A two-day outage that left millions of Skype users unable to use the popular Internet phone call program was caused by an abnormally high number of restarts after people had downloaded a Windows security update through Microsoft Corp's automatic updates, the company said on Monday.
The worldwide outage, which began last Thursday and ended on Saturday, left millions of Skype users unable to log on to make phone calls or send instant messages.
Luxembourg-based Skype Ltd, part of online auction giant eBay Inc, has more than 220 million users in total but typically has 5 million to 6 million users online at any given time. In January, Skype reported that it had counted 9 million users online at one time.
In an update to users on Skype's Heartbeat blog, employee Villu Arak said the disruption was not because of hackers or any other malicious activity.
Instead, he said that the disruption "was triggered by a massive restart of our users' computers across the globe within a very short timeframe as they re-booted after receiving a routine set of patches through Windows Update," he wrote at 1:45pm.
"The high number of restarts affected Skype's network resources. This caused a flood of log-in requests, which, combined with the lack of peer-to-peer network resources, prompted a chain reaction that had a critical impact," Arak wrote.
Arak did not blame Microsoft for the troubles and said the outage ultimately rested with Skype. Arak said Skype's network normally has an ability to heal itself in such cases, but a previously unknown glitch in Skype's software prevented that from occurring quickly enough.
In a statement, Microsoft described its patch as routine and said that the disruption resulted from a bug in Skype software.
The outage was a critical moment for the company, founded in 2003 by Niklas Zennstrom and Janus Friis, and was the first major outage since October 2005 when its service was down only for a few hours.