Create a more secure guest sharing environment

In this article, we'll walk through a variety of options for creating a more secure guest sharing environment in Microsoft 365. These are examples to give you an idea of the options available. You can use these procedures in different combinations to meet the security and compliance needs of your organization.

This article includes:

• Setting up multi-factor authentication for guests.
• Setting up a terms of use for guests.
• Setting up quarterly guest access reviews to periodically validate whether guests continue to need permissions to teams and sites.
• Restricting guests to web-only access for unmanaged devices.
• Configuring a session timeout policy to ensure guests authenticate daily.
• Creating a sensitive information type for a highly sensitive project.
• Automatically assigning a sensitivity label to documents that contain a sensitive information type.
• Automatically removing guest access from files with a sensitivity label.

Some of the options discussed in this article require guests to have an account in Azure Active Directory. To ensure that guests are included in the directory when you share files and folders with them, use the SharePoint and OneDrive integration with Azure AD B2B Preview.

Note that we won't discuss enabling guest sharing settings in this article. See Collaborating with people outside your organization for details about enabling guest sharing for different scenarios.

Set up multi-factor authentication for guests

Multi-factor authentication greatly reduces the chances of an account being compromised. Since guests may be using personal email accounts that don't adhere to any governance policies or best practices, it's especially important to require multi-factor authentication for guests. If a guest's username and password is stolen, requiring a second factor of authentication greatly reduces the chances of unknown parties gaining access to your sites and files.

In this example, we'll set up multi-factor authentication for guests by using a conditional access policy in Azure Active Directory.

To set up multi-factor authentication for guests

1. Go to Azure conditional access policies.
2. On the Conditional Access | Policies blade, click New policy.
3. In the Name field, type a name.
4. Select the Users link.
5. Select Select users and groups, and then select the Guest or external users check box.
6. In the dropdown, select B2B collaboration guest users and B2B collaboration member users.
7. Select the Cloud apps or actions link.
8. Select All cloud apps on the Include tab.
9. Select the Grant link.
10. On the Grant blade, select the Require multi-factor authentication check box, and then click Select.
11. Under Enable policy, click On, and then click Create.

Now, guest will be required to enroll in multi-factor authentication before they can access shared content, sites, or teams.

More information

Planning an Azure AD multi-factor authentication deployment

Set up a terms of use for guests

In some situations guests may not have signed non-disclosure agreements or other legal agreements with your organization. You can require guests to agree to a terms of use before accessing files that are shared with them. The terms of use can be displayed the first time they attempt to access a shared file or site.

To create a terms of use, you first need to create the document in Word or another authoring program, and then save it as a .pdf file. This file can then be uploaded to Azure AD.

To create an Azure AD terms of use

1. Sign in to Azure as a Global Administrator, Security Administrator, or Conditional Access Administrator.

2. Navigate to Terms of use.

3. Click New terms.

   

4. Type a Name.

5. For Terms of use document, browse to the pdf file that you created and select it.

6. Select the language for your terms of use document.

7. Type a display name.

8. Set Require users to expand the terms of use to On.

9. Under Conditional Access, in the Enforce with Conditional Access policy template list choose Create conditional access policy later.

10. Click Create.

Once you've created the terms of use, the next step is to create a conditional access policy that displays the terms of use to guests.

To create a conditional access policy

1. Go to Azure conditional access policies.
2. On the Conditional Access | Policies blade, click New policy.
3. In the Name box, type a name.
4. Select the Users link.
5. Select Select users and groups, and then select the Guest or external users check box.
6. In the dropdown, select B2B collaboration guest users and B2B collaboration member users.
7. Select the Cloud apps or actions link.
8. On the Include tab, select Select apps, and then click the Select link.
9. On the Select blade, select Office 365, then click Select.
10. Select the Grant link.
11. On the Grant blade, select Guest terms of use, and then click Select.
12. Under Enable policy, click On, and then click Create.

Now, the first time a guest attempts to access content or a team or site in your organization, they will be required to accept the terms of use.

Note

Using Conditional Access requires an Azure AD Premium P1 license. For more information, see What is Conditional Access.

More information

Azure Active Directory terms of use

Set up guest access reviews

With access reviews in Azure AD, you can automate a periodic review of user access to various teams and groups. By requiring an access review for guests specifically, you can help ensure guests do not retain access to your organization's sensitive information for longer than is necessary.

To set up a guest access review

1. On the Identity Governance page, in the left menu, click Access reviews.

2. Click New access review.

3. Choose the Teams + Groups option.

4. Choose the All Microsoft 365 groups with guest users option. Click Select group(s) to exclude if you want to exclude any groups.

5. Choose the Guest users only option, and then click Next: Reviews.

6. Under Select reviewers, choose Group Owner(s).

7. Click Select fallback reviewers, choose who should be the fallback reviewers, and then click Select.

8. Choose a Duration (in days) for the review to be open for comments.

9. Under Specify recurrence of review, choose Quarterly.

10. Select a start date and duration.

11. For End, choose Never, and then click Next: Settings.

    

12. On the Settings tab, review the settings for compliance with your business rules.

    

13. Click Next: Review + Create.

14. Type a Review name and review the settings.

15. Click Create.

More information

Manage guest access with Azure AD access reviews

Create an access review of groups or applications in Azure AD access reviews

Set up web-only access for guests with unmanaged devices

If your guests use devices that are not managed by your organization or another organization that you have a trust relationship with, you can require them to access your teams, sites, and files by using a web browser only. This reduces the chance that they might download sensitive files and leave them on an unmanaged device. This is also useful when sharing with environments that use shared devices.

For Microsoft 365 Groups and Teams, this is done with an Azure AD conditional access policy. For SharePoint, this is configured in the SharePoint admin center. (You can also use sensitivity labels to restrict guests to web-only access.)

To restrict guests to web-only access for Groups and Teams:

1. Go to Azure conditional access policies.

2. Click New policy.

3. In the Name box, type a name.

4. Click the Users link.

5. Select Select users and groups, and then select the Guest or external users check box.

6. In the dropdown, select B2B collaboration guest users and B2B collaboration member users.

7. Click the Cloud apps or actions link.

8. On the Include tab, select Select apps, and then click the Select link.

9. On the Select blade, select Office 365, and then click Select.

10. Click the Conditions link.

11. On the Conditions blade, click the Client apps link.

12. On the Client apps blade, click Yes for Configure, and then select the Mobile apps and desktop clients, Exchange ActiveSync clients, and Other clients settings. Clear the Browser check box.

    

13. Click Done.

14. Click the Grant link.

15. On the Grant blade, select Require device to be marked as compliant and Require Hybrid Azure AD joined device.

16. Under For multiple controls, select Require one of the selected controls, and then click Select.

17. Under Enable policy, click On, and then click Create.

More information

SharePoint and OneDrive unmanaged device access controls for administrators

Configure a session timeout for guests

Requiring guests to authenticate on a regular basis can reduce the possibility of unknown users accessing your organization's content if a guest's device isn't kept secure. You can configure a session timeout conditional access policy for guests in Azure AD.

To configure a guest session timeout policy

1. Go to Azure conditional access policies.
2. Click New policy.
3. In the Name box, type Guest session timeout.
4. Click the Users link.
5. Select Select users and groups, and then select the Guest or external users check box.
6. In the dropdown, select B2B collaboration guest users and B2B collaboration member users.
7. Click the Cloud apps or actions link.
8. On the Include tab, select Select apps, and then click the Select link.
9. On the Select blade, select Office 365, and then click Select.
10. Click the Session link.
11. On the Session blade, select Sign-in frequency.
12. Choose 1 and Days for the time period, and then click Select.
13. Under Enable policy, click On, and then click Create.

Create a sensitive information type for a highly sensitive project

Sensitive information types are predefined strings that can be used in policy workflows to enforce compliance requirements. The Microsoft Purview compliance portal comes with over one hundred sensitive information types, including driver's license numbers, credit card numbers, bank account numbers, etc.

You can create custom sensitive information types to help manage content specific to your organization. In this example, we'll create a custom sensitive information type for a highly sensitive project. We can then use this sensitive information type to automatically apply a sensitivity label.

To create a sensitive information type

1. In the Microsoft Purview compliance portal, in the left navigation, select Data classification, and then select the Sensitive info types tab.
2. Click Create sensitive info type.
3. For Name and Description, type Project Saturn, and then click Next.
4. Select Create pattern.
5. On the New pattern panel, select Add primary element, and then select Keyword list.
6. Type an ID such as Project Saturn.
7. In the Case insensitive box, type Project Saturn, Saturn, and then select Done.
8. Select Create, and then select Next.
9. Choose a confidence level, and then select Next.
10. Select Create.
11. Select Done.

For more information, see Custom sensitive information types.

Create an auto-labeling policy to assign a sensitivity label based on a sensitive information type

If you are using sensitivity labels in your organization, you can automatically apply a label to files that contain defined sensitive information types.

To create an auto-labeling policy

1. Open the Microsoft Purview admin center.
2. In the left navigation, click Information protection.
3. On the Auto-labeling tab, click Create auto-labeling policy.
4. On the Choose info you want this label applied to page, choose Custom and then click Custom policy.
5. Click Next.
6. Type a name and description for the policy and click Next.
7. On the Choose locations where you want to apply the label page, turn on SharePoint sites and click Choose sites.
8. Add the URLs for the sites where you want to turn on auto-labeling and click Done.
9. Click Next.
10. On the Set up common or advanced rules page, choose Common rules and click Next.
11. On the Define rules for content in all locations page, click New rule.
12. On the New rule page, give the rule a name, click Add condition, and then click Content contains.
13. Click Add, click Sensitive info types, choose the sensitive info types that you want to use, click Add, and then click Save.
14. Click Next.
15. Click Choose a label, select the label you want to use, and then click Add.
16. Click Next.
17. Leave the policy in simulation mode and choose if you want it to automatically turn on.
18. Click Next.
19. Click Create policy, and then click Done.

With the policy in place, when a user types "Project Saturn" into a document, the auto-labeling policy will automatically apply the specified label when it scans the file.

For more information, see Apply a sensitivity label to content automatically.

Create a DLP policy to remove guest access to highly sensitive files

You can use Microsoft Purview Data Loss Prevention (DLP) to prevent unwanted guest sharing of sensitive content. Data loss prevention can take action based on a file's sensitivity label and remove guest access.

To create a DLP rule

1. In the Microsoft Purview admin center, go to the Data loss prevention page.

2. On the Policies tab, click Create policy.

3. Choose Custom and then Custom policy.

4. Click Next.

5. Type a name for the policy and click Next.

6. On the Locations to apply the policy page turn off all settings except SharePoint sites and OneDrive accounts, and then click Next.

7. On the Define policy settings page, click Next.

8. On the Customize advanced DLP rules page, click Create rule and type a name for the rule.

9. Under Conditions, click Add condition, and choose Content is shared from Microsoft 365.

10. In the dropdown, choose with people outside my organization.

11. Under Conditions, click Add condition, and choose Content contains.

12. Click Add, choose Sensitivity labels, choose the labels you want to use, and click Add.

    

13. Under Actions click Add an action and choose Restrict access or encrypt the content in Microsoft 365 locations.

14. Select the Restrict access or encrypt the content in Microsoft 365 locations check box and then choose the Block only people outside your organization option.

    

15. Turn user notifications On, and then select the Notify users in Office 365 service with a policy tip check box.

16. Click Save and then click Next.

17. Choose your test options and click Next.

18. Click Submit, and then click Done.

It's important to note that this policy doesn't remove access if the guest is a member of the site or team as a whole. If you plan to have highly sensitive documents in a site or team with guest members, consider these options:

• Use private channels and only allowing members of your organization in the private channels.
• Use shared channels to collaborate with people outside your organization while only having people from your organization in the team itself.

Additional options

There are some additional options in Microsoft 365 and Azure Active Directory that can help secure your guest sharing environment.

• You can create a list of allowed or denied sharing domains to limit who users can share with. See Restrict sharing of SharePoint and OneDrive content by domain and Allow or block invitations to B2B users from specific organizations for more information.
• You can limit which other Azure Active Directory tenants your users can connect to. See Use tenant restrictions to manage access to SaaS cloud applications for information.
• You can create a managed environment where partners can help manage guest accounts. See Create a B2B extranet with managed guests for information.

See Also

Limit accidental exposure to files when sharing with guests

Best practices for sharing files and folders with unauthenticated users

Create a B2B extranet with managed guests