Darkshell is a distributed denial of service (DDoS) botnet targeting Chinese websites. It was found in 2011 and was first analyzed by Arbor Networks. McAfee Labs recently analyzed a few new samples that turned out to be variants of Darkshell, and we found extensive variations in network traffic and control commands.
The Darkshell bot follows a fairly standard installation process by copying itself into the System32 directory with a name that appears to be legitimate, for example, C:\WINDOWS\system32\WinHe803.exe. It then sends the system information of the infected machine to its control server in encrypted format. Once the control server receives the information, it responds with the victim’s address and the type of DDoS attack to perform.
Here are a few of the MD5 hashes we analyzed:
The binaries we analyzed were compiled in a way that it makes them hard to reverse engineer (and ease our analysis). Each binary contained a lot of junk code and made multiple calls between the junk codes to complete a single task. We also found that the binaries used antidebugging and antidisassembly techniques to evade disassembly and reversing. The code was written in C++.
Let’s dig into some detailed analysis of the new variant of Darkshell. The binary executed fake code along with a debugger detection check, and exited the process while debugging. The binary accessed heap flags from the Process Environment Block (PEB) structure to detect our debugger. (Heap flags are set to “0?50000062” when a process is being debugged.) The following figure shows the actual code from the botnet binary:
If the flags are set to 0?50000062, the bot will detect the debugger and will exit the process. Once we bypassed this defense, the botnet started its decryption routine with the help of the hard-coded XOR key shown below:
First, the binary decrypted 1917 (hex) bytes of the code with the preceding XOR key, starting at the address 0?00401000. Next, it decrypted all the strings using the same XOR key. Following that, the bot built its import address table using the “LoadLibrary()” and “GetProcAddress()” functions. We found it interesting that the bot did not call LoadLibrary() and GetProcAddress() in the usual way. In this sample, it first pushed the address of the LoadLibrary() function on the stack and then returned to it, as shown in the next image:
After the LoadLibrary() function, the sample returned to a piece of code that the OllyDbg 1.1 debugger failed to recognize.
This maneuver means the binary uses an antidisassembly technique. To get around this obstacle, we tried OllyDbg 2.0, which successfully assembled the code as follows:
The malware used a similar move with the GetProcAddress() function, pushing the address on the stack and returning to it. This way the bot built its import address table and jumped to original entry point, as we can see in the next image:
By looking at the preceding code, we can easily say that this is a generic Microsoft Visual C++ executable entry point. Finally, we dumped the process and fixed the import table to unpack it in the original format. Here are the strings from the unpacked binary:
Now we have our unpacked binary. Rather than dive into the reverse engineering, we will focus on network control activity. The bot sends a TCP packet of 228 bytes in encrypted format to its control server. Here is the packet capture:
We identified the routine in use-a fairly simple XOR and substitution-with the help of the hard-coded value “7DB” in following snippet:
The XOR key is generated and data is encrypted in the “encryption_routine” function seen above.
The preceding encryption algorithm can be translated as follows:
Encrypted Byte = (Original Byte ^ XOR Key) + XOR Key
And hence the decryption algorithm is:
Original Byte = (Encrypted Byte – XOR Key) ^ XOR Key
Applying this decryption algorithm on the encrypted packet results in the following:
Here’s an analysis of the Darkshell control structure of 228 bytes:
char Processor; // Processor information
char Memory; // Memory information
char OS; // Operating System information
char Version; // Bot version information
As we see above, the bot sends processor, memory, and operating system information along with its version, “VIP0410” in this sample. Once the control server receives this information, it replies with 124 bytes of data that contain the victim’s address and method for launching the DDOS attack. The response packet is not encrypted, as we see below:
The first 4 bytes describe the type of attack. In the preceding case the value “0?00000400” launches an HTTP GET request with a small-header DDOS attack on the victim, using port number 80. The port value is specified at offset “0x009E,” which is 50 (or 80 in decimal).
The response attack structure of the 124 bytes:
DWORD dwCode; // attack method
char Target; // URL of target, NULL-terminated/extended
DWORD Port // Port to attack
DWORD ThreadCount // number of threads to create
DWORD dwMilliseconds // Sleep (in milliseconds)
DWORD socketCounter1 // Counter to create sockets
DWORD socketCounter2 // Counter to create sockets
Once the bot receives the response, it parses the attack method and creates multiple threads with attack methods on the infected machine, according to the thread count. The bot supports multiple attack methods, including SYN flood, UDP flood, ICMP flood, SuperSYN flood, GET requests flood, etc. The bot can also download and execute malicious binaries from the control servers.
An HTTP GET requests attack with a small header looks like this:
Here are a few of the control domains we identified:
Further investigation revealed that the Darkshell botnet source code is available online. We found the www.darkshellnew.com domain, which calls itself the official Darkshell website. It hosts various versions of Darkshell botnet builder that can be downloaded free with source code. Here is a screenshot of the homepage (when converted from Chinese to English):
Here are the different versions of Darkshell botnet builders available to download:
Our research shows that variants of the Darkshell botnet are still evolving, with features such as antidebugging and antidisassembly techniques to make reverse engineering more time consuming. The botnet can launch DDOS attacks using different methods and can flood websites. Further, the presence of free Darkshell builders with source code on the Internet opens up the evolution of other variants with other mechanisms.
I would like to thank my colleague Amit Malik for contributing to this botnet research.
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