Last year, we reported about PlugX a breed of Remote Access Trojan (RAT) used in certain high-profile APT campaigns. We also noted some of its noteworthy techniques, which include its capability to hide its malicious codes by decrypting and loading a backdoor “executable file” directly into memory, without the need to drop the actual “executable file”.
Recently, we uncovered a RAT using the same technique. The new sample detected by Trend Micro as BKDR_RARSTONE.A is similar (but not) PlugX, as it directly loads a backdoor “file” in memory without dropping any “file”. However, as we proceeded with our analysis, we found that BKDR_RARSTONE has some tricks of its own.
We obtained the sample through a spear phishing email that contains a specially-crafted .DOC file (detected as TROJ_ARTIEF.NTZ). This Trojan drops and executes BKDR_RARSTONE.A, which in turn drops the following files:
- %System%\ymsgr_tray.exe – copy of BKDR_RARSTONE.A
- %Application Data%\profile.dat – blob file containing malware routines
BKDR_RARSTONE.A then executes the dropped copy ymsgr_tray.exe. This backdoor then opens a hidden Internet Explorer process, in which it injects the codes contained in profile.dat.
As with PlugX, the injected code decrypts itself in memory. Once decrypted it “downloads” a .DLL file from its C&C server and again loads it in the memory space of the hidden Internet Explorer process. This “downloaded” file is actually not dropped onto the system, but instead directly loaded in memory, making file-based detection ineffective.
Typical of a backdoor, BKDR_RARSTONE.A connects to specific sites and can perform several routines, which include enumerating files and directories, downloading, executing, and uploading files, and updating itself and its configuration.
Worth noting among its backdoor routine is its ability to get installer properties from Uninstall Registry Key entries. It does this to get hold of information about the installed applications in the affected system, as well as to know how to uninstall certain applications. This can be handy in silently uninstalling applications, which may interfere with the backdoor’s routine, e.g. anti-malware software and the likes.
Another interesting feature of this backdoor is the communication method it uses, specifically SSL. This use of SSL has a two-fold advantage: it guarantees that communication between the C&C and infected system is encrypted, at the same time it blends in with normal traffic.
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