You don't want this Sleepwalker backdoor on your Windows machine

Like a sleeper cell awaiting activation, a never-before-seen Windows backdoor dubbed Sleepwalker waits silently in memory for one specifically crafted network packet to wake it up and deliver commands using the malware's 23-instruction language.

The commands can do everything from running code directly in memory to moving data off the computer.

Malware researcher Dominik Reichel discovered the passive backdoor, which also has its own command language, and detailed Sleepwalker in a technical analysis on Monday.

“What makes it worth writing up is what that packet carries: not a readable command, but a short program written in a command language of the backdoor’s own design,” Reichel said.

“Its 23 instructions cover scheduling, several ways to move data, staged file delivery and running code directly in memory.

Recovering the encryption key is not enough to understand one of these programs.

The internal command language must be reverse engineered as well.” In addition to having its own command language, it's also notable that the remote host can be a VMware VMCI target instead of a normal network address.

“Taken as a whole, the approach here is consistent with a targeted, well-resourced operation rather than an opportunistic one,” Reichel wrote.

The malware, hidden inside a 64-bit Windows DLL file, impersonates Microsoft's dpapi.dll, part of Windows' data protection API for protecting sensitive data.

It exports the same seven functions as the real dpapi.dll, but attempts to forward calls to a file named dpapisvc.dll, which is not a real Windows component.

The file also has a forged ESET Management Agent version resource, and loads via side-loading into ERAAgent.exe, the Windows executable for ESET Management Agent.

After confirming that its host process is named ERAAgent.exe, Sleepwalker goes to sleep inside the computer's memory, which also helps it remain hidden from traditional anti-virus tools.

Unlike most backdoors, which call back to an attacker-controlled command-and-control (C2) server and start receiving commands, Sleepwalker lies in wait, checking every packet that passes through the network looking for a specific pattern - this is called a magic packet.

Once it sniffs out a packet that matches the exact pattern, the backdoor decrypts the data and treats it as a command.