Threat actors have been observed leveraging a now-patched security flaw in Microsoft Windows to deploy an open-source information stealer called Phemedrone Stealer.

"Phemedrone targets web browsers and data from cryptocurrency wallets and messaging apps such as Telegram, Steam, and Discord," Trend Micro researchers Peter Girnus, Aliakbar Zahravi, and Simon Zuckerbraun said.

"It also takes screenshots and gathers system information regarding hardware, location, and operating system details. The stolen data is then sent to the attackers via Telegram or their command-and-control (C&C) server."

The attacks leverage CVE-2023-36025 (CVSS score: 8.8), a security bypass vulnerability in Windows SmartScreen, that could be exploited by tricking a user into clicking on a specially crafted Internet Shortcut (.URL) or a hyperlink pointing to an Internet Shortcut file.

The actively-exploited shortcoming was addressed by Microsoft as part of its November 2023 Patch Tuesday updates.

The infection process involves the threat actor hosting malicious Internet Shortcut files on Discord or cloud services like FileTransfer.io, with the links also masked using URL shorteners such as Short URL.

The execution of the booby-trapped .URL file allows it to connect to an actor-controlled server and execute a control panel (.CPL) file in a manner that circumvents Windows Defender SmartScreen by taking advantage of CVE-2023-36025.

"When the malicious .CPL file is executed through the Windows Control Panel process binary, it in turn calls rundll32.exe to execute the DLL," the researchers said. "This malicious DLL acts as a loader that then calls on Windows PowerShell to download and execute the next stage of the attack, hosted on GitHub."

The follow-on payload is a PowerShell loader ("DATA3.txt") that acts as a launchpad for Donut, an open-source shellcode loader that decrypts and executes Phemedrone Stealer.

Written in C#, Phemedrone Stealer is actively maintained by its developers on GitHub and Telegram, facilitating the theft of sensitive information from compromised systems.

The development is once again a sign that threat actors are getting increasingly flexible and quickly adapting their attack chains to capitalize on newly disclosed exploits and inflict maximum damage.

"Despite having been patched, threat actors continue to find ways to exploit CVE-2023-36025 and evade Windows Defender SmartScreen protections to infect users with a plethora of malware types, including ransomware and stealers like Phemedrone Stealer," the researchers said.