Use MailItemsAccessed to investigate compromised accounts

Article
06/13/2024

A compromised user account (also called an account takeover) is a type of attack when an attacker gains access to a user account and operates as the user. These types of attacks sometimes cause more damage than the attacker may have intended. When investigating compromised email accounts, you have to assume that more mail data was compromised than may be indicated by tracing the attacker's actual presence. Depending on the type of data in email messages, you have to assume that sensitive information was compromised or face regulatory fines unless you can prove that sensitive information wasn't exposed. For example, HIPAA-regulated organizations face significant fines if there's evidence that patient health information (PHI) was exposed. In these cases, attackers are unlikely to be interested in PHI, but organizations still must report data breaches unless they can prove otherwise.

To help you with investigating compromise email accounts, we're now auditing accesses of mail data by mail protocols and clients with the MailItemsAccessed mailbox-auditing action. This new audited action helps investigators better understand email data breaches and help you identify the scope of compromises to specific mail items that may been compromised. The goal of using this new auditing action is forensics defensibility to help assert that a specific piece of mail data wasn't compromised. If an attacker gained access to a specific piece of mail, Exchange Online audits the event even though there's no indication that the mail item was read.

Tip

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The MailItemsAccessed mailbox-auditing action

The MailItemsAccessed action is part of Audit (Standard) functionality. It's part of Exchange mailbox auditing and is enabled by default for users that are assigned an Office 365 E3/E5 or Microsoft 365 E3/E5 license.

The MailItemsAccessed mailbox-auditing action covers all mail protocols: POP, IMAP, MAPI, EWS, Exchange ActiveSync, and REST. It also covers both types of accessing mail: sync and bind.

Auditing sync access

Sync operations are only recorded when a mailbox is accessed by a desktop version of the Outlook client for Windows or Mac. During the sync operation, these clients typically download a large set of mail items from the cloud to a local computer. The audit volume for sync operations is huge. So, instead of generating an audit record for each mail item that's synched, we generate an audit event for the mail folder containing items that were synched and assume that all mail items in the synched folder have been compromised. The access type is recorded in the OperationProperties field of the audit record.

See step 2 in the Use MailItemsAccessed audit records for forensic investigations section for an example of displaying the sync access type in an audit record.

Auditing bind access

A bind operation is an individual access to an email message. For bind access, the InternetMessageId of individual messages is recorded in the audit record. The MailItemsAccessed audit action records bind operations and then aggregates into a single audit record. All bind operations that occur within a 2-minute interval are aggregated in a single audit record in the Folders field within the AuditData property. Each message that was accessed is identified by its InternetMessageId. The number of bind operations that were aggregated in the record is displayed in the OperationCount field in the AuditData property.

See step 4 in the Use MailItemsAccessed audit records for forensic investigations section for an example of displaying the bind access type in an audit record.

Throttling of MailItemsAccessed audit records

If more than 1,000 MailItemsAccessed audit records are generated in less than 24 hours, Exchange Online stops generating auditing records for MailItemsAccessed activity. When a mailbox is throttled, MailItemsAccessed activity won't be logged for 24 hours after the mailbox was throttled. If the mailbox was throttled, there's a potential that mailbox could have been compromised during this period. The recording of MailItemsAccessed activity is resumed following a 24-hour period.

Here's a few things to keep in mind about throttling:

Less than 1% of all mailboxes in Exchange Online are throttled
When a mailbox is throttling, only audit records for MailItemsAccessed activity aren't audited. Other mailbox auditing actions aren't affected.
Mailboxes are throttled only for Bind operations. Audit records for sync operations aren't throttled.
If a mailbox is throttled, you can probably assume there was MailItemsAccessed activity that wasn't recorded in the audit logs.

See step 1 in the Use MailItemsAccessed audit records for forensic investigations section for an example of displaying the IsThrottled property in an audit record.

Use MailItemsAccessed audit records for forensic investigations

Mailbox auditing generates audit records for access to email messages so that you can be confident that email messages haven't been compromised. For this reason, in circumstances where we're not certain that some data has been accessed, we assume that it has by recording all mail access activity.

Using MailItemsAccessed audit records for forensics purposes is typically performed after a data breach has been resolved and the attacker has been evicted. To begin your investigation, you should identify the set of mailboxes that they've been compromised and determine the time frame when attacker had access to mailboxes in your organization. Then, you can use the Search-UnifiedAuditLog or Search-MailboxAuditLog cmdlets in Exchange Online PowerShell to search audit records that correspond to the data breach.

You can run one of the following commands to search for MailItemsAccessed audit records:

Unified audit log:

Search-UnifiedAuditLog -StartDate 01/06/2020 -EndDate 01/20/2020 -UserIds <user1,user2> -Operations MailItemsAccessed -ResultSize 1000

Mailbox audit log:

Search-MailboxAuditLog -Identity <user> -StartDate 01/06/2020 -EndDate 01/20/2020 -Operations MailItemsAccessed -ResultSize 1000 -ShowDetails

Tip

One primary difference between these two cmdlets in that you can use the Search-UnifiedAuditLog cmdlet to search for audit records for activity performed by one or more users. That's because UserIds is a multi-value parameter. The Search-MailboxAuditLog cmdlet searches the mailbox audit log for a single user.

Here are the steps for using MailItemsAccessed audit records to investigate a compromised user attack. Each step shows the command syntax for the Search-UnifiedAuditLog or Search-MailboxAuditLog cmdlets.

Check whether the mailbox has been throttled. If so, this would mean that some mailbox-auditing records wouldn't have been logged. In the case that any audit records have the "IsThrottled" is "True," you should assume that for a 24-hour period afterwards that record was generated, that any access to the mailbox wasn't audited and that all mail data has been compromised.

To search for MailItemsAccessed records where the mailbox was throttled, run the following command:

Unified audit log:
```
Search-UnifiedAuditLog -StartDate 01/06/2020 -EndDate 01/20/2020 -UserIds <user1,user2> -Operations MailItemsAccessed -ResultSize 1000 | Where {$_.AuditData -like '*"IsThrottled","Value":"True"*'} | FL
```
Mailbox audit log:
```
Search-MailboxAuditLog -StartDate 01/06/2020 -EndDate 01/20/2020 -Identity <user> -Operations MailItemsAccessed -ResultSize 10000 -ShowDetails | Where {$_.OperationProperties -like "*IsThrottled:True*"} | FL
```
Check for sync activities. If an attacker uses an email client to downloaded messages in a mailbox, they can disconnect the computer from the Internet and access the messages locally without interacting with the server. In this case, mailbox auditing wouldn't be able to audit these activities.

To search for MailItemsAccessed records where the mail items were accessed by a sync operation, run the following command:

Unified audit log:
```
Search-UnifiedAuditLog -StartDate 01/06/2020 -EndDate 02/20/2020 -UserIds <user1,user2> -Operations MailItemsAccessed -ResultSize 1000 | Where {$_.AuditData -like '*"MailAccessType","Value":"Sync"*'} | FL
```
Mailbox audit log:
```
Search-MailboxAuditLog -StartDate 01/06/2020 -EndDate 01/20/2020 -Identity <user> -Operations MailItemsAccessed -ResultSize 10000 -ShowDetails | Where {$_.OperationProperties -like "*MailAccessType:Sync*"} | FL
```

Check sync activities to determine in any of them have occurred in the same context as the one used by the attacker access the mailbox. Context is identified and differentiated by the IP address of the client computer used to access the mailbox and the mail protocol. For more information, see the Identifying the access contexts of different audit records section.

Use the properties listed below to investigate. These properties are located in the AuditData or OperationProperties property. If any of the syncs occur in the same context as the attacker activity, assume the attacker has synced all mail items to their client, which means the entire mailbox has probably been compromised.

Property	Description
ClientInfoString	Describes protocol, client (includes version)
ClientIPAddress	IP address of the client machine.
SessionId	Session ID helps to differentiate attacker actions vs day-to-day user activities on the same account (useful for compromised accounts)
UserId	UPN of the user reading the message.

Check for bind activities. After performing steps 2 and step 3, you can be confident that all other access to email messages by the attacker is captured in the MailItemsAccessed audit records that have a MailAccessType property with a value of "Bind".

To search for MailItemsAccessed records where the mail items were accessed by a Bind operation, run the following command.

Unified audit log:
```
Search-UnifiedAuditLog -StartDate 01/06/2020 -EndDate 01/20/2020 -UserIds <user1,user2> -Operations MailItemsAccessed -ResultSize 1000 | Where {$_.AuditData -like '*"MailAccessType","Value":"Bind"*'} | FL
```
Mailbox audit log:
```
Search-MailboxAuditLog -StartDate 01/06/2020 -EndDate 01/20/2020 -Identity <user> -Operations MailItemsAccessed -ResultSize 10000 -ShowDetails | Where {$_.OperationProperties -like "*MailAccessType:Bind*"} | FL
```
Email messages that were accessed are identified by their internet message ID. You can also check to see if any audit records have the same context as the ones for other attacker activity. For more information, see the Identifying the access contexts of different audit records section.

You can use the audit data for bind operations in two different ways:
- Access or collect all email messages the attacker accessed by using the InternetMessageId to find them and then checking to see if any of those messages contains sensitive information.
- Use the InternetMessageId to search audit records related to a set of potentially sensitive email messages. This is useful if you're concerned only about a few messages.

Filtering of duplicate audit records

Duplicate audit records for the same bind operations that occur within an hour of each other are filtered out to remove auditing noise. Sync operations are also filtered out at one-hour intervals. The exception to this deduplication process occurs if, for the same InternetMessageId, any of the properties described in the following table are different. If one of these properties is different in a duplicate operation, a new audit record is generated. This process is described in more detail in the next section.

Property	Description
ClientIPAddress	IP address of the client computer.
ClientInfoString	The client protocol, client used to access the mailbox.
ParentFolder	The full folder path of the mail item that was accessed.
Logon_type	The logon type of the user who performed the action. The logon types (and their corresponding Enum value) are Owner (0), Admin (1), or Delegate (2).
MailAccessType	Whether the access is a bind or a sync operation.
MailboxUPN	The UPN of the mailbox where the message being read is located.
User	The UPN of the user reading the message.
SessionId	The Session ID helps to differentiate attacker actions and day-to-day user activities in the same mailbox (in the case of account compromise) For more information about sessions, see Contextualizing attacker activity within sessions in Exchange Online.

Identifying the access contexts of different audit records

It's common that an attacker may access a mailbox at the same time the mailbox owner is accessing it. To differentiate between access by the attacker and the mailbox owner, there are audit record properties that define the context of the access. As previously explained, when the values for these properties are different, even when the activity occurs within the aggregation interval, separate audit records are generated. In the following example, there are three different audit records. Each one is differentiated by the Session ID and ClientIPAddress properties. The messages that were accessed are also identified.

Audit record 1	Audit record 2	Audit record 3
ClientIPAddress1 SessionId2	ClientIPAddress2 SessionId2	ClientIPAddress1 SessionId3
InternetMessageIdA InternetMessageIdD InternetMessageIdE InternetMessageIdF	InternetMessageIdA InternetMessageIdC	InternetMessageIdB

If any of the properties listed in the table in the previous section are different, a separate audit record is generated to track the new context. Accesses are sorted into the separate audit records depending on the context in which the activity took place.

For example, in audit records shown in the following screenshot, though we're accessing mail from EWSEditor and OWA simultaneously, the access activity is collated in different audit records depending on the context in which the access took place. In this case, the context is defined by different values for the ClientInfoString property.

Different audit records based on context.

Here's the syntax for the command shown in the previous screenshot:

Search-MailboxAuditLog -Identity admin -ShowDetails -Operations MailItemsAccessed -ResultSize 2000 | Select LastAccessed,Operation,AuditOperationsCountInAggregatedRecord,ClientInfoString

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