It didn't surprise me that I found discrepancies. Our termination process relies on e-mail -- HR sends a notice to a distribution list, which has a lot of people on it, and the administrators of the various systems are supposed to disable account access for the people who have left the company. But there's no real check on that process -- if the administrator doesn't follow through with removing the access, nobody knows, until someone like me comes along later and checks.
I made a list of our known SaaS services and their assigned administrators and spoke with each of those people to highlight the importance of removing access for departed employees in a timely fashion. It wasn't too difficult to get them to remove the accounts I flagged as requiring removal. The termination process really requires nothing more than disabling the accounts (or deleting them on services that don't support enabling and disabling access). So I don't think there's any good reason for falling behind as we have, other than basic human sloppiness.
So we got the accounts cleaned up, and access is up to date and correct now. But clearly I'm going to need to check up on the termination process more often.
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