Windows 10 apps abused by BazarLoader 'call me back' attack

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A new attack campaign abuses the appxbundle format used by the Windows 10 app installer, in order to deliver malware via highly targeted spam campaigns, shares researchers.

Cybersecurity sleuths from Sophos said they became aware of the campaign after one of its own employees was targeted with the spam email that used social engineering techniques to get victims to open a URL.

Through a novel delivery mechanism, the link led to a malicious payload, which the researchers claim belongs to a malware family variously known as BazarBackdoor and BazarLoader, in order to prep the victim’s computer for future attacks.

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Novel delivery mechanism

In their breakdown of the delivery mechanism, the researchers note that instead of directing potential victims to a website, the phishing lure instead asks the web browser to invoke a tool that’s used by the Windows Store application, called AppInstaller.exe, to download and execute “whatever’s on the other end of that link.”

The researchers note that the attackers used two different web addresses for hosting this fake page in Microsoft’s cloud storage service, most likely in a bid to lend it some authenticity.

Once the campaign manages to install the BazarBackdoor malware it profiles the system, identifies its public facing IP address, and relays this information to its command and control (C2) server. The backdoor is now all set to receive further malicious payloads.

“Spamming a security company with malicious emails featuring a novel attack technique might not have been the best decision by the operators,” said Andrew Brandt, principal researcher at Sophos.

He adds that malware that comes in application installer bundles is not commonly seen in attacks. However, now that the technique has been demonstrated, the researchers expect it to “attract wider interest” from both sides of the fence. 

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Mayank Sharma

With almost two decades of writing and reporting on Linux, Mayank Sharma would like everyone to think he’s TechRadar Pro’s expert on the topic. Of course, he’s just as interested in other computing topics, particularly cybersecurity, cloud, containers, and coding.