By Dinesh Devadoss and Phil Stokes

Researchers looking into a new APT group targeting gambling sites with a variety of cross-platform malware recently identified a version of oRAT malware targeting macOS users and written in Go. While neither RATs nor Go malware are uncommon on any platform, including the Mac, the development of such a tool by a previously unknown APT is an interesting turn, signifying the increasing need for threat actors to address the rising occurrence of Macs among their intended targets and victims. In this post, we dig deeper into the technical details of this novel RAT to understand better how it works and how security teams can detect it in their environments.

oRAT Distribution

The oRAT malware is distributed via a Disk Image masquerading as a collection of Bitget Apps. The disk image contains a package with the name Bitget Apps.pkg and the distribution identifier com.adobe.pkg.Bitget.

The disk image and installer package are notable for two reasons: neither has a valid developer signature, and the latter doesn’t actually install any files and only contains a preinstall script, a succinct bash shell script whose purpose is to deliver a payload to the /tmp directory, give the payload executable permissions, and then launch it.

Precisely what kind of lure the threat actors use to convince targets to download and launch the dropper is unknown at this time, but given that the target would need to override default security warnings from Gatekeeper, it is likely either that the users are sourcing the malware from an environment where this is typical (e.g., a 3rd-party software distribution site that regularly delivers unsigned software) or users have been pre-groomed to bypass Gatekeeper during a social engineering engagement of some kind.

In either case, the fact that there’s no deliverable from the user’s perspective is a risky gamble on the part of the threat actors. After running the installer and finding that it did not provide whatever they were expecting, users are likely to become suspicious. This might suggest the campaign was broadly targeted and that the threat actors were playing a numbers game, happy to sweep up opportunistic infections as they occurred.

The oRAT Payload

Things get more interesting when we examine the darwinx64 payload dropped in the /tmp folder. The binary doesn’t define any Symbols, and outputting the list of Sections tells us that the file has been packed with UPX.

Packed files like this are opaque to static analysis, but fortunately standard UPX is very easy to unpack thanks to the UPX utility itself. Dumping the strings tells us that it was packed with UPX 3.96, the most recently released version available.

The packed binary is around 3MB in size, but after unpacking we are presented with a massive ~10MB file. Such large file sizes are typical of cross-platform malware, particularly when binaries are compiled in Go, since they contain the entire run-time for the language along with a number of supporting libraries.

Fortunately, from a reverse engineering perspective, we can easily ignore most of the standard code that is common to all Go bins and focus on what is unique to the sample at hand. For IDA Pro users, see here; for r2 users, we can start by printing out a list of the functions flagged with sym._main.

In Go binaries, the program code entrypoint is at main.main, and we can work our way through there to see what other functions, packages and modules are called. Below, we see that the main.main function calls out to another custom package, orat_utils.

The orat_utils package contains several interesting functions and gives us an entry into understanding how the RAT works.

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