Over the weekend, users of Microsoft's Windows Live Messenger instant-messaging service found themselves unable to send and receive links to torrent site the Pirate Bay, as reported by TorrentFreak, leading to speculation that Microsoft had singled out the Pirate Bay as a target for censorship.

Though the block now appears to be lifted, at the time it was in place, attempts to send links to the site resulted in an error message that read "The link you tried to send was blocked because it was reported as unsafe." This was experienced with both Microsoft's official client and third-party software.

Was this a deliberate attempt to block a site infamous for its role in distributing pirated media? Probably not. Windows Live Messenger has long used Microsoft's Smart Screen system to filter the URLs used in Messenger. Malware that distributes itself through Messenger is not uncommon—a friend appears to message you with a link to some great new Web page/application/whatever, with the link leading to some kind of malicious code—and Microsoft uses the ability to block these URLs to hinder the spread of such malware.

According to a Microsoft spokesman, "We block instant messages if they contain malicious or spam URLs based on intelligence algorithms, third-party sources, and/or user complaints. Pirate Bay URLs were flagged by one or more of these and were consequently blocked."

The filtering doesn't care about the content of the blocked sites per se, just that it has been flagged as malicious by some combination of user reports and algorithmic analysis. The spokesman continues, "Our filtering systems can block all URLs from a given domain if we observe a sufficient number of abusive URLs from that domain. We do this to protect our users from likely additional abusive URLs in the same domain."

While this might raise some privacy concerns—Messenger conversations are not subject to end-to-end encryption, and Microsoft could, in principle, read messages sent over the system—it doesn't appear to be any deliberate attempt to censor the Pirate Bay or block access to its content.