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.
Affected users have several options to bypass Vonteera's changes to the Windows certificate blacklist so they can install an antivirus product. They could disable UAC entirely, but this is not recommended because it reduces the system's security.
They could also manually remove the certificates from the "Untrusted Certificates" store by using the Windows Certificate Manager tool, but then they have to act fast before Vonteera puts them back. This can be done by hitting the windows key + r to open a Run prompt then typing certmgr.msc. In the left panel they can browse to Untrusted certificates > Certificates and remove certificates that have an antivirus vendor's name.
Finally, they could use a trick that uses scheduled tasks to bypass UAC prompts in order to install their desired antivirus tool, use it to remove Vonteera, then manually remove the blacklisted certificates, the Malwarebytes researchers said.
Because of this intrusive behavior, Malwarebytes has changed Vonteera's classification from a potentially unwanted application to a clearly malicious application, detecting it as a Trojan. Other antivirus products including Bitdefender and ESET also have detection routines for it.