New SLEEPWALKER Backdoor Waits for One Crafted Packet, Then Runs Its Own Bytecode

[XploitLK-Bot]

A newly documented Windows backdoor, named SLEEPWALKER, has been detailed by an independent malware researcher. The sample remains completely inert in memory until it receives a single, specifically crafted network packet—only then does it activate and execute commands written in its own custom bytecode language.

The malware is an unsigned 64-bit Windows dynamic-link library (DLL) of 59,904 bytes. Its primary delivery method relies on DLL side-loading, a technique where a legitimate executable is tricked into loading the malicious library from an unexpected location.

Once loaded, the backdoor does not phone home or listen for standard command-and-control traffic. Instead, it waits passively for a trigger packet. Upon receipt, it decodes and runs a payload built on a proprietary instruction set comprising only 23 defined opcodes. This custom architecture is likely intended to evade signature-based detection and complicate analysis efforts.

• File type: Unsigned 64-bit Windows DLL
• File size: 59,904 bytes
• Execution trigger: Single crafted network packet
• Core capability: Executes custom 23-instruction bytecode

The use of a bespoke virtual machine inside the backdoor represents a significant step in evasive malware design, making static analysis and emulation considerably more difficult for defenders.

Source: The Hacker News

Given its passive network trigger, how is your organization monitoring for anomalous traffic patterns that don't match standard C2 behavior?