Proxylife security researchers discovered that QakBot Malware (aka QBot) operators started abusing the Windows 7 calculator application to side-load malicious payload onto the infected computers from July 11, 2022. The DLL Side-Loading is a common type of attack that exploits the process of handling Dynamic Link Libraries in Windows, which includes modifying the legitimate DLL and placing it in a folder where the OS loads it instead of the legitimate one. The attack chain starts with an email mass spamming campaign where the email contains an HTML file that directs you to download the password-protected Zip Archive with an ISO file. After mounting, the ISO file shows a “.LNK" file masquerading as a PDF file and the reason for the password is to evade antivirus detection. The ISO file that has a '.LNK' file contains a copy of ‘calc.exe’ (Windows calculator), alongside two DLL files named 'WinowsCodecs[.]dll' and '7533[.]dll' (payload). Once the user clicks the shortcut points in a calculator it triggers the infection by executing the 'calc.exe' via command prompt. When loaded, the Windows 7 Calculator file automatically searches for and attempts to load the legitimate WindowsCodecs DLL file. However, it does not check for the DLL in certain hard-coded system paths and will load any DLL with the same name if placed in the same folder as the 'Calc.exe' executable. The hackers take advantage of this bug by creating their own malicious WindowsCodecs.dll file that launches the other [numbered].dll file, which is the QBot malware. By installing QakBot through legitimate programs like Windows Calculator, security software may not detect the malware when loaded. However, the DLL sideloading fails to work in Windows 10 Calc.exe and later versions, forcing threat actors to bundle it to Windows Version 7.
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