Previously I showed you an anti-debug technique using the macOS exception ports API to detect the presence of a debugger. Today, I’m going to show you a different exception-port-based anti-debug trick which actually detaches the debugger by stealing the exception ports it is using. Some readers may be familiar with similar tricks done on Windows, however it’s very rare to see something like this done on macOS, though I have seen this kind of thing in the wild so here goes.
Continuing my series on anti-debug technique and how to defeat them, today we have a mach exception ports based trick on macOS.
For those unfamiliar with how the macOS flavor of Unix does things, these ports are used by debuggers to handle exceptions like breakpoints and bad-access. With this knowledge, a piece of software that does not want to be debugged, like a piece of malware trying to prevent analysis, can check if these ports are open and do something else in response, like shutdown.
The code for mach exception handling is fairly complex and poorly documented, but I’ve created a fairly simple example of debugger-detection code so we can dive right in.
Every reverse engineer who handles software for macOS knows about
ptrace(PT_DENY_ATTACH, 0, 0, 0), the infamous kernel-enforced anti-tracing DRM feature added to OS X years back (somewhere around Leopard) and most-notably used in iTunes. There are plenty of resources out there on how to bypass the common use of this feature, ranging from using a debugger to loading up a custom kernel-extension, but clever hackers have found new ways to abuse this feature to try to prevent researchers from debugging their malicious code.
I debated publishing this for a while as this information could misused, but since these techniques are being used in malware in the wild, I think it’s important to document how to defeat them.
For a long time Apple has had a debugger-detection function on their developer site. It’s not very hard to defeat with a debugger, but since this technique is sometimes used by malware to try to prevent debugging, I decided to document how to defeat it.
Recently I was asked if captchas.net is still secure today. My guess was it probably was not very secure, when compared to the fuzzy text of reCAPTCHA, but I wasn’t sure by how much, so I decided to look into it a bit more.
As you may know, Google has deprecated their old reCAPTCHA V1 API, in favor of their new reputation and image recognition based system. Apparently this may also in-part be because advances in text recognition software is starting to make the fuzzy text challenge obsolete.
As a research experiment, I decided to try my hand at solving captchas.net captchas with only software, and see how well I could do.
Recently while reverse engineering a piece of malicious macOS software to study what exactly it does, I noticed something odd in the binary. There seemed to be a little signature or message in the binary.
(c) 2014 - Cryptic Apps SARL - Disassembling not allowed
Necessity is the mother of pull requests, so that’s what I did.
Recently it became well-known that websites can leak information about a user based on if that user is logged in or not with this example that can demonstrate the issue. I’d recommend reading that page for a quick overview of how it can be done (but note that it is not the only way, as I will discuss below). So how can we properly prevent this information leak?
It’s not every day you get to release a freelance project as open source, but in this case I’m pleased to be able to release flshm as open source on GitHub.
This project was developed for a client who wanted to add some new features to an existing Flash-based project, but was running into some limitations due to the sand-boxing features which are even present when publishing a desktop application.