A CIO update has just been posted on Skype’s blog, explaining the cause of their major downtime last week. The issue turned out to be a bug in the Windows version of Skype (version 5.0.0152), keeping messages from being properly processed on Skype’s servers. Ouch.

Skype’s outage last week raised quite a few questions for the company seeking an IPO, amongst other areas of opportunistic growth. Having avoided such a massive outage for a few years running now, this last debacle was an unwelcome holiday surprise, affecting upwards of 10 million users worldwide.

As a follow-up to last week’s outage, Skype has posted a detailed response regarding the outage, its cause, how the company handled it, and how Skype plans to keep this situation from happening again in the future, which is really the key for Skype handling this very public situation. Some highlights from Skype’s blog this morning:

What was the cause for the failure?

On Wednesday, December 22, a cluster of support servers responsible for offline instant messaging became overloaded. As a result of this overload, some Skype clients received delayed responses from the overloaded servers. Because of a bug identified in a version of the Skype for Windows client (version 5.0.0152), the delayed responses from the overloaded servers were not properly processed, causing Windows clients running the affected version to crash.

Users running either the latest Skype for Windows (version 5.0.0.156), older versions of Skype Windows (4.0 versions), Skype for Mac, Skype for iPhone, Skype on your TV, and Skype Connect or Skype Manager for enterprises were not affected by this initial problem.

However, around 50% of all Skype users globally were running the 5.0.0.152 version of Skype for Windows, and the crashes caused approximately 40% of those clients to fail. These clients included 25–30% of the publicly available supernodes, also failed as a result of this problem.

What is Skype doing to prevent this from happening again?

We understand how important the reliability, security and quality of our software is to Skype users around the world, and we work hard to maintain high standards, as well as develop new features and products.

First, we will continue to examine our software for potential issues, and provide ‘hotfixes’ where appropriate, for download or automatic delivery to our users. Since a bug was identified in Skype for Windows (version 5.0.0.152), we had provided a fix to v5.0 of our Windows software prior to the incident, and we will provide further updates for download this week. We will also be reviewing our processes for providing ‘automatic’ updates to our users so that we can help keep everyone on the latest Skype software. We believe these measures will reduce the possibility of this type of failure occurring again.

Second, we are learning the lessons we can from this incident and reviewing our processes and procedures, looking in particular for ways in which we can detect problems more quickly to potentially avoid such outages altogether, and ways to recover the system more rapidly after a failure.

Third, while our Windows v5 software release was subject to extensive internal testing and months of Beta testing with hundreds of thousands of users, we will be reviewing our testing processes to determine better ways of detecting and avoiding bugs which could affect the system.

Finally, as we continue to grow, we will keep under constant review the capacity of our core systems that support the Skype user base, and continue to invest in both capacity and resilience of these systems. An investment program we initiated a year ago has significantly increased our capacity already and more investment is planned for 2011 both to support the ongoing roll out of our paid and enterprise products, and to continue to support the growth of our core Skype software that we know millions of users rely on every day.