Suspicious File Created in Outlook Temporary Directory — Sigma Rule

Rule metadata and linked tags

Author: Florian Roth (Nextron Systems), Swachchhanda Shrawan Poudel (Nextron Systems). Source status: experimental; severity: high. Source dates: 2025-07-22 / not supplied.

{
  "product": "windows",
  "category": "file_event"
}

Pinned original Sigma rule · Detection Rule License 1.1

Source SHA-256: e3ffc5b745d0132f7f8665414ff6ea3e4f2f3d9e099d86ce947d1e7aa4467371

Detection logic

Original source YAML. Source-tag agreement is not proof of complete semantic coverage. This rule has not been compiled for a SIEM backend or validated against live telemetry here.

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

Original YAML and metadata in JSON

False positives

• Opening of headers or footers in email signatures that include SVG images or legitimate SVG attachments

Source 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/