Suspicious File Created in Outlook Temporary Directory

calendar Jul 29, 2025 · attack.initial-access attack.t1566.001  ·
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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.

Sigma rule (View on GitHub)

 1title: Suspicious File Created in Outlook Temporary Directory
 2id: fabb0e80-030c-4e3e-a104-d09676991ac3
 3related:
 4    - id: f748c45a-f8d3-4e6f-b617-fe176f695b8f
 5      type: obsolete
 6status: experimental
 7description: |
 8    Detects the creation of files with suspicious file extensions in the temporary directory that Outlook uses when opening attachments.
 9    This can be used to detect spear-phishing campaigns that use suspicious files as attachments, which may contain malicious code.    
10references:
11    - https://vipre.com/blog/svg-phishing-attacks-the-new-trick-in-the-cybercriminals-playbook/
12    - https://thecyberexpress.com/rogue-rdp-files-used-in-ukraine-cyberattacks/
13    - https://www.microsoft.com/en-us/security/blog/2024/10/29/midnight-blizzard-conducts-large-scale-spear-phishing-campaign-using-rdp-files/
14author: Florian Roth (Nextron Systems), Swachchhanda Shrawan Poudel (Nextron Systems)
15date: 2025-07-22
16tags:
17    - attack.initial-access
18    - attack.t1566.001
19logsource:
20    product: windows
21    category: file_event
22detection:
23    selection_extension:
24        TargetFilename|endswith:
25            - '.cpl'
26            - '.hta'
27            - '.iso'
28            - '.rdp'
29            - '.svg'
30            - '.vba'
31            - '.vbe'
32            - '.vbs'
33    selection_location:
34        - TargetFilename|contains:
35              - '\AppData\Local\Packages\Microsoft.Outlook_'
36              - '\AppData\Local\Microsoft\Olk\Attachments\'
37        - TargetFilename|contains|all:
38              - '\AppData\Local\Microsoft\Windows\'
39              - '\Content.Outlook\'
40    condition: all of selection_*
41falsepositives:
42    - Opening of headers or footers in email signatures that include SVG images or legitimate SVG attachments
43level: high

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