I've been upgrading our sharded cluster, (from 2.4 to 2.6.10, and then 2.6.10 to 3.0.5). I had finished the 2.6.10 upgrade, and everything was working well. I had started the v3 upgrade, and had got to step for of the guide [ http://docs.mongodb.org/manual/release-notes/3.0-upgrade/#upgrade-sharded-clusters ] and had upgraded one of the mongos instances when I realised there was a problem - my app was no longer able to authenticate via this new v3 mongos.

I then checked the v2.6.10 mongos instances, and realised that, although me app (via the js driver) was able to authenticate via these instances, I was not able to properly authenticate using the shell (via either the -u -p method, or the db.auth() method).{{ [ errmsg: "auth failed", code: 18 ] }}.

The log entry for this error was Failed to authenticate xxxuser@xxxdb with mechanism MONGODB-CR: AuthenticationFailed UserNotFound User "xxxuser@xxxdb" not found

If I conect a shell to the replica set primary, I'm able to authenticate with this user normally (using either -u -p or db.auth() ).

One thing (not sure if relevant), but if I do 'show dbs' via the 2.6.10 mongos, the admin db is showing as '(empty)'.

I'm at a dead end now, and stuck half way through the upgrade, so any ideas you might have would be very gratefully received.