Suspicious Email Attachment Extensions

SPLUNK detection rule by David Dorsey, Splunk · Apache-2.0

The following analytic detects emails containing attachments with suspicious file extensions. It leverages the Email data model in Splunk, using the tstats command to identify emails where the attachment filename is not empty. This detection is significant for SOC analysts as it highlights potential phishing or malware delivery attempts, which are common vectors for data breaches and malware infections. If confirmed malicious, this activity could lead to unauthorized access to sensitive information, system compromise, or data exfiltration. Immediate review and analysis of the identified emails and attachments are crucial to mitigate these risks.

name: Suspicious Email Attachment Extensions
id: 473bd65f-06ca-4dfe-a2b8-ba04ab4a0084
version: 12
creation_date: '2020-04-29'
modification_date: '2026-05-13'
author: David Dorsey, Splunk
status: experimental
type: Anomaly
description: The following analytic detects emails containing attachments with suspicious file extensions. It leverages the Email data model in Splunk, using the tstats command to identify emails where the attachment filename is not empty. This detection is significant for SOC analysts as it highlights potential phishing or malware delivery attempts, which are common vectors for data breaches and malware infections. If confirmed malicious, this activity could lead to unauthorized access to sensitive information, system compromise, or data exfiltration. Immediate review and analysis of the identified emails and attachments are crucial to mitigate these risks.
data_source: []
search: |
    | tstats `security_content_summariesonly` count min(_time) as firstTime max(_time)
    as lastTime from datamodel=Email.All_Email where All_Email.file_name="*"

    by All_Email.src_user All_Email.file_name All_Email.file_size All_Email.message_id
       All_Email.message_info All_Email.process All_Email.process_id All_Email.orig_dest
       All_Email.orig_recipient

    | `drop_dm_object_name(All_Email)`
    | `security_content_ctime(firstTime)`
    | `security_content_ctime(lastTime)`
    | lookup update=true is_suspicious_file_extension_lookup file_name OUTPUT suspicious
    | search suspicious=true
    | `suspicious_email_attachment_extensions_filter`
how_to_implement: |
    You need to ingest data from emails. Specifically, the sender's address and the file names of any attachments must be mapped to the Email data model.
    **Splunk Phantom Playbook Integration**\nIf Splunk Phantom is also configured in
    your environment, a Playbook called \"Suspicious Email Attachment Investigate and
    Delete\" can be configured to run when any results are found by this detection search.
    To use this integration, install the Phantom App for Splunk `https://splunkbase.splunk.com/app/3411/`,
    and add the correct hostname to the \"Phantom Instance\" field in the Adaptive Response
    Actions when configuring this detection search. The finding event will be sent to
    Phantom and the playbook will gather further information about the file attachment
    and its network behaviors. If Phantom finds malicious behavior and an analyst approves
    of the results, the email will be deleted from the user's inbox.'"
known_false_positives: No false positives have been identified at this time.
references: []
intermediate_findings:
    entities:
        - field: user
          type: user
          score: 20
          message: Email attachment $file_name$ with suspicious extension from $src_user$
analytic_story:
    - Data Destruction
    - Emotet Malware DHS Report TA18-201A
    - Hermetic Wiper
    - Suspicious Emails
asset_type: Endpoint
mitre_attack_id:
    - T1566.001
product:
    - Splunk Enterprise
    - Splunk Enterprise Security
    - Splunk Cloud
category: application
security_domain: network

[[ currentRule.title && currentRule.title.length > 60 ? currentRule.title.slice(0, 60) + '…' : currentRule.title ]]

57bdc823-ec52-4f5c-aaf9-752341a8b9d8

Propose edits, view related rules, and engage with the community through comments.

Executable code, not a declarative rule

This submission is arbitrary [[ currentRule.format ? currentRule.format.toUpperCase() : '' ]] code, not a declarative detection rule — review it as you would any script before running it, and note that it cannot be safely aggregated or auto-executed alongside YARA/Sigma/Suricata/CRS rules.

Rejected pattern present

[[ reason ]]

What does this mean?

Format & documentation suggestions

Not a security issue — this rule is valid and was accepted as-is. These are optional fields/conventions its own format recommends, mainly affecting its quality score and how well it documents itself for other reviewers.

[[ reason ]]

What do these messages mean?

[[ currentRule.title && currentRule.title.length > 60 ? currentRule.title.slice(0, 60) + '…' : currentRule.title ]]

[[ currentRule.title ]]

Community Discussion