Original release date: January 28, 2019
The CERT Coordination Center (CERT/CC) has released information to address NTLM relay attacks affecting Microsoft Exchange 2013 and newer versions. A remote attacker could exploit this vulnerability to take control of an affected system.
Microsoft Exchange 2013 and newer fail to set signing and sealing flags on NTLM authentication traffic, which can allow a remote attacker to gain the privileges of the Exchange server.
|Microsoft Exchange supports a API called Exchange Web Services (EWS). One of the EWS API functions is called PushSubscription,
which can be used to cause the Exchange server to connect to an
arbitrary website. Connections made using the PushSubscription feature
will attempt to negotiate with the arbitrary web server using NTLM
authentication. Starting with Microsoft Exchange 2013, the NTLM
authentication over HTTP fails to set the NTLM Sign and Seal flags. The lack of signing makes this authentication attempt vulnerable to NTLM relay attacks.|
Microsoft Exchange is by default configured with extensive privileges with respect to the Domain object in Active Directory. Because the Exchange Windows Permissions group has WriteDacl access to the Domain object, this means that the Exchange server privileges obtained using this vulnerability can be used to gain Domain Admin privileges for the domain that contains the vulnerable Exchange server.
|An attacker that has credentials for an Exchange mailbox and also has the ability to communicate with both a Microsoft Exchange server and a Windows domain controller may be able to gain domain administrator privileges. It is also reported that an attacker without knowledge of an Exchange user's password may be able to perform the same attack by using an SMB to HTTP relay attack as long as they are in the same network segment as the Exchange server.|
|The CERT/CC is currently unaware of a practical solution to this problem. Please consider the following workarounds:|
|Disable EWS push/pull subscriptions|
If you have an exchange server that does not leverage EWS push/pull subscriptions, you can block the PushSubscription API call that triggers this attack. In an Exchange Management Shell window, execute the following commands:
Restart-WebAppPool -Name MSExchangeServicesAppPool
Remove privileges that Exchange has on the domain object
Please note that the following workaround was not developed by CERT and is not supported by Microsoft. Please test any workarounds in your environment to ensure that they work properly.
https://github.com/gdedrouas/Exchange-AD-Privesc/blob/master/DomainObject/Fix-DomainObjectDACL.ps1 is a PowerShell script that can be executed on either the Exchange Server or Domain Controller system. By default this script will check for vulnerable access control entries in the current active directory. When executed with Domain Admin privileges and the -Fix flag, this script will remove the ability for Exchange to write to the domain object.
Note that if you encounter an error about Get-ADDomainController not being recognized, you will need to install and import the ActiveDirectory PowerShell module, and then finally run Fix-DomainObjectDACL.ps1 :
If the script reports that faulty ACE were found, run:
PowerShell may be configured to block the execution of user-provided .ps1 files. If this is the case, first find your current PowerShell execution policy: