A criminal gang recently found an effective way to spread malware that drains online bank accounts. According to a blog post published Monday, they bundled the malicious executable inside a file that installed a legitimate administrative tool available for download.

The legitimate tool is known as Ammyy Admin and is used to provide remote access to a computer so someone can work on it even when they don't have physical access to it. According to Monday's blog post, members of a criminal enterprise known as Lurk somehow managed to tamper with the Ammyy installer so that it surreptitiously installed a malicious spyware program in addition to the legitimate admin tool people expected. To increase their chances of success, the criminals modified the PHP script running on the Ammyy Web server, suggesting they had control over the website.

What resulted was a highly effective means for distributing the banking Trojan. That's because the legitimate tool Ammyy provided was in many ways similar to the banking Trojan in that they both provided remote access to the computer they ran on. As researchers from antivirus provider Kaspersky Lab explained:

We should note that attacks of this type (Watering Hole) are very effective, and doubly dangerous if they target the users of a remote administration software tool: administrators using such a tool might presume that a malware (or malicious activity) detection event reported by their security software is a false positive triggered by the presence of the remote administration tool itself, and allow the detected activity. Moreover, they could disable protection or add the malicious program to the tracking and checking exemption list, thus allowing it to infect the computer. Kaspersky Lab products detect this type of legitimate software (remote administration tools), but with a ‘not-a-virus’ verdict, displaying a yellow detection notification window. This is done in order to keep the user informed when remote access software is launched on a computer, because this type of software was used by Lurk operators without the victim’s knowledge or consent, and is still used by cybercriminals distributing other malware adapted to steal money.

Kaspersky Lab researchers say the Ammyy website has been breached several times. Even after removing the malicious code earlier this year, it somehow managed to come back. In June, after a law enforcement crackdown shut down the Lurk gang, the Ammyy site started distributing a new malicious program that had no ties to Lurk.

"This suggests the malicious actors behind the Ammyy Admin website breach are offering the chance to buy a place on their Trojan dropper in order to spread malware from ammyy.com," Kaspersky Lab researchers wrote.

The take away is that website infections can have serious consequences and are often extremely hard to remove. Sites that are caught distributing malware should probably not be trusted again.

Listing image by Wikipedia