One of the most popular programs used by some to illegally share files under copyright has patched a serious software vulnerability.

The problem affects the P-to-P (peer-to-peer) program uTorrent as well as BitTorrent Mainline, another program based on the uTorrent code. It has been classified as "highly critical," the second most severe ranking of risk, by Secunia, a security vendor in Denmark.

Both programs use the BitTorrent protocol, which has become the most popular method of file sharing worldwide, according to iPoque, a company based in Leipzig, Germany, that specializes in traffic-management appliances for ISPs. The programs collect pieces of a particular file from other computers around the world and assemble it.

The vulnerability can be exploited if a user downloads a malicious torrent, which is a text file that coordinates the downloading of content. The problem causes a stack overflow, which can allow an attacker to upload other malicious software to a PC.

The bug was in the software for at least two years, wrote Rhys Kidd [cq], who is credited with the find and has written a short paper describing the problem.

Users should upgrade to the latest version, 1.8, which also includes support for the Internet Protocol v6, which increases the number of IP addresses available on the Internet. Utorrent will also prompt users to download the upgrade. The latest version of BitTorrent is available on its Web site.