Recently one of my customers had a user’s account compromised. Because the user had a weak password (name of the local soccer team followed by a sequential number) his account was compromised using a password spray attack.

One way (of many) to avoid this is to use multi-factor authentication (MFA). Besides a regular username and password, MFA uses another authentication option, like a text message, an app on your mobile device or an ‘app password’ when the other two options cannot be used for some reason.

Note. MFA is available in three versions. There’s an MFA for admin accounts (MFA for admin accounts), there’s a full version as part of the Azure AD Premium subscription and there’s a lightweight version part of all Office 365 business subscriptions called the Multi-Factor Authentication for Office 365. This version can only be used with Office 365 services and is the one I used for this blog.

MFA Requirements

Before you start with implementing MFA in your environment, make sure your Office 365 tenant and your devices meet the following criteria.

Office 2016 clients support MFA natively using Active Directory Authentication Library (ADAL), the same is true for browsers. Not all Office 365 tenants have ADAL functionality enabled by default, especially the older tenants have not. To check if your tenants support this, open the Exchange Management Shell for Exchange Online and enter the following command:

Get-OrganizationConfig | Format-Table name, *OAuth*

If it returns False as shown in the screenshot below (I have an older tenant) you can use the following command to enable it on an organization level:

Set-OrganizationConfig -OAuth2ClientProfileEnabled:$true

The same should is true for Skype for Business Online clients. To check the ADAL support in your tenant, open a Skype for Business Online PowerShell command and execute the following command:

Get-CsOAuthConfiguration | select *client*

If it’s not enabled (as shown in the following screenshot) you can use the following command to enable it:

Set-CsOAuthConfiguration -ClientAdalAuthOverride Allowed

For older apps, or apps that do not support MFA through ADAL, you can use an AppPassword. This is a special password, especially for the device you work on. You can only use the AppPassword on this specific device, and not on any other device. For other devices you need to have an additional AppPassword. An AppPassword is created the first time you use MFA on a device which will be shown later in this blog.

For mobile device I strongly recommend installing the Microsoft Authenticator to help you make authentication life a bit easier. The MFA information is stored on this specific device and is only available on this specific machine. The Microsoft Authenticator is available via the App Store on your device.

Enable MFA

To enable MFA for cloud accounts, open the Microsoft Online Portal (https://portal.office.com) and logon using the tenant admin account. Select Active Users, but before you select any user click on the More drop-down box and select Multifactor Authentication Setup as shown in the following screenshot:

In the next window, select the user you want MFA to enable for and click enable in the right pane. In the confirmation box click enable multi-factor auth.

In the same Window you can also change the service settings as shown below:

In the service settings, you can change variables like whether or not to have users create AppPasswords, what authentication options can be used and the timeframe to remember MFA authentications:

Make sure the Allow users to create app passwords to sign in to non-browser apps radio button is checked. At the same time have a look at the Allow users to remember multi-factor authentication on devices they trust option. Using this option, you can set the number of days before the user has to use MFA again for authentication. In the mean time the device is trusted and MFA is not needed.

Using MFA on your clients

After enabling MFA and you logon to for example OWA an additional pop-up is presented where additional information is requested. The mobile phone number is already in Active Directory and thus prepopulated, and a text message is sent to this number.

After entering the validation code you are officially logged on. The last step is that an AppPassword is presented, you can use this AppPassword on this device for application that do not support the Microsoft MFA (through ADAL).

When Outlook 2016 is started an additional pop-up is shown where the validation code has to be entered. Since the MFA is remembered for 30 days you won’t see this again the upcoming 4 weeks (and two days 😊).

The same is true for OneDrive for Business and Skype for Business clients, they need to authenticate as well. My work laptop is Azure AD joined, and the advantage here is that I only need to logon once, and need to enter the SMS authentication only once since both tokens are stored on the device.

When enabling MFA as shown below it is enforced on mobile device as well. I have an iPhone with iOS 12.2 and this supports MFA natively. The next time the device needs to authenticate (can take some time after enabling MFA) a validation code is sent to the device. This can be a bit challenging since you need to enter this code on the device itself. The Microsoft Authenticator (or any authenticator) can help you here, especially if you have multiple profiles with multiple (MFA enabled) mailboxes.

The next 30 days (or whatever timeframe you’ve entered) you won’t get the MFA validation challenge, unless you change the password, then the MFA is triggered, and you need to authenticate again. Remember that the token is stored on the local device, so if you want to check your email on another device you have a authenticate using MFA on that device as well. And this is what will frustrate the bad guys in Nigeria (screenshot below shows where out attacker was hiding) since they don’t have access to your device, so even with a weak password (still not recommended!) you should be much safer.

Summary

You can use Multi Factor Authentication as an additional measure against hackers that want to use user credentials to access your environment. Since an additional authentication method is needed, it is much more difficult to get access to your environment for the bad guys. Since they don’t have access to the mobile device it’s hardly impossible to misuse the mailbox.

The next logical step would be to implement a password less authentication, but that’s for a future blog 😊

More information

• Set up multi-factor authentication – https://docs.microsoft.com/en-us/office365/admin/security-and-compliance/set-up-multi-factor-authentication?view=o365-worldwide
• Use Microsoft authenticator with Office 365 – https://support.office.com/en-us/article/use-microsoft-authenticator-with-office-365-1412611f-ad8d-43ab-807c-7965e5155411?ui=en-US&rs=en-US&ad=US
• How to get Azure Multi-Factor Authentication – https://docs.microsoft.com/en-us/azure/active-directory/authentication/concept-mfa-licensing
• Multi-Factor Authentication pricing – https://azure.microsoft.com/en-us/pricing/details/multi-factor-authentication/
• Azure Active Directory Documentation – https://docs.microsoft.com/en-us/azure/active-directory/