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Perhaps openssh should simply delay looking up the user, sounds like it could cause disclosure about user's existing if openssh causes the system to perform network searches before authentication.

With the caching, I am not sure whether it can disclosure something. OpenSSH does not know where does the information pulled from getpwnam() come from (local passwd or AD) and additionally, it really needs to know these information before authentication, for example to know where to look for authorized keys file in the users home directory.

Andreas, is this still an issue? Do you have somebody who could verify the proposed patch I have in comment #5?

After evaluating this issue, there are no plans to address it further or fix it in an upcoming release.  Therefore, it is being closed.  If plans change such that this issue will be fixed in an upcoming release, then the bug can be reopened.