What would I do (I assume that you are using third-party LDAP, and that there is no built-in way to do this, I did not check) I configured MADAM and implement a custom IUserSecurityAuthority by expanding Madam.UserSecurityAuthorityBase to verify credentials for LDAP.
All you have to do is override several authentication methods from LDAP. If you use an LDAP membership provider to authenticate forms, you can skip the implementation of this separately and use the MembershipSecurityAuthority example included in MADAM.
An excerpt from this example is here:
protected override bool ValidateUser(string userName, string password) { return MembershipProvider.ValidateUser(userName, password); }
Potentially Useful LDAP User Verification Code
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