O365 OneDrive Anonymous File Accessed

// Copyright 2025 Google LLC. Licensed under Apache-2.0.

rule o365_onedrive_anonymous_file_accessed {

  meta:
    author = "Google Cloud Security"
    description = "Anonymous links can be used to export files from OneDrive. While this isn't always a sign of malicious activity, some organizations do not support the use of anonymous links because of the risk of data leakage. This rule detects when anonymous links are used to access files from OneDrive."
    rule_id = "mr_f713e2b0-31b1-4165-8764-59a5e2c5eabc"
    rule_name = "O365 OneDrive Anonymous File Accessed"
    tactic = "TA0010"
    technique = "T1048.002"
    type = "hunt"
    platform = "azure"
    data_source = "o365"
    severity = "Medium"
    priority = "Medium"

  events:
    $file.metadata.event_type = "USER_RESOURCE_ACCESS"
    $file.metadata.product_event_type = "FileAccessed"
    $file.metadata.product_name = "Office 365"
    $file.metadata.vendor_name = "Microsoft"
    (
        $file.principal.user.userid = /^urn:spo:anon#/ or
        $file.principal.user.userid = "anonymous"
    )
    $file.principal.ip = $ip

  match:
    $ip over 5m

  outcome:
    $risk_score = 35
    $event_count = count_distinct($file.metadata.id)
    $referral_url = array_distinct($file.network.http.referral_url)
    $user_agent = array_distinct($file.network.http.user_agent)
    $principal_application = array_distinct($file.principal.application)
    $principal_ip = array_distinct($file.principal.ip)  //IP is a Microsoft IP address not the individual who accessed the file
    $target_application = array_distinct($file.target.application)
    $principal_user_userid = array_distinct($file.principal.user.userid)
    $target_file_full_path = array_distinct($file.target.file.full_path)
    $target_url = array_distinct($file.target.url)

  condition:
    $file
}