Responding to a Compromised Email Account in Office 365

Summary Learn how to recognize and respond to a compromised email account in Office 365.

What is a Compromised Email Account in Office 365?

Access to Office 365 mailboxes, data and other services, is controlled through the use of credentials, for example a user name and password or PIN. When someone other than the intended user steals those credentials, the stolen credentials are considered to be compromised. With them the attacker can sign in as the original user and perform illicit actions. Using the stolen credentials, the attacker can access the user’s Office 365 mailbox, SharePoint folders, or files in the user's OneDrive. One action commonly seen is the attacker sending emails as the original user to recipients both inside and outside of the organization. When the attacker emails data to external recipients, this is called data exfiltration.

Symptoms of a Compromised Office 365 Email Account

Users might notice and report unusual activity in their Office 365 mailboxes. Here are some common symptoms:

  • Suspicious activity, such as missing or deleted emails.
  • Other users might receive emails from the compromised account without the corresponding email existing in the Sent Items folder of the sender.
  • The presence of inbox rules that weren't created by the intended user or the administrator. These rules may automatically forward emails to unknown addresses or move them to the Notes, Junk Email, or RSS Subscriptions folders.
  • The user's display name might be changed in the Global Address List.
  • The user's mailbox is blocked from sending email.
  • The Sent or Deleted Items folders in Microsoft Outlook or Outlook on the web (formerly known as Outlook Web App) contain common hacked-account messages, such as "I'm stuck in London, send money."
  • Unusual profile changes, such as the name, the telephone number, or the postal code were updated.
  • Unusual credential changes, such as multiple password changes are required.
  • Mail forwarding was recently added.
  • An unusual signature was recently added, such as a fake banking signature or a prescription drug signature.

If a user reports any of the above symptoms, you should perform further investigation. The Microsoft 365 Security & Compliance Center and the Azure Portal offer tools to help you investigate the activity of a user account that you suspect may be compromised.

  • Office 365 Unified Audit Logs in the Security & Compliance Center - Review all the activities for the suspected account by filtering the results for the date range spanning from immediately before the suspicious activity occurred to the current date. Do not filter on the activities during the search.
  • Use the Azure AD Sign-in logs and other risk reports that are available in the Azure AD portal. Examine the values in these columns:
    • Review IP address
    • sign-in locations
    • sign-in times
    • sign-in success or failure

How to secure and restore email function to a suspected compromised Office 365 account and mailbox

Even after you've regained access to your account, the attacker may have added back-door entries that enable the attacker to resume control of the account.

You must perform all the following steps to regain access to your account the sooner the better to make sure that the hijacker doesn't resume control your account. These steps help you remove any back-door entries that the hijacker may have added to your account. After you perform these steps, we recommend that you run a virus scan to make sure that your computer isn't compromised.

Step 1 Reset the user's password

Warning

Do not send the new password to the intended user through email as the attacker still has access to the mailbox at this point.

  1. Follow the Reset an Office 365 business password for someone else procedures in Admins: Reset Office 365 business passwords

Notes:

  • Make sure that the password is strong and that it contains upper and lowercase letters, at least one number, and at least one special character.
  • Don't reuse any of your last five passwords. Even though the password history requirement lets you reuse a more recent password, you should select something that the attacker can't guess.
  • If your on-premises identity is federated with Office 365, you must change your password on-premises, and then you must notify your administrator of the compromise.

Tip

It is highly recommended that you enable Multi-Factor Authentication (MFA) in order to prevent compromise, especially for accounts with administrative privileges. You can learn more here.

Step 2 Remove suspicious email forwarding addresses

  1. Open the Microsoft 365 admin center > Active Users.
  2. Find the user account in question and expand Mail Settings.
  3. For Email forwarding, click Edit.
  4. Remove any suspicious forwarding addresses.

Step 3 Disable any suspicious inbox rules

  1. Sign in to the user's mailbox using Outlook on the web.
  2. Click on the gear icon and click Mail.
  3. Click Inbox and sweep rules and review the rules.
  4. Disable or delete suspicious rules.

Step 4 Unblock the user from sending mail

If the suspected compromised mailbox was used illicitly to send spam email, it is likely that the mailbox has been blocked from sending mail.

  1. To unblock a mailbox from sending mail, follow the procedures in Removing a user, domain, or IP Address from a block list after sending spam email.

Step 5 Optional: Block the user account from signing-in

Important

You can block the suspected compromised account from signing-in until you believe it is safe to re-enable access.

  1. Go to the Microsoft 365 admin center.
  2. In the Microsoft 365 admin center, select Users.
  3. Select the employee that you want to block, and then choose Edit next to Sign-in status in the user pane
  4. On the Sign-in status pane, choose Sign-in blocked and then Save.
  5. In the Admin center, in the lower-left navigation pane, expand Admin Centers and select Exchange.
  6. In the Exchange admin center, navigate to Recipients > Mailboxes.
  7. Select the user, and on the user properties page, under Mobile Devices, click Disable Exchange ActivcSync and Disable OWA for Devices and answer yes to both.
  8. Under Email Connectivity, Disable and answer yes.

Step 6 Optional: Remove the suspected compromised account from all administrative role groups

Note

Administrative role group membership can be restored after the account has been secured.

  1. Sign in to the Microsoft 365 admin center with a global administrator account and open Active Users.
  2. Find the suspected compromised account and manually check to see if there are any administrative roles assigned to the account.
  3. Open the Security & Compliance Center.
  4. Click Permissions.
  5. Manually review the role groups to see if the suspected compromised account is a member of any of them. If it is: a. Click the role group and click Edit Role Group. b. Click Chose Members and Edit to remove the user from the role group.
  6. Open the Exchange admin center
  7. Click Permissions.
  8. Manually review the role groups to see if the suspected compromised account is a member of any of them. If it is: a. Click the role group and click Edit. b. Use the members section to remove the user from the role group.

Step 7 Optional: Additional precautionary steps

  1. Make sure that you verify your sent items. You may have to inform people on your contacts list that your account was compromised. The attacker may have asked them for money, spoofing, for example, that you were stranded in a different country and needed money, or the attacker may send them a virus to also hijack their computers.
  2. Any other service that used this Exchange account as its alternative email account may have been compromised. First, perform these steps for your Office 365 subscription, and then perform these steps for your other accounts.
  3. Make sure that your contact information, such as telephone numbers and addresses, is correct.

Secure Office 365 like a cybersecurity pro

Your Office 365 subscription comes with a powerful set of security capabilities that you can use to protect your data and your users. Use the Office 365 security roadmap: Top priorities for the first 30 days, 90 days, and beyond to implement Microsoft recommended best practices for securing your Office 365 tenant.

  • Tasks to accomplish in the first 30 days. These have immediate affect and are low-impact to your users.
  • Tasks to accomplish in 90 days. These take a bit more time to plan and implement but greatly improve your security posture.
  • Beyond 90 days. These enhancements build in your first 90 days work.

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