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Adware program Vonteera blocks security products with simple Windows UAC trick

Lucian Constantin | Nov. 24, 2015
The program turns digital code signatures against anti-malware products.

Windows user account control
Windows User Account Control configuration panel. Click on image to enlarge. Credit: Lucian Constantin

A well-known adware program is preventing users from installing antivirus products by leveraging a Windows feature that was designed for security.

The program, known as Vonteera, abuses the digital signature check performed by the Windows User Access Control (UAC) for executable files.

UAC prompts users for confirmation whenever a program wants to make a system change that requires administrator-level privileges. It therefore prevents malware from silently gaining full system access if executed from a limited user account.

Depending on whether an executed file is digitally signed by a trusted publisher, the UAC displays confirmation prompts indicating different levels of risk. For example, if the file is unsigned, or is signed with a self-generated certificate that Windows can't link back to a trusted certificate authority, the UAC prompt will have a yellow exclamation mark.

However, if the file is signed with a certificate that was blacklisted, UAC will simply block the file from running and a red warning will be displayed.

It seems that the creators of Vonteera, whose purpose is to hijack browsers and display ads, have figured out that they can abuse this UAC behavior to prevent users from installing security products.

The program copies 13 digital certificates that were used to sign antivirus programs and security tools to the "Untrusted Certificates" store in Windows, researchers from security firm Malwarebytes said in a blog post.

The blacklisted certificates are from Avast Software, AVG Technologies, Avira, Baidu, Bitdefender, ESET, ESS Distribution, Lavasoft, Malwarebytes, McAfee, Panda Security, Trend Micro and ThreatTrack Security.

Vonteera creates a service that periodically checks if these certificates are present in the "Untrusted Certificates" store and adds them back if they're not.

Fortunately, this blacklisting of vendor certificates is only partially effective, said Bogdan Botezatu, a senior e-threat analyst at antivirus vendor Bitdefender. The technique only prevents new product installations or the execution of stand-alone removal tools that need administrator privileges. System drivers and services created by antivirus products that are already running would not be affected, he said.

However, if the user already has an antivirus running and Vonteera has managed to make these changes, it means that the product already failed to detect it and the user would need to install another tool to remove it -- one that might now be blocked.

Vonteera is quite persistent and intrusive, so users would have a hard time getting rid of it manually. The program creates multiple scheduled tasks to ensure its execution and to display advertisements on a regular basis. It also registers a system service, installs rogue extensions in Internet Explorer and Google Chrome and alters the browsers' shortcuts to automatically open a URL when clicked.

 

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