O365 OneDrive Anonymous Link Accessed

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

rule o365_onedrive_anonymous_link_accessed {

  meta:
    author = "Google Cloud Security"
    description = "Anonymous links can be used to access 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_7e67005a-d5fc-4497-8e81-e6797b2f54e9"
    rule_name = "O365 OneDrive Anonymous Link 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 = "AnonymousLinkUsed"
    $file.metadata.product_name = "Office 365"
    $file.metadata.vendor_name = "Microsoft"
    $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)
    $target_application = array_distinct($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
}