Amazon Web Services (AWS) has provided more details on a major Elastic Compute Cloud (EC2) Instance reboot that’s set to take place in the coming days, and the good news is it doesn’t have anything to do with the Bash – or Shellshock – vulnerability revealed earlier this week.

In fact, as was previously suggested, the instances update is related to an issue with the Xen hypervisor. As to what that issue is, Amazon won’t be giving any specific details until after the issue has been resolved.

In its blog post, Amazon says about ten percent of its EC2 instances will be affected by a “timely security and operational update”.

“As we explained in emails to the small percentage of our customers who are affected and on our forums, the instances that need the update require a system restart of the underlying hardware and will be unavailable for a few minutes while the patches are being applied and the host is being rebooted,” Amazon’s Jeff Barr wrote.

It should be noted that anyone who uses the Xen hypervisor is affected by this issue, which includes Rackspace.

Any link to the Bash bug – a “catastrophic” new flaw affecting Linux and OS X systems – has been ruled out by AWS, meaning the timing of the two events is just a coincidence.

However, Forbes.com contributor Ben Kepes quotes an anonymous industry insider with “visibility over the broader impacts of this issue” as saying there’s a lot more to Amazon’s reboot than it’s prepared to admit:

“AWS is furiously rebooting pretty much every instance in every az [availability zone] to get the patch in in the next 5 days,” claims the source.

However bad the true situation is, Amazon is in a race against time to fix it. All of the updates must be completed before October 1, when details of the Xen flaw will be made public with the update XSA-108 release. Hopefully AWS and/or the Xen community will provide more details at that time.

As for AWS customers, the big take away is that ‘some’ instances will need to be rebooted, and it will happen sometime in the next five days. RightScale, a cloud consultancy, says the updates should begin around 10 PM ET on Thursday and run through Sept. 30 at 7:59 PM ET. Most customers won’t need to do anything, expect be ready for their instances to go offline for a few minutes if they’ve been notified by AWS.

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