Changed Block Tracking (CBT) is a VMware feature introduced in vSphere 4.0 in order to implement native incremental backup at source level. VMware VDAP uses this technology and so all backup and recovery software designed for VMware use it for speed the backup. But last week a critical issue has been discovered on all vSphere 6  version (included the lates Update 1a) where Changed Block Tracking (CBT) data on ESXi 6 cannot be trusted.