Suspicious File Created in Outlook Temporary Directory

Detects the creation of files with suspicious file extensions in the temporary directory that Outlook uses when opening attachments. This can be used to detect spear-phishing campaigns that use suspicious files as attachments, which may contain malicious code.

title: Suspicious File Created in Outlook Temporary Directory
id: fabb0e80-030c-4e3e-a104-d09676991ac3
related:
    - id: f748c45a-f8d3-4e6f-b617-fe176f695b8f
      type: obsolete
status: experimental
description: |
    Detects the creation of files with suspicious file extensions in the temporary directory that Outlook uses when opening attachments.
    This can be used to detect spear-phishing campaigns that use suspicious files as attachments, which may contain malicious code.
references:
    - https://vipre.com/blog/svg-phishing-attacks-the-new-trick-in-the-cybercriminals-playbook/
    - https://thecyberexpress.com/rogue-rdp-files-used-in-ukraine-cyberattacks/
    - https://www.microsoft.com/en-us/security/blog/2024/10/29/midnight-blizzard-conducts-large-scale-spear-phishing-campaign-using-rdp-files/
author: Florian Roth (Nextron Systems), Swachchhanda Shrawan Poudel (Nextron Systems)
date: 2025-07-22
tags:
    - attack.initial-access
    - attack.t1566.001
logsource:
    product: windows
    category: file_event
detection:
    selection_extension:
        TargetFilename|endswith:
            - '.cpl'
            - '.hta'
            - '.iso'
            - '.rdp'
            - '.svg'
            - '.vba'
            - '.vbe'
            - '.vbs'
    selection_location:
        - TargetFilename|contains:
              - '\AppData\Local\Packages\Microsoft.Outlook_'
              - '\AppData\Local\Microsoft\Olk\Attachments\'
        - TargetFilename|contains|all:
              - '\AppData\Local\Microsoft\Windows\'
              - '\Content.Outlook\'
    condition: all of selection_*
falsepositives:
    - Opening of headers or footers in email signatures that include SVG images or legitimate SVG attachments
level: high

• T1566.001Spearphishing Attachment

• https://vipre.com/blog/svg-phishing-attacks-the-new-trick-in-the-cybercriminals-playbook/↗
• https://thecyberexpress.com/rogue-rdp-files-used-in-ukraine-cyberattacks/↗
• https://www.microsoft.com/en-us/security/blog/2024/10/29/midnight-blizzard-conducts-large-scale-spear-phishing-campaign-using-rdp-files/↗