Amazon Web Services will be updating a substantial number of its cloud servers, and has asked customers to re-launch their instances in the coming days, according to AWS consultants. Amazon has not said why the reboot is happening, but AWS watchers believe it is to patch a security issue. The company will be updating many of its servers between Thursday, Sept. 25 at 10 PM ET and Tuesday Sept. 30 at 7:59 PM ET, according to Amazon partner and cloud consultancy RightScale.

Amazon Web Services has kicked off a large scale maintenance reboot of Elastic Compue Cloud (EC2) instances across the world in what appears to be a response to a critical security flaw. Amazon has told EC2 customers by email that it’s gearing up to reboot a range of EC2 instance types across all availability zones over the next few days, and that users won’t be able to do anything to stop the process. Amazon occasionally schedules instances for a reboot to apply patches, upgrades, or maintain a host, but in this case hasn’t told users what’s behind this month’s overhaul, causing some to speculate that it is applying a pre-released but embargoed (XSA-108) fix for a bug in the open source hypervisor, Xen.