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Auditing Inbox rules (and looking for hidden rules) with EWS in OnPrem Exchange

 After the events of the last weeks around the latest zero day vulnerabilities in Exchange  and once you've finished cleaning up any back doors that may have been left on servers its a good idea to review some other less known but established ways bad actors may hide persistent access within Mailboxes. One of these are Inbox Rules (but Mail Flow rules could also be used) and a more advanced method is the hidden Inbox rule exploit that was first talked about https://blog.compass-security.com/2018/09/hidden-inbox-rules-in-microsoft-exchange/ and I covered it in https://gsexdev.blogspot.com/2019/05/audting-inbox-rules-with-ews-and-graph.html and somebody else https://mgreen27.github.io/posts/2019/06/09/O365HiddenRules.html there are a number of tools and techniques around detecting these types of rule but are all focused more toward Office365 as that was where at the time this exploit was being mostly employed. In my post at the time I modified the Microsoft script https://github.com/gscales/O365-InvestigationTooling/blob/master/Get-AllTenantRulesAndForms.ps1 so it would include the PidTagRuleMsgProvider property in its final report so you could easy spot any null, empty or custom values that would effectively hide rules from being enumerated via other more conventional methods. Because this tool was designed for o365 it can't easily be run against onPrem Exchange so I've put together a modified version that can be and posted it up here.  https://github.com/gscales/Powershell-Scripts/blob/master/EWShiddenRuleEnum.ps1  I've retained the funky Encoded compressed version of the EWS Managed API dll that the original used and extended some of the properties to include things like the display-name of the user and the last modified date time of the rule object. To use this script you need to have EWS Impersonation enabled for the user you either pass into the script or the user the script is running under. https://docs.microsoft.com/en-us/exchange/client-developer/exchange-web-services/how-to-configure-impersonation

Mostly the changes to the script involved (other then cleanup) where around the remote power shell differences (this script will try to open a remote ps connection if one isn't already established) and passing in the hostname of the EWS endpoint.

To use this script first import the script module

Import-module ./EWShiddenRuleEnum.ps1

Then 

Invoke-EnumerateAllInboxRules

parameters that can/must be used

EWSHostName this is the EWS endpoint hostname the script doesn't use autodiscover so if your not running directly on the destination server you must pass this in

ExchangePSHostName this is the servername that the Remote powershell session will be open against if your not running directly on the destination server you must pass this in

Credentials are the credentials that will be used in EWS to connect and impersonate mailboxes, the remote powershell credentials used are the running users. (credentials can be entered in either UPN format or downlevel format domain\username and this can be environment dependent as to what works)

Filter this allows you to filter the Get-Mailbox query with any valid filter you would normally use in the PowerShell eg Invoke-EnumerateAllInboxRules -filter '{servername -eq hostname}'

The script will produce a CSV report of rules and forms in the current directory, you can open this in Excel and apply filters to make it easy to read and interpret. 

If you don't go down the path of rebuilding Exchange servers that where exploited make sure you also look at any Transport Agents you have installed and verify the assemblies, its a a lesser known method of persistent access but it has been used before https://www.bankinfosecurity.com/researchers-spies-exploit-microsoft-exchange-backdoor-a-12459 to some success.   




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All sample scripts and source code is provided by for illustrative purposes only. All examples are untested in different environments and therefore, I cannot guarantee or imply reliability, serviceability, or function of these programs.

All code contained herein is provided to you "AS IS" without any warranties of any kind. The implied warranties of non-infringement, merchantability and fitness for a particular purpose are expressly disclaimed.