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31 January 2019

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             AUSCERT External Security Bulletin Redistribution

  Microsoft Exchange 2013 and newer are vulnerable to NTLM relay attacks
                              31 January 2019


        AusCERT Security Bulletin Summary

Product:           Microsoft Exchange Server 2013
                   Microsoft Exchange Server 2016
                   Microsoft Exchange Server 2019
Publisher:         Microsoft
Operating System:  Windows
Impact/Access:     Increased Privileges -- Existing Account
Resolution:        Mitigation

Original Bulletin: 

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Microsoft Exchange 2013 and newer are vulnerable to NTLM relay attacks

Vulnerability Note VU#465632

Original Release Date: 2019-01-28 | Last Revised: 2019-01-30


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 PushSubscriptionRequest, which can be used to
cause the Exchange server to connect to an arbitrary website. Connections made
using the PushSubscriptionRequest function 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 an Exchange user.


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 PushSubscriptionRequest API call that triggers
this attack. In an Exchange Management Shell window, execute the following

    New-ThrottlingPolicy -Name NoEWSSubscription -ThrottlingPolicyScope
    Organization -EwsMaxSubscriptions 0
    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.

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 :

    Import-Module ServerManager
    Add-WindowsFeature RSAT-AD-PowerShell
    Import-Module ActiveDirectory

If the script reports that faulty ACE were found, run:

    .\Fix-DomainObjectDACL.ps1 -Fix

PowerShell may be configured to block the execution of user-provided .ps1
files. If this is the case, first find your current PowerShell execution


Temporarily allow the execution of the Fix-DomainObjectDACL.ps1 script by

    Set-ExecutionPolicy unrestricted

Once you are finished running the Fix-DomainObjectDACL.ps1script, set the
policy back to the original value as reported by Get-ExecutionPolicy:

    Set-ExecutionPolicy [POLICY]

Consider additional workarounds

The blog post for this vulnerability contains several mitigations that may also
help protect against this and similar vulnerabilities.

CVSS Metrics

    Group     Score            Vector
Base          8.3   AV:A/AC:L/Au:N/C:C/I:C/A:C
Temporal      7.5   E:F/RL:W/RC:C
Environmental 7.5   CDP:ND/TD:H/CR:ND/IR:ND/AR:ND


  o https://dirkjanm.io/abusing-exchange-one-api-call-away-from-domain-admin/
  o https://www.thezdi.com/blog/2018/12/19/
  o https://docs.microsoft.com/en-us/dotnet/api/microsoft.exchange.webservices.data.pushsubscription?view=exchange-ews-api+
  o https://docs.microsoft.com/en-us/exchange/client-developer/web-service-reference/pushsubscriptionrequest
  o https://docs.microsoft.com/en-us/previous-versions/office/developer/exchange-server-2010/dd877045(v%3Dexchg.140)
  o https://msdn.microsoft.com/en-us/library/cc236702.aspx
  o https://msdn.microsoft.com/en-us/library/cc236707.aspx


This vulnerability was publicly disclosed by Dirk-jan Mollema.

This document was written by Will Dormann.

Other Information

CVE IDs:                     None
Date Public:                 2019-01-21
Date First Published:        2019-01-28
Date Last Updated:           2019-01-30 19:30 UTC
Document Revision:           28

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