Splunk

Email files written outside of the Outlook directory

Last updated a day ago on 2026-09-18

About

The following analytic detects email files (.pst or .ost) being created outside the standard Outlook directories. It leverages the Endpoint.Filesystem data model to identify file creation events and filters for email files not located in "C:\Users\*\My Documents\Outlook Files\*" or "C:\Users\*\AppData\Local\Microsoft\Outlook*". This activity is significant as it may indicate data exfiltration or unauthorized access to email data. If confirmed malicious, an attacker could potentially access sensitive email content, leading to data breaches or further exploitation within the network.

Platform: Splunk
Tags: Data Source: SplunkDomain: ApplicationRule Type: QueryOS: Any
Severity: medium
Risk Score: 47
False Positives: Administrators and users sometimes prefer backing up their email data by moving the email files into a different folder. These attempts will be detected by the search. You should confirm that the activity is legitimate and modify the search to add exclusions, as necessary.
Source: View on GitHub(external, opens in a new tab or window)

Definition

Rule Language: SPL
Rule Type: query

✄𐘗text code block:
✄𐘗| tstats `security_content_summariesonly`
  count values(Filesystem.file_path) as file_path
        min(_time) as firstTime
        max(_time) as lastTime

FROM datamodel=Endpoint.Filesystem WHERE

Filesystem.action IN (
    "created",
    "modified"
)
Filesystem.file_name IN (
    "*.pst",
    "*.ost"
)
NOT Filesystem.file_path IN (
    "C:\\Users\\*\\My Documents\\Outlook Files\\*",
    "C:\\Users\\*\\AppData\\Local\\Microsoft\\Outlook*"
)

BY Filesystem.action Filesystem.dest Filesystem.file_access_time
   Filesystem.file_create_time Filesystem.file_hash Filesystem.file_modify_time
   Filesystem.file_name Filesystem.file_path Filesystem.file_acl
   Filesystem.file_size Filesystem.process_guid Filesystem.process_id
   Filesystem.user Filesystem.vendor_product

| `drop_dm_object_name("Filesystem")`
| `security_content_ctime(firstTime)`
| `security_content_ctime(lastTime)`
| `email_files_written_outside_of_the_outlook_directory_filter`

View this rule on Splunk

This rule was sourced from the official Splunk public repository. View the original rule on GitHub(external, opens in a new tab or window).