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Extenbro DNS-Changer Used in Adware Campaign

A recently observed DNS-changer Trojan is being used in an adware campaign to prevent users from accessing security-related websites, Malwarebytes reveals.

A recently observed DNS-changer Trojan is being used in an adware campaign to prevent users from accessing security-related websites, Malwarebytes reveals.

While the tactic isn’t new, this threat features some additional tricks and the actors behind this attack have been known to use aggressive techniques in the past. In this instance, they simply attempt to prevent users from removing their adware from the compromised systems.

Dubbed Extenbro, the Trojan is delivered via a bundler. Once on the compromised device, the malware changes the DNS settings to prevent the system from accessing the websites of security vendors. Users can only notice the addition of four DNS servers by heading to the Advanced DNS tab in Windows.

To ensure that the DNS changes live past computer reboots, the malware creates a randomly-named Scheduled Task that points to a fixed-location folder, Malwarebytes reveals.

Additionally, the Trojan adds a certificate to the set of Windows Root certificates. The certificate has no “Friendly Name” and is supposedly registered to abose[at]reddit[dot]com.

The threat also disables IPv6 by changing the registry value DisabledComponents under the key HKEY_LOCAL_MACHINESYSTEMCurrentControlSetservicesTCPIP6Parameters. Thus, it forces the system to use the new DNS servers.

On top of that, the Trojan makes a change in the Firefox user.js file and configures the browser to use the Windows Certificate Store where its root certificate was added, Malwarebytes’ security researchers explain.

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The researchers also provide a detailed step-by-step guide to how the malware can be removed from the system and the modifications reset to the original settings.

To restore their DNS settings and download a security tool, users should head to Advanced TCP/IP Settings and select the DNS tab, where the four DNS servers added by the malware are listed.

Users should simply remove those to be able to visit security sites again, but should not reboot the system, as that could restore the malicious settings by the Scheduled Task belonging to the Trojan.

To change the Firefox settings back to their originals, users should type about:config in the address bar, search for security.enterprise_roots.enabled and change it to the default setting, “False.”

Related: ‘MaMi’ Mac Malware Hijacks DNS Settings

Related: Hundreds Targeted in Recent Roaming Mantis Campaign

Written By

Ionut Arghire is an international correspondent for SecurityWeek.

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