an attacker were able to hook this function, they would be able to sniff and steal your keystrokes."

Typically, to do hooking you have to write hooking code inside a target process, he says. With BoundHook, the code is not used to execute the hook itself but to cause an error, like a boundary exception error in the process. From there an attacker can take complete control of the thread execution, Naim notes. "If you control the thread execution, you can do anything you want by the name of the target process. For example, if it's Word.exe, you can steal credentials or send information to the Internet through this process." Most antivirus tools are not equipped to detect the malicious activity that is enabled via BoundHook, according to CyberArk.

While Microsoft has downplayed BoundHook just as it did with GhostHook, Naim insists CyberArk's latest technique indeed poses a threat. "The first thing to note is that this technique is most likely to be used by nation-state attackers, or very well financed criminal organizations that are looking for infiltrations that last."

In the current threat environment, gaining administrative privileges on an endpoint system is something that administrators should assume even the most basic attacker can accomplish, he says. In most cases, all it takes is for a single individual to click on the wrong link or fall for a phishing scam.

Techniques such as the one that CyberArk demonstrated this week are important because they show how attackers can improve their dwell-time on a compromised network, Naim notes. "Techniques like this are incredibly powerful in helping attackers disappear after the initial infection point — allowing them to build in backdoors and plan their attacks in de facto stealth mode."

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