New ASLR-busting JavaScript is about to make drive-by exploits much nastier

February 18, 2017 – 7:01 AM

For a decade, every major operating system has relied on a technique known as address space layout randomization to provide a first line of defense against malware attacks. By randomizing the computer memory locations where application code and data are loaded, ASLR makes it hard for attackers to execute malicious payloads when exploiting buffer overflows and similar vulnerabilities. As a result, exploits cause a simple crash rather than a potentially catastrophic system compromise.

Now, researchers have devised an attack that could spell the end of ASLR as the world knows it now. The attack uses simple JavaScript code to identify the memory addresses where system and application components are loaded. When combined with attack code that exploits vulnerabilities in browsers or operating systems, the JavaScript can reliably eliminate virtually all of the protection ASLR provides. The technique, which exploits what’s known as a side channel in the memory cache of all widely used modern CPUs, is described in a research paper published on Wednesday. The researchers have dubbed the technique ASLR Cache or AnC for short.
“Fundamentally insecure”

The researchers said the side channel attack is much more damaging than previous ASLR bypasses, because it exploits a micro-architectural property of the CPU’s that’s independent of any operating system or application running on it. Whereas heap spraying and other forms of ASLR bypass can often be mitigated by software tweaks, there isn’t much that can stop or lessen the effects of the JavaScript, which targets a CPU’s MMU, or memory management unit. That’s because CPU caching behavior and strong address space randomization are mutually exclusive. (Apple, however, recently hardened its Safari browser to partially mitigate such attacks. It’s also possible to prevent JavaScript from running in a browser, but such blocking often severely degrades a site’s usability.)


You must be logged in to post a comment.