You use Exchange Web Services (EWS) Managed API to connect to a coexistence environment of Microsoft Exchange Server 2013 and Microsoft Exchange Server 2010. If you have the DefaultNetworkCredentials option selected, you can access others people's mailboxes and read the message content in those mailboxes.
This issue occurs when the initial token in the SOAP envelope is ignored during EWS proxy. In this situation, the token is not processed by using permission checking.
To fix this issue, install Update Rollup 12 for Exchange Server 2010 Service Pack 3.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
Learn about the terminology that Microsoft uses to describe software updates.