We highlighted in our quarterly threat roundup how various ransomware variants and other similar threats like CryptoLocker that now perform additional routines such as using different languages in their warning and stealing funds from cryptocurrency wallets. The addition of mobile ransomware highlights how these threats are continuously improved over time.
We recently encountered another variant that used the Windows PowerShell feature in order to encrypt files. This variant is detected as TROJ_POSHCODER.A. Typically, cybercriminals and threat actors have used Windows Powershell to go undetected on an affected system, making detection and analysis harder. However, once detected, using PowerShell made it easier to analyze as this malware is also hardcoded. Decrypting and analyzing this malware was not too difficult, particularly compared to other ransomware variants.
Since it uses Powershell, TROJ_POSHCODER.A is script-based, which is not common for ransomware. It uses AES to encrypt the files, and RSA4096 public key cryptography to exchange the AES key. When executed, it adds registry entries, encrypts files, and renames them to {filename}.POSHCODER. It also drops UNLOCKYOURFILES.html into every folder. Once all files on the infected system are encrypted, it displays the following image:
Figure 1. Instructions on how users can supposedly retrieve their files
Once users followed the instructions stated in the ‘ransom note,’ they will see the image below informing them to install the Multibit application that will allow them to have their own Bitcoin-wallet account for 1 Bitcoin. When they purchase the application, they are instructed to submit the form that contains information like email address, and BTC address and ID. Users will supposedly get the decryptor that will help encrypt the files.
Figure 2. Users need to fill this form once they avail of the Multibit application
Currently, POSHCODER uses English for its ransom notes and primarily affects users in the United States.
Ransomware and other similar threats are continuously improving as exemplified by the emergence of POSHCODER. Trend Micro protects users from this threat via its Smart Protection Network that detects the malicious file.
