Figure 1: Recent Locky spam campaign
Figure 2. Locky spam ZIP attachment containing JS downloader
The new downloader has a custom network communication protocol. In our tests, it only downloads the Locky ransomware as its payload. This malware seems to be in its early development stage as it only supports commands for download and execution of an executable and deletion of itself. This means the malware can also update its own binary, leading to the possiblity of more commands being supported.
The malware communicates with its command and control (C2) over HTTP using a custom encryption algorithm. The first beacon to the hard-coded C2 asks for a task to be executed by the malware. An example of the unencrypted message sent to C2 is formatted, as shown in Figure 3.
Figure 3. Raw message format
ID1 – derived from HDD Volume Serial Number
ID2 – 2222222222 (hard-coded value)
ID3 – random generated number
ID4 – derived from bit-masked OS version and system architecture
time – UTC time the message is created
type – getjob (hard-coded value)
This beacon string is encrypted with the custom algorithm shown in Figure 4 before sending it to its C2. The custom encryption is composed of XOR and bit shifts.
Figure 4. Custom string encryption
After encryption, an ‘A’ (0x41h) character is appended to the encrypted message. The beacon request is delivered via an HTTP POST request. In this sample, it reaches out to hxxp://raprockacademy.com/api, as shown in Figure 5.
Figure 5. Encrypted HTTP POST request and C2 response
The C2 server responds with an encrypted message that tells the malware what action to take. Decrypting the C2 response is possible with the Python code shown in Figure 6.
Figure 6. C2 reponse decryptor
The decrypted message shows a URL to download a binary and, in this case, an updated Locky binary.
Figure 7. Decrypted message
The ‘command’ field can be ‘UPDATE’, ‘NOTASKS’, and ‘DEL’ – ‘NOTASKS’ being no further instructions from the C2 for the moment and ‘DEL’ for deletion of the downloader from the victim machine through drop and execute of a batch file.
Further inspection of this malware reveals several small DLL files embedded in the binary. These DLLs may be used depending on the OS environment of the compromised system. The following is a brief description of the embedded DLLs:
1. 32-bit and 64-bit DLLs, which executes a file via the CreateProcessW API.
2. 64-bit binary used for bypassing User Account Control (UAC). Debug symbol path is not stripped in the binary:
3. 64-bit binary which can elevate privileges for a specified process.
The Locky sample downloaded (MD5: 357c162a35c3623d1a1791c18e9f56e7) has updated its DGA. The DGA has the following differences:
We provide an update to the shared DGA code from our previous blog, as shown in Figure 8.
Figure 8. Updated Locky Domain Generation Algorithm
The actors behind the Locky ransomware are actively seeking new ways to successfully install their malware on victim computers. That may be one of the reasons this new downloader is used and being introduced to the current distribution framework. This downloader can be a new platform for installing other malware (“Pay-per-Install”).