Trustwave SpiderLabs’ spam traps have identified an increase in threats packaged in password-protected archives with about 96% of these being spammed by the Emotet Botnet. In the first half of 2022, we identified password-protected ZIP files as the third most popular archive format used by cybercriminals to conceal malware. This is significant because one of the most difficult obstacles threat actors face when conducting this type of spam campaign is to convince the target to open the archive using the provided password.
The SpiderLabs team noticed an interesting attachment in this spam campaign. Disguised as an invoice, the attachment in either ZIP or ISO format, contained a nested self-extracting (SFX) archive. The first archive is an SFX RAR (RARsfx) whose sole purpose is to execute a second RARsfx contained within itself. The second RARsfx is password-protected but despite that, no user input is necessary to extract and execute its content. In some samples, the nested SFX archive is encapsulated further in another archive.
Figure 1: Email sample unpacked with MailMarshal
As mentioned in our previous blog, the main factor in the success of delivering threats via password-protected files like Emotet is the email recipient’s intuition. The user must be persuaded to open the archive using the password enclosed in the email. This blog will present a campaign that attempts to override this ‘supply-a-password’ hurdle.
The Nested Self-extracting RAR
Self-extracting archives are commonly used to distribute malware. Setting an archive as SFX makes the archive executable. This archive format is convenient as the content of the archive can be unpacked without employing any archiving tools. Importantly, for this attack, SFX archives also provide the ability to run script commands.
The first in-archive SFX we collected makes use of either a PDF or Excel icon to appear legitimate, and has three components:
- Batch file – the launcher of the RARsfx component
- RARsfx archive – password-protected container of the payload
- Images or PDF file – decoy file
Figure 2: The script commands and icon of the RARsfx contained in the attachment Payment.gz in Figure 1
The script commands from the parent RARsfx silently extract these components to the %AppData% folder with existing files overwritten. Then two components are run. The batch file is launched first followed by an image or PDF file.
Figure 3: The process flow
The execution of the batch file leads to the installation of the malware lurking within the password-protected RARsfx. The batch script specifies the password of the archive and destination folder where the payload will be extracted. Along with this process, a command prompt is invoked, and the decoy image or PDF attempts to hide this from view.
Figure 4: The command prompt invoked by the batch file from the RARsfx in Figure 1
Figure 5: Malicious RARsfx in action with image decoy
In later samples, some of the RARsfx archives do not have a decoy file, and moreover, the destination path of the RARsfx components is changed to the %temp% folder.
Figure 6: The email sample containing a RARsfx with no decoy component
The Payload
The password-protected RARsfx contains one file, an executable payload. The executable is extracted and executed from the %AppData% folder. All the executables in this campaign are .NET compiled and obfuscated with ConfuserEX, a free and open-source protector for .NET applications.
The payloads we observed from this campaign are CoinMiner and QuasarRat.
Figure 7: The CoinMiner payload from the spam in Figure 1 as viewed in Detect It Easy tool which shows protection with ConfuserEX
CoinMiner is a threat that mines cryptocurrency by using resources of the infected system. This malware can also exhibit credential stealer behavior since it can read user data in web browsers and access Microsoft Outlook profiles. Such versatility is achieved through its modular design, and it is up to the command-and-control server or operator to specify what job to do next.
In the samples we analyzed, once the CoinMiner is executed, a copy of itself is created in the %AppData% folder and a VBS script is dropped at the startup location as a persistence mechanism. CoinMiner used Windows Management Instrumentation (WMI) to gather information from the system such as hardware information and antivirus software installed. Such a technique is often used to avoid sandboxing and to hinder analysis. It utilized free dynamic DNS domains for accessing its C2 server.
Another payload we came across is the infamous Quasar RAT, which is an open-source remote access trojan (RAT) which is publicly available on GitHub. Quasar RAT is widely used in campaigns conducted by threat actors and is a tool of choice due to its powerful capabilities. The Cybersecurity and Infrastructure Security Agency (CISA) provided a technical analysis of Quasar https://www.cisa.gov/uscert/ncas/analysis-reports/AR18-352A.
The Quasar sample we had used the threat actor’s domain and free dynamic DNS domain for accessing its C2 server. Its mutex follows the default Quasar pattern, “QSR_MUTEX_[0-9A-Za-z]{18}”.
Conclusion
The self-extracting archive has been around for a long time and eases file distribution among end users. However, it poses a security risk since the file contents are not easily verifiable, and it can run commands and executables silently. The attack technique we detailed only requires one click, and no password input is required to compromise a target. As a result, threat actors can perform a multitude of attacks like crypto jacking, data theft, ransomware, etc.
IOCs
Files in initial archive
archive Payment.gz (914609 bytes):
FF86161334B70BCC2A5D638AD2AB2BF3980DC457 (SHA1)
Confirmacion Mensaje.zip (878036 bytes):
DB7A08AB199F7F341F90D05A6B09846C6D43F8CB (SHA1)
Confirmacion Mensaje.img (1572864 bytes):
4C0F487D60FC4F1ADB29128EC9FD044C10E8653F (SHA1)
Muestras_772022.img (2228224 bytes):
9A5021D6679E2E8ECF900D6B5FF2CDA9715E83DB (SHA1)
07122022.IMG (1966080 bytes):
0B9381FFC9AE3A7749484D0AD788E91452DAE86B (SHA1)
Files in RARsfx
Balance_Payment.exe (1034902 bytes):
541BF4E5FA5FBEC25374BEF131C59070DA9D3C4A (SHA1)
Confirmacion Mensaje Swift Operado.exe (995833 bytes):
B4E93ED6ED7038CDD70F4791B73EED849F49275B (SHA1)
Muestras.exe (1664430 bytes):
C29C5382074F64176FE3904D24492A4DAC45C123 (SHA1)
pago12072022.exe (1401744 bytes):
925E4A825C139DF9535A4D4649A4CF64656E3194 (SHA1)
Batch Files
jhyuonsdjhj.bat (114 bytes):
48741C6E3E736FD5083F0DEF8E3741CE4E60B944 (SHA1)
uvjjjukvijhyujhj.bat (119 bytes):
88444E90913017BA77CC06FECCA3E659840633C6 (SHA1)
tomjkoohmggg.bat (152 bytes):
CF00D27AF07A40613A7932CD3090D89A8C6B0569 (SHA1)
cjgxhgikjkgbg.bat (178 bytes):
206ED91982F2011EC20B76CB07AA7BD48E6F81CC (SHA1)
Decoy Files
556yu67.PNG (9511 bytes):
E638A4040AE2553410CE77A88968CD7321F55063 (SHA1)
888yu67.PNG (9511 bytes):
E638A4040AE2553410CE77A88968CD7321F55063 (SHA1)
samples.jpeg (96532 bytes):
D9A78F491AF45BB3DC02E764217D89DD95F4F17C (SHA1)
Files in password-protected RARsfx
yrqs.sfx.exe (1034798 bytes):
A602E13CCE53CE9D589D9C6386058204BD3CE978 (SHA1)
dtccnppbk.sfx.exe (995790 bytes):
CC63815B9B77FCDD7E3EFAAB3664CA5DB15328AA (SHA1)
server1.sfx.exe (1207134 bytes):
5F0C88D8BE30996E3D623E54F49A9E00ADEF3A2E (SHA1)
server1.sfx.exe (1201086 bytes):
325E84243E2E901C3CAAEAC533D8931E5C15F043 (SHA1)
Payload Files
yrqs.exe (614912 bytes):
F625A44582E4C790F3A90E5A7B896AA660166A60 (SHA1)
dtccnppbk.exe (591872 bytes):
1118A8A12CD2D59A32A8B6DCFFF279F20351FB9B (SHA1)
server1.exe (929792 bytes):
78F4166B7611428E076ADCDCCDA34F73DC95CE37 (SHA1)
server1.exe (955904 bytes):
0A36CAD9F18249D42E05685B00330583C20D0BC3 (SHA1)