QBot phishing uses Windows Calculator sideloading to infect devices

01 August 2022

The operators of the QBot malware have been using the Windows Calculator to side-load the malicious payload on infected computers.

DLL side-loading is a common attack method that takes advantage of how Dynamic Link Libraries (DLLs) are handled in Windows. It consists of spoofing a legitimate DLL and placing it in a folder from where the operating system loads it instead of the legitimate one.

QBot, also known as Qakbot is a Windows malware strain that started as a banking trojan but evolved into a malware dropper, and is used by ransomware gangs in the early stages of the attack to drop Cobalt Strike beacons.

Security researcher ProxyLife recently discovered that Qakbot, has been abusing the the Windows 7 Calculator app for DLL side-loading attacks since at least July 11. The method continues to be used in malspam campaigns.

New QBot infection chain

To help defenders protect against this threat, researchers documented the latest QBot infection chain.

The emails used in the latest campaign carry an HTML file attachment that downloads a password-protected ZIP archive with an ISO file inside.

The password for opening the ZIP file is shown in the HTML file, and the reason for locking the archive is to evade antivirus detection.

The ISO contains a .LNK file, a copy of 'calc.exe' (Windows Calculator), and two DLL files, namely WindowsCodecs.dll and a payload named 7533.dll.

When the user mounts the ISO file, it only displays the .LNK file, which is masqueraded to look like a PDF holding important information or a file that opens with Microsoft Edge browser.

However, the shortcut points to the Calculator app in Windows, as seen in the properties dialog for the files.

Clicking the shortcut triggers the infection by executing the Calc.exe through the Command Prompt.

When loaded, the Windows 7 Calculator automatically searches for and attempts to load the legitimate WindowsCodecs DLL file. However, it does not check for the DLL in certain hard coded paths, and will load any DLL with the same name if placed in the same folder as the Calc.exe executable.

The threat actors take advantage of this flaw by creating their own malicious WindowsCodecs.dll file that launches the other [numbered].dll file, which is the QBot malware.

By installing QBot through a trusted program like the Windows Calculator, some security software may not detect the malware when it is loaded, allowing the threat actors to evade detection.

It should be noted, that this DLL sideloading flaw no longer works in Windows 10 Calc.exe and later, which is why the threat actors bundle the Windows 7 version.

QBot has been around for more than a decade, with origins going as far back as 2009 [1234]. While campaigns delivering it are not frequent, it was observed being distributed by Emotet botnet in the past to drop ransomware payloads.

Among the ransomware families that QBot delivered are RansomExx, Maze, ProLock, and Egregor. More recently, the malware dropped Black Basta ransomware.

Related News

The Benefits of Building a Mature and Diverse Blue Team

15 Aug 2022

A few days ago, a friend and I were having a rather engaging conversation that sparked my excitement. We were discussing my prospects of becoming a red teamer as a natural career progression. The reason I got stirred up is not that I want to change either my job or my position, as I am a happy camper being part of blue team.

Read More

Hackers scan for vulnerabilities within 15 minutes of disclosure

08 Aug 2022

System administrators have even less time to patch disclosed security vulnerabilities than previously thought, as a new report shows threat actors scanning for vulnerable endpoints within 15 minutes of a new CVE being publicly disclosed.

Read More

New Luna ransomware targets Windows, Linux and ESXi systems

25 Jul 2022

A new ransomware family dubbed Luna can be used to encrypt devices running several operating systems, including Windows, Linux, and ESXi systems.

Read More