CPU spiking on Microsoft Exchange Activesync

We had the MS Exchange ActiveSync app pool (W3WP process in task manager) spiking up to 99% and staying there. Recycling the ActiveSync pool only temporarily fixed it. It turned out that our HTTP redirect for OWA had been inherited by all virtual directories including the ActiveSync one. Due to a bug, this can happen even if you select not to apply the redirect to all sub-directories. Once we remove the redirect things started working again.