Validate user password against active directory.
Based on forum solution.
Install-Package MissingCode.Umbraco.AdPasswordChecker
<add name="ADConnectionString" connectionString="LDAP://127.0.0.1/OU=Users,DC=Domain" />
<add key="owin:appStartup" value="MissingCode.Umbraco.AdPasswordChecker.AdPasswordCheckerOwinStartup,MissingCode.Umbraco.AdPasswordChecker"/>