Last week we promised to explain in detail how the “Blackbeard” Trojan infiltrates and hide itself in a victim’s system, especially on its 64-bit variant. Everything described in this blogpost happens just before Pigeon (clickbot payload) gets downloaded and executed. The most interesting aspects are the way it bypasses the Windows’ User Access Control (UAC) security feature and switches the run of 32-bit code of the Downloader to 64-bit code of the Payload. And finally, how the persistence is achieved.
From 32-bit Loader to 64-bit Payload
As almost all other malware, this downloader is encapsulated with a cryptor. After removing the first layer cryptor, we can see that the downloader is written in a robust way. The same code can be run under either a 32-bit or 64-bit environment, which the code itself decides on the fly based on the entrypoint of the unpacked layer. Authors can therefore encapsulate their downloader in either a 32-bit or 64-bit cryptor and it will get executed well in both environments.