Extenbro Trojan replaces DNS and blocks access to antivirus sites

Malwarebytes Labs specialists discovered Extenbro Trojan, which not only replaces DNS for displaying advertisements, but also does not allow the user to visit anti-virus and other security products.

With this feature, user cannot download and install any protective program and get rid of malware. Researchers warn that by doing so, malware puts infected machines at risk from other types of attacks, effectively leaving infected systems unprotected.

“Similar malicious behavior has already been demonstrated by Vonteera malware, which used system certificates to disable anti-virus solutions on infected machines”, — experts say.

Extenbro is mainly distributed as part of various promotional bundles.

After installing such a “software package”, the user will find that on his machine appeared unwanted advertisements that originate not from the sites he visits; a new start page may appear in the browser. Finally, Malwarebytes Labs experts found that Extenbro is distributed as part of malware from the Trojan.IstartSurf bundle family.

In order to demonstrate unwanted advertising, Extenbro changes the DNS settings in the victim’s system. Moreover, the researchers note that

“Moreover, if a user opens the settings and switches to the Advanced DNS tab, he will find that four new DNS servers have appeared in the system at once, and not two, as is usually the case. Unfortunately, not all users will be able to switch to advanced settings and pay attention to the fact that there are not two, but four servers at once”, – note the specialists of Malwarebytes Labs.

What is worse, if the victim tries to get rid of the fraudulent DNS servers by deleting them, the malware will re-add them to the settings after each system restart, since at the infection stage, a scheduled task with a random name is created for this.

Additionally, malware completely disables the IPv6 system so that the victim cannot bypass the malicious DNS servers and protect his computer. Another “safety net” is adding a new root certificate to Windows root certificates and making changes to the Firefox user.js file (the security.enterprise_roots.enabled parameter is set to true).

This forces Firefox to use the Windows certificate store, where the attacker’s root certificate was added.

In the Malwarebytes Labs blog, you can find instructions on how to remove such malware from the system, and there were also published all the necessary indicators of compromise.