Skype has blamed last week's Windows security updates for triggering a bug in its software that brought down the Internet telephony service for more than 48 hours.
Service was restored late Friday.
The Skype peer-to-peer network became unstable and suffered a critical disruption" beginning last Thursday, said spokesman Villu Arak in a statement posted on the company's site. "The disruption was triggered by a massive restart of our users' computers across the globe within a very short time frame as they rebooted after receiving a routine set of patches through Windows Update."
According to Arak, the large number of restarts started a chain reaction that brought down the service. After users' machines rebooted and came back online, Skype was hit with a "flood" of attempted log-ins that, combined with a smaller-than-usual number of systems available to handle the peer-to-peer traffic, caused the blackout.
Although Skype fingered Tuesday's Windows updates for triggering the outage, it said the root cause was "a previously unseen software bug within the network resource-allocation algorithm" that prevented the network from recovering on its own, as it was supposed to do.
"Skype has now identified and already introduced a number of improvements to its software to ensure that our users will not be similarly affected in the unlikely possibility of this combination of events recurring," Arak added. The company did not specify what that combination of events may have been and did not explain how this month's updates were different from past rounds of patches.
For example, Microsoft's security updates have been on their current schedule of the second Tuesday of each month since October 2003, before Skype left beta testing. And required restarts are the norm for many of Microsoft's security updates. Nor was the quantity of restarts last week -- five of August's nine updates required a reboot -- out of line with previous months. In July, for instance, four of six updates also restarted affected PCs.
Some Windows users weren't buying the explanation. "If a 'massive restart' was the cause of the problem, it should have happened long ago," said Marcus McCurdy in a comment posted on the istartedsomething blog. "Definitely calling BS on this one."