Microsoft researchers shared the example of a password-stealing Trojan bundled up with a game tool used for customizing the massively popular Dota2 Warcraft battle arena mod.
Made by an unknown third party and offered for free on an unofficial site, the tool works but the malicious bundle also drops and runs the Trojan, which then begins to harvest stored passwords for a variety of FTP and IM clients and web browsers. The stolen passwords are then encrypted and sent to the bundle author either by email or FTP.
To show you how easy this malicious bundling is done, the researchers have tracked down the malware builder tool used in this case, which is publicly available for download and obviously geared towards Russian-speaking users (click on the screenshot to enlarge it):
"Once a trojan is created with the builder, an author can choose to bundle the malware with legitimate tools, software or images", they point out.
The final product (bundle) is usually distributed via download sites, and the link to the download page sent out by email or comment spamming.