DOUBLECUP Malware Uses Clever Trick to Hide PowerShell Payload in PNG Files
The DOUBLECUP malware appends a PowerShell payload after PNG files, using a simple text trick to enable easy extraction via the FINDSTR command.
New malware that utilizes steganography often captures the attention of security researchers, but the latest variant of DOUBLECUP malware presents a less sophisticated, yet still effective, approach to hiding its malicious payload. Instead of employing true steganographic techniques to embed code within image pixels, DOUBLECUP appends a PowerShell payload directly after a PNG file.
This method, while not technically steganography, employs a clever trick to facilitate the payload's execution. The appended PowerShell script begins with a Carriage-Return and Newline sequence (0x0D 0x0A). In Windows environments, this sequence signifies the end of a text line, a detail that proves crucial for the malware's evasion strategy.
The significance of this newline sequence lies in its ability to bypass the need for a custom extractor. Traditional methods for extracting hidden payloads often require specialized tools to decode or de-obfuscate the embedded code. However, DOUBLECUP's payload can be readily identified and extracted using standard Windows command-line utilities.
Specifically, the Windows FINDSTR command, which functions similarly to grep on Unix-like systems, can be leveraged to locate the payload. By using FINDSTR with a unique identifier present in the script, an attacker can efficiently extract the cleartext PowerShell commands.
Once extracted, the payload can be piped directly into the PowerShell interpreter for execution. This streamlined process allows attackers to deploy their malicious scripts with minimal technical overhead, making the malware easier to distribute and execute in targeted attacks.
While the technique might not be as technically complex as true steganography, its effectiveness stems from its simplicity and reliance on built-in system tools. This approach can help DOUBLECUP evade detection by security solutions that might be looking for more complex encoding or embedding mechanisms.
The discovery highlights a common theme in malware development: attackers often find simple, yet effective, ways to leverage existing system functionalities for malicious purposes. Researchers at the SANS Internet Storm Center noted this technique, emphasizing that while not true steganography, the method is a noteworthy evasion tactic.
Further analysis of DOUBLECUP's operations and its broader campaign objectives is ongoing. Security professionals are advised to remain vigilant against such file-appending techniques and ensure that their systems are protected against PowerShell-based threats.