Tag Archives: oauth

Looks like you don’t have permission to schedule meetings for this account: A deep-dive into Teams delegate meeting scheduling in a hybrid environment

After implementing Teams last year we were faced with a number of issues to work through. One of the issues we were having was the inability of mailbox delegates to schedule meetings on behalf of mailbox owners. If you are a hybrid Exchange user the very first step is making sure you have OAuth configured between your on-premises environment and Exchange online. The Hybrid Configuration Wizard should be taking care of this for you and I wrote an article on properly securing it externally. You will also need create and configure a service principal to allow Teams to interact with your on-premises Exchange environment. Steps 2-3 in this article must be preformed to set this up. If one of these items are not configured you will receive the error ‘Sorry, but we can’t connect to the server right now. Please try again later.

Teams delegate invitations through Outlook - Microsoft Community

There is already a lot of decent information around troubleshooting the OAuth side of things here, so I’m going to focus on the other pieces that aren’t always covered…

Once these pre-requisites are configured you may find users are still receiving errors when trying to schedule Teams meetings on behalf of other users. An error that many of our users were experiencing was ‘Looks like you don’t have permission to schedule meetings for this account. Talk to the owner to get permission and try again.

Scheduling Teams Meetings as Delegate for another mailbox. - Microsoft Tech  Community

During testing we found that Teams was looking for a specific set of permissions on the owner’s mailbox when generating the Teams meeting in Outlook. In order to create a meeting as a delegate the delegate must be a true ‘delegate’ under the owner’s mailbox AND have either ‘Author‘ or ‘Editor‘ access to the calendar like the example below. If the user is not present in this list or if they are present but show a calendar permission such as ‘Custom‘ in this dialog they will receive an error like the one above when creating the Teams portion of the meeting even if they technically have all the required permissions to create a meeting on behalf of the organizer. Many times permissions are granted either by giving full access (Add-MailboxPermission) to the mailbox, by granting folder level permissions (Add-MailboxFolderPermission), and by granting Send-On-Behalf/SendAs permissions.

To understand why Teams requires such a specific configuration let’s look at what is happening in the background… When you are creating a new Teams meeting via Outlook the Teams back-end actually does a OAuth-authenticated EWS (Exchange Web Services) ‘GetDelegate‘ call to your on-premises Exchange environment to verify that the user creating the meeting actually has access to the mailbox. The we

<?xml version="1.0" encoding="utf-8"?>"
<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/"
               xmlns:t="http://schemas.microsoft.com/exchange/services/2006/types">
  <soap:Header>
    <t:RequestServerVersion Version="Exchange2016"/>
  </soap:Header>
  <soap:Body>
    <GetDelegate xmlns="http://schemas.microsoft.com/exchange/services/2006/messages"
                 xmlns:t="http://schemas.microsoft.com/exchange/services/2006/types"
                 IncludePermissions="true">
      <Mailbox>
        <t:EmailAddress>mailboxowner@domain.com</t:EmailAddress>
      </Mailbox>
    </GetDelegate>
  </soap:Body>
</soap:Envelope>

Exchange will respond with delegate list and associated permissions (similar to the view you see in Outlook).

<?xml version="1.0" encoding="utf-8"?>
<!-- Note: EwsEditor has replaced the "utf-16" text in the first line with"utf-8" in order for the XML to render in the response web control. -->
<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/">
    <s:Header>
        <h:ServerVersionInfo MajorVersion="15" MinorVersion="1" MajorBuildNumber="1979" MinorBuildNumber="3" Version="V2017_07_11" xmlns:h="http://schemas.microsoft.com/exchange/services/2006/types" xmlns="http://schemas.microsoft.com/exchange/services/2006/types" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" />
    </s:Header>
    <s:Body xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
        <GetDelegateResponse ResponseClass="Success" xmlns="http://schemas.microsoft.com/exchange/services/2006/messages">
            <ResponseCode>NoError</ResponseCode>
            <ResponseMessages>
                <DelegateUserResponseMessageType ResponseClass="Success">
                    <ResponseCode>NoError</ResponseCode>
                    <DelegateUser>
                        <UserId xmlns="http://schemas.microsoft.com/exchange/services/2006/types">
                            <SID>S-1-5-21-2061111111-1111111111-222222222-33333</SID>
                            <PrimarySmtpAddress>delegateuser@domain.com</PrimarySmtpAddress>
                            <DisplayName>User, Delegate</DisplayName>
                        </UserId>
                        <DelegatePermissions xmlns="http://schemas.microsoft.com/exchange/services/2006/types">
                            <CalendarFolderPermissionLevel>Editor</CalendarFolderPermissionLevel>
                            <TasksFolderPermissionLevel>None</TasksFolderPermissionLevel>
                            <InboxFolderPermissionLevel>None</InboxFolderPermissionLevel>
                            <ContactsFolderPermissionLevel>None</ContactsFolderPermissionLevel>
                            <NotesFolderPermissionLevel>None</NotesFolderPermissionLevel>
                        </DelegatePermissions>
                        <ReceiveCopiesOfMeetingMessages xmlns="http://schemas.microsoft.com/exchange/services/2006/types">false</ReceiveCopiesOfMeetingMessages>
                        <ViewPrivateItems xmlns="http://schemas.microsoft.com/exchange/services/2006/types">false</ViewPrivateItems>
                    </DelegateUser>
                </DelegateUserResponseMessageType>
            </ResponseMessages>
            <DeliverMeetingRequests>NoForward</DeliverMeetingRequests>
        </GetDelegateResponse>
    </s:Body>
</s:Envelope>

The important things here are that the user is present and has the either ‘Author‘ or ‘Editor‘ permissions to the calendar. Remember, even having ‘Custom‘ permissions to the calendar will result in an error during meeting creation.

Another issue you can run into is one where you have already added the user as a delegate with the appropriate calendar permissions, but still receive the error ‘Looks like you don’t have permission to schedule meetings for this account. Talk to the owner to get permission and try again.‘ To troubleshoot this, let’s look at the mailbox from the perspective of Teams and perform the EWS ‘GetDelegate‘ call ourselves. We can use EwsEditor to do this.

In this example we receive an empty response from EWS even though we have properly configured the delegate permissions. This is indicative of mailbox corruption and to resolve this we have to understand how delegates are actually stored in the mailbox. Delegates actually translate to a number of things, but at the center of all of this is the ‘LocalFreebusy‘ object in the mailbox which can only be viewed with a tool like MFCMapi. The reason EWS is returning a blank list is because the object in the mailbox is corrupt and missing the delegate information. In order to resolve this we will use MFCMapi to delete the object. You can perform this on behalf of the user if you have full mailbox permissions and write access to their AD account, but in this example we will run as the mailbox owner using their default MAPI profile. To reset/repair the delegates list we will perform the following steps:

  • Before anything, record all of the users delegates from Outlook along with all the permissions for each one
  • Go to QuickStart -> Open Folder -> Local Freebusy
  • Choose the item with the subject ‘LocalFreebusy
  • Choose ‘Delete message‘ and when prompted with options choose ‘Permanent delete passing DELETE_HARD_DELETE…’
  • Go back into Outlook and re-create the user’s delegate(s)

After recreating the user’s delegates we can issue the EWS call again and see if they now show properly. The response should contain every delegate user along with their ‘Author‘ or ‘Editor‘ calendar permissions. Once you have verified EWS is returning the correct information you can try creating new Teams meeting request.

Properly securing your on-prem Exchange 2016 environment when using Hybrid Modern Authentication

In the past many organizations completely blocked or limited external access to on-premises Exchange servers because of the lack of multi-factor authentication. Protocols like OutlookAnywhere (also known as RPC-over-HTTP, now MAPI-over-HTTP) and EWS had no native methods to accomplish multi-factor authentication. Failure to protect these protocols from external exposure has led to many breaches like FIN4 and London Blue.

HMA to the rescue… In 2017 Microsoft finally answered this deficiency with Hybrid Modern Authentication. I briefly touched on modern authentication in two previous articles (here and here). With Hybrid Modern Authentication Microsoft gave you the ability to use new technologies like modern authentication and conditional access for on-premises Exchange. Clients will connect using modern authentication by default once Exchange is on a supported version, supported clients are implemented, and the configuration is implemented. The issue here is that legacy Windows authentication is still available. You can simply disable modern authentication in the client or use a different client and you are now connected to on-premises Exchange with a simple username and password completely bypassing conditional access. Conditional access is only invoked when you are authenticating with modern authentication. Exchange 2019 implemented Authentication Policies which allow you turn off legacy authentication methods. If you are using Exchange 2019, you can use these to lock down your environment.

We were in the situation where we wanted to allow secure external access to Exchange (mainly for OutlookAnywhere, but also Outlook Mobile), but we couldn’t have any legacy authentication exposure. The solution we came up with was creating a set of externally facing Exchange 2016 mailbox servers (think Client Access Servers from the pre-Ex2016 days) that have all legacy authentication methods disabled (only OAuth available). These servers are the only ones exposed to the internet. The protocols we want to expose but lock down are ActiveSync (needed for Outlook Mobile), EWS (Exchange Web Services), MAPI, and OAB (Offline Address Book). To lock these down we ran the following against the externally facing servers:

$Servers = @(Get-MailboxServer excas01)
$Servers = $Servers + (Get-MailboxServer excas02)
$Servers | Get-ActiveSyncVirtualDirectory | Set-ActiveSyncVirtualDirectory -BasicAuthEnabled $false
$Servers | Get-WebServicesVirtualDirectory | Set-WebServicesVirtualDirectory -WindowsAuthentication $false
$Servers | Get-MapiVirtualDirectory | Set-MapiVirtualDirectory -IISAuthenticationMethods @('OAuth')
$Servers | Get-OabVirtualDirectory | Set-OabVirtualDirectory -WindowsAuthentication $false

After this is completed, Windows and basic authentication should now fail for these virtual directories.

IMPORTANT: It is VERY important to regularly check that these settings are still in place. You should always re-run these commands after any kind of Exchange update. If you do not do this, you could inadvertently expose your Exchange environment. A simple script could be run on a schedule to check and report on any changes to the authentication configuration of these virtual directories.

The second step is disabling or blocking the other virtual directories that do not need to be accessed externally. For us, these were ECP, OWA, PowerShell, and RPC. We have an on-premises load balancer with SSL bridging configured for our Exchange environment, so we used that to block access to these virtual directories. Another option is to use IP restrictions in IIS on these virtual directories. A third option is to disable the virtual directories via PowerShell. For those of you who want to allow secure access to OWA (Outlook Web Access) you can use Azure App Proxy to accomplish this or an ADC like NetScaler or F5 Big-IP.

The final step in this configuration is allowing the O365 servers to reach an unaltered version of EWS for the IntraOrganizationConnector used for Exchange Online to pull free/busy data (and other data like photos) from your on-premises environment. I found that for some reason the IntraOrganizationConnector fails to authenticate from EXO->on-premises when it uses the modified virtual directory even though all OAuth tests pass. I also use this configuration for my MRS endpoint when doing mailbox migrations since MRS wants to do traditional Windows authentication to EWS. If you are using the Microsoft Hybrid Agent, you shouldn’t have to do this since Azure App Proxy is taking care of the MRS and free/busy communication. I have still have an ongoing ticket open with Microsoft to understand the root cause of this. The workaround is fairly simple:

  • Create a namespace that can be used for EXO->on-premises communications. (Ex. exocomm.domain.com)
  • Configure this namespace to point to your regular INTERNAL and unaltered mailbox servers
  • Lock down this namespace in your firewall, so that ONLY Microsoft O365 servers can reach it. NOTE: This is very important and failure to do so will undermine all of the work done above and leave you exposed. We use a combination of PaloAlto firewalls and MineMeld to accomplish this, but this can be accomplished with a static/maintained ACL as well.
  • Configure the IntraOrganizationConnector in EXO to not use Autodiscover and to use this new namespace as its endpoint with the following commands:
Get-IntraOrganizationConnector | Set-IntraOrganizationConnector -TargetSharingEpr "https://exocomm.domain.com/ews/Exchange.asmx"
Get-IntraOrganizationConnector | Set-IntraOrganizationConnector -DiscoveryEndpoint $null