SharePoint Tip #36. Do you know “a cause of HTTP 400 Bad Request error when login to site” ?

Sometimes, you may find that you get “HTTP 400 Bad Request” error, or “HTTP 401.1 – Unauthorized: Logon Failed” error when navigate to local SharePoint sites and asked for credentials tree times.

This mostly happens for Web sites that use Integrated Authentication and have a name that is mapped to the local loopback address. More details in KB896861.

Such situation is “behavior-by-design” and caused by Windows security updates, when authentication fails if the FQDN (fully qualified SND name) or the custom host header does not match the local computer name.

Solution: run REGEDIT and create DWORD “DisableLoopbackCheck” equal “1” in HKEY_LOCAL_MACHINESYSTEMCurrentControlSetControlLsa.

PS: I actually had all my sites inaccessible after bunch of security updates on Windows 2008 before and after installing SharePoint SP2.


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  1. Jauchter Said,

    June 11, 2009@ 12:34 pm      Reply

    I am running into a similar problem. – My share point site works internally buy no external. After checking all settings I made this registry entry and I am able to authenticate locally on the machine but still unable to get the site to pull up authentication on the outside. Granted there could be other reasons for this but I have gone over the settings a dozen times. Would this reqistry entry effect the ability for users on the outside to connect to the share point site?

  2. laflour Said,

    June 11, 2009@ 12:47 pm      Reply

    Who are the “externals” and how are they logging in?!
    Did you extend you web app or you are in the same network?

  3. ramesh Said,

    December 28, 2010@ 10:07 am      Reply


    i am also facing the same problem… in my case the externals are my client at different location and they are accessing through a public ip… i extended my website with the public ip as header… still i am getting the same error

  4. Michael Said,

    December 28, 2010@ 10:17 am      Reply

    ramesh, did you configure ISA and added new entry to AAM? Contact me via Contact form if still experiencing issues

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