Microsoft employs a server-side SPIM filter on its Messenger network for blocking malicious URLs in order to keep its users safe from the spread of malware. The system doesn't always work, however. Most recently, Windows Live Messenger users have reported that TinyURL.com, a popular URL-shortening service that gets over 1.5 billion hits each month, is being blocked. Since the site can be potentially used to help send users to malicious websites, Microsoft may have done this intentionally. It may even be possible that there is currently a piece of malware out there that uses TinyURL to redirect users to a malicious site. Either way, TinyURL is being blocked regardless of what site a given URL redirects to:
Microsoft has blocked domains on the its instant-messaging service before, but last time, when YouTube and DeviantArt was blocked, Microsoft blamed its third-party partner that manages the URL blocking and explained that the blocking of a specific URL or whole domain is determined by multiple factors. Microsoft apologized for the blunder, but did not further detail how legitimate URLs were mistaken for harmful ones. This time, it's not clear if Microsoft considers TinyURL to be a legitimate site; we'll keep you posted as this story develops.
A Microsoft spokesperson told Ars: “We are very serious in our efforts to block virus, malware and other harmful URLs from being passed on to our customers. We’re continually working to improve this process so that we can keep our customers safe without having a negative impact on your Messenger service. We are aware that Windows Live Messenger is currently blocking the URL tinyurl.com. This URL is being blocked unintentionally and we are working to take the appropriate steps to remedy the situation as quickly as possible. We sincerely apologize for any inconvenience and disruption this may be causing our customers.”
As of 4:00pm PDT, TinyURL.com was unblocked from Windows Live Messenger.
Further readingMess.be: Microsoft bans TinyURL.com links from instant messages