PC users are being threatened by a new password-stealing Trojan, purporting to come from Microsoft. The malware, dubbed Trojan-PSW.Win32.Sinowal.u by anti-virus software developer Kaspersky Lab, is in German although, according to Kaspersky, the Trojan originated in Russia.

The Trojan, which has been disguised as an official Microsoft Windows update. is an example of a generation of Trojans whose frequency is on the rise, said Roel Schouwenberg, a senior research engineer with the company. The Sinowal family of malware was first detected in December, and first seeded on malicious websites.

If a user visited the site and did not have a properly patched browser, the software would install itself, allowing it to harvest login and password information for some European banks' websites, Schouwenberg said. The Sinowal family of malware may have been created in Russia, since the malware code contains some Russian, he said.

The latest spam messages have a ".de" e-mail address. Rather than depending on a browser exploit to install itself, the latest version of Sinowal tries to trick users into installing it. The message, written in German, claims that a new worm is on the loose, and that the recipient should run the attached file to protect their system.

Schouwenberg said the malware writers may have decided to send it by mass e-mail if the browser exploit approach wasn't working as well.

The Sinowal Trojan is a type of "man-in-the-middle" malware. Even if a user has started a Secure Sockets Layer (SSL) transaction with a bank, the Sinowal Trojan can insert HTML code that causes a pop-up window asking for a user name and password. It is programmed to react to certain bank websites.

"This is something we are going to see more and more and really make life hard," Schouwenberg said.

It's unique since it then sends that information immediately to the hacker's server rather than storing the information for periodic transmission, Schouwenberg said. The Trojan is also capable of checking for updates of itself.