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WAF issues after updating to 9.709-3

FrancWest

Hi,

anyone else noticed that after updating to 9.709-3 Exchange Web Services is not working anymore? We get HTTP Error 500 when connecting to EWS published trhrough WAF. Also, the virtual server changes to orange when this error occurs. Accessing EWS through the browser shows the service page after authentication, but when interacting with EWS by using the Exchange Remote Connectivity Analyzer or EWS Editor generates the HTTP 500 error and the WAF rule turns orange.

When directly connecting to EWS and bypassing UTM works fine and we can interact with EWS.

Before the update everything worked fine.

Franc.



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  • in reply to Jay Jay +4 verified

    It's related to the handling of the '100 Continue' message in the HTTP protocol. This message is sent by a Server after receiving the headers for a request to indicate that it is ready to receive the body of the request from the client - it allows a server to check the headers and potentially reject the request before the client unecessarily sends all the data. Normally, the client would send an additional "Expect:" header to indicate that it is going to wait for the server to send this message before it sends the request body.

    The recent update to Apache changed how this process is handled by the proxy, in a way that didn't work well with Exchange.

    The old behaviour was that the proxy would itself respond to the client with a '100 Continue' and would not wait for the server to do so. The new behaviour of Apache forwards the "Expect:" header to the server and waits for the server to respond with 100 Continue before passing the "100 Continue" to the client. This change defends against potential issues where a client could send a very large request, which the WAF proxy would have to buffer until the server is ready for it.

    The change should be fine for servers that respond to 'Expect' headers and use 100 Continue strictly according to the HTTP specification, but it seems that Exchange does not.

    Changing the configuration option as specified reverts this behaviour so that the proxy itself responds with a "100 Continue" message instead of waiting for the Server to do so. The risk of this behaviour is that the proxy has to buffer the entire body of the request before the server is ready to receive it. This shouldn't be a problem in most situations because request bodies are usually quite small.

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  • 0

    I must admit, I have SEVERAL services, including Exchange 2016 sitting behind my WAF, and so far, this update has not caused these issues. From time to time, an update may break Lets Encrypt renewals or new cert sign up processes, but one way or another I do manage to get that working again. I followed the usual update process from within the management console, so just thought I would share my experience. Hopefully they fix whatever is causing these issues for you guys. 

  • 0 in reply to David Florey

    Hi David, as far as I know, it's not an on-prem, but hybrid issue, like described by several people.

    e.g. Connection to on-prem EWS behind WAF. Since it's hybrid there is a redirect to O365. Problem is somewhere with the redirect, maybe auth, maybe response (awaited XML - like in our custom application), etc. You can see in logs. 

    Don't mix things up, please. That doesn't help in solution finding. 
    Btw my on-prem EWS behind WAF works also fine, but as said, its not the topic.

  • 0 in reply to Alex_F

    Alex, what suggestions do folks have in the Microsoft Exchange community?  That might give us a clue on what to do in the UTM.

    Cheers - Bob

     
    Sophos UTM Community Moderator
    Sophos Certified Architect - UTM
    Sophos Certified Engineer - XG
    Gold Solution Partner since 2005
    MediaSoft, Inc. USA
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  • 0 in reply to Alex_F

    Alex, what suggestions do folks have in the Microsoft Exchange community?  That might give us a clue on what to do in the UTM.

    Cheers - Bob

     
    Sophos UTM Community Moderator
    Sophos Certified Architect - UTM
    Sophos Certified Engineer - XG
    Gold Solution Partner since 2005
    MediaSoft, Inc. USA
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