"HTTP 400 Bad Request" error when proxying HTTP requests from an Exchange Server to a previous version of Exchange Server


When a user tries to proxy an HTTP request from a Microsoft Exchange Server 2016 server that is running client access service or a Microsoft Exchange Server 2013 Client Access server (CAS) to a previous version of Exchange Server, the user may receive the following error message:

This error (HTTP 400 Bad Request) means that Internet Explorer was able to connect to the web server, but the webpage could not be found because of a problem with the address.

Additionally, the HTTPERR logs on the Microsoft Exchange Server 2010 or Microsoft Exchange Server 2007 Client Access server may show one of the following for the HTTP resource the user was requesting: 

2014-07-24 16:48:06 53335 443 HTTP/1.1 GET /owa/ 400 - RequestLength -

2014-07-24 16:48:06 53335 443 HTTP/1.1 GET /owa/ 400 - FieldLength - 

Also, you may see the following in the Exchange Server <Exchange Server Install Path>\Logging\HttpProxy\<Http resource> logs on the Exchange Server 2013 Client Access server:  

2014-07-24T16:56:17.806Z,ddf5379e-4a97-4833-b331-36328b9f8b58,15,0,913,7,,Owa,outlook.Wingtiptoys.com,/owa/,,Negotiate,True,WINGTIPTOYS\user003,,Sid~S-1-5-21-3205615561-4199783494-2467053687-1128,Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.1; WOW64; Trident/4.0; SLCC2; .NET CLR 2.0.50727; .NET4.0C; .NET4.0E; .NET CLR 3.5.30729; .NET CLR 3.0.30729),,WINGTIP-E2K13,400,400,,GET,Proxy,wingtip-e2k10.wingtiptoys.com,14.03.0123.000,IntraForest,WindowsIdentity-ServerCookie,Server~WINGTIP-E2K10.Wingtiptoys.com~1937997947~07/24/2014 17:01:18,,,0,342,1,,4,14,,0,,0,,0,0,,0,46.8744,0,,,,18,0,1,0,0,1,38,1,20,20,20,24,43,,,BeginRequest=2014-07-24T16:56:17.759Z;CorrelationID=<empty>;ProxyState-Run=None;ProxyToDownLevel=True;BeginGetResponse=2014-07-24T16:56:17.791Z;OnResponseReady=2014-07-24T16:56:17.806Z;EndGetResponse=2014-07-24T16:56:17.806Z;ProxyState-Complete=ProxyResponseData;EndRequest=2014-07-24T16:56:17.806Z;,WebExceptionStatus=ProtocolError;ResponseStatusCode=400;WebException=System.Net.WebException: The remote server returned an error: (400) Bad Request. at System.Net.HttpWebRequest.EndGetResponse(IAsyncResult asyncResult) at Microsoft.Exchange.HttpProxy.ProxyRequestHandler.<>c__DisplayClass2a.<OnResponseReady>b__28();  


This issue may occur if the user is a member of many Active Directory groups. This issue may occur during the proxy process from Exchange Server 2016 or Exchange Server 2013 CAS to Exchange Server 2010 CAS.


To fix this issue, use one of the following methods:

  • Reduce the number of Active Directory groups that are assigned to the user.

  • On every Exchange 2010 CAS, locate the following subkey:


  • Under this subkey, increase the MaxFieldLength and MaxRequestBytes entries by using the values in the following table.

    Value name Value type Value data Value base
    MaxFieldLength DWORD 65536 Decimal
    MaxRequestBytes DWORD 65536 Decimal


    • This change requires a restart of the CAS.
    • If the entries do not exist for MaxFieldLength and MaxRequestBytes, create the entries manually.

More Information


Changing these registry keys might be extremely dangerous. Increasing the values allow larger HTTP packets to be sent to IIS, which in turn might cause Http.sys to use more memory and might increase vulnerability to malicious attacks.

  • The recommended value for Exchange Server coexistence is 65536.


    The value should be 65536 for Exchange Server. It should not be 65534, as indicated in KB 2020943. That setting is for Internet Information Services (IIS). This difference is because of additional requirements for Exchange Server.

  • In some cases, a MaxFieldLength value of 65536may not fix the issue. If this occurs, we recommend that you reduce the size of the user's access token by removing groups instead of increasing the value.

  • Increasing the value of MaxRequestBytes to be greater 65536 has risks. Therefore, we do not recommend that you do this. These risks are discussed in detail in KB 820129. This key is assigned a warning code of 1 to indicate a high risk for changing the default value.