There are multiple ways updates can be deployed through WSUS to client machines (“client machines” mean clients of the WSUS server - the machines may be running either client or server operating systems). This posting describes these mechanisms and the way they can be controlled by the administrator in order to ensure unexpected changes do not occur.
· Explicit approval. An administrator can explicitly approve an update for installation to a group of machines.
· Auto-reapprove revisions. By default, when a new revision of an approved update is synchronized to the WSUS server we move the approval to the new revision. Normally this is what customers want, since new revisions never contain new binaries, just fixes to the metadata that describe how to automate the installation of the update. However we had one incident when a new revision of the Windows Desktop Search update changed the metadata so that the new revision was offered to *all* machines but the old revision was offered only to machines with older versions of Desktop Search installed, which caused it to be deployed more widely than expected for many customers (see http://blogs.technet.com/wsus/archive/2007/10/25/wds-revision-update-expanded-applicability-rules-auto-approve-revisions.aspx for details). Since then, we’ve added processes to ensure this type of change will not happen again. The administrator has direct control over this and can disable the option to auto-reapprove revisions.
o Warning: turning off auto-reapprove revisions can create problems if the administrator has “definition updates” (signatures) in their synchronization options, because definition updates get created and expired fairly quickly and the expired ones won’t get auto-unapproved. As described in KB 938947, this can quickly lead to having too many updates approved which can cause problems for client-server communication. If auto-reapprove revisions is turned off, the administrator will need to manage revisions themselves; looking for older revisions that are approved and either unapproving them (if the new revision is marked “expired”) or move the approval to the new revision. We have provided a PowerShell sample script at http://www.microsoft.com/technet/scriptcenter/scripts/sus/server/susvms09.mspx that can be used to manage revisions.