Vertexnet Botnet Spotted Using AutoIt

Researchers at McAfee found new malware samples using AutoIt, joining the growing ranks of malware using the scripting language.

According to McAfee, the samples belonged to the Vertexnet botnet. The malware uses multiple layers of obfuscation – but once decoded, connects to a control server to accept commands and transfer stolen data.

In McAfee’s analysis, the sample is packed using a custom packer. Once executed, it drops three files in the %TEMP% folder.

“These files are compiled using aut2exe,” McAfee researcher Hardik Shah blogged. “The malware next executes the file botnet.exe from the temp folder. This file is written in AutoIt.”

The file uses numerous encrypted, obfuscated variables. Decoding them revealed that the script calls various Windows APIs using the AutoIt DLL functions DllStructCreate, DllStructGetPtr, DllCall and others.

“The technique of running an executable from memory through an AutoIt script is well documented…To summarize, it first creates a process with with the CREATE_SUSPENDED flag,” Shah wrote. “Next it uses GetThreadContext to get the CONTEXT structure. Subsequently, it uses WriteProcessMemory,SetThreadContext and allocates memory for the data.”

“Then it resumes the thread,” Shah continued. “After dumping the data in WriteProcessMemory calls, we get a Visual Basic file, which uses the RunPE method to execute the payload. After dumping the data in WriteProcessMemory calls, we get a Visual Basic file, which uses the RunPE method to execute the payload.”

In May, Trend Micro researchers noted both an increase in the amount of malware using AutoIt as a scripting language as well as the presence of AutoIt hacker tools online. 

“The increased usage of AutoIt is likely attributed to the fact that AutoIt is scalable, very similar to Basic, and is outrageously easy to code in,” Trend Micro Senior Threat Researcher Kyle Wilhoit blogged at the time. “This ease of use takes the learning curve off learning more complex languages such as Python. This opens up a wide array of possibilities to hackers that may not otherwise expose themselves to a scripting language. In addition, the ability to host code on Pastebin, natively compile, and run applications in stand-alone executable files makes it very quick to develop in. Finally, the ability to natively support UPX packing in AutoIt makes obfuscation easy for AutoIt applications.”

McAfee’s write-up can be found here.