Detection rules › YARA-L
O365 OneDrive Anonymous File Downloaded
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 download files from OneDrive.
MITRE ATT&CK coverage
| Tactic | Techniques |
|---|---|
| Exfiltration | T1048.002 Exfiltration Over Alternative Protocol: Exfiltration Over Asymmetric Encrypted Non-C2 Protocol |
Event coverage
| Provider | Event |
|---|---|
| M365-SharePointFileOperation | FileDownloaded |
Rules detecting the same action
Other rules on this platform that filter on the same API call or operation.
Rule body yaral
/*
* Copyright 2025 Google LLC
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* https://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
rule o365_onedrive_anonymous_filedownload {
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 download files from OneDrive."
rule_id = "mr_f713e2b0-31b1-4165-8764-59a5e2c5eabb"
rule_name = "O365 OneDrive Anonymous File Downloaded"
tactic = "TA0010"
technique = "T1048.002"
type = "hunt"
platform = "azure"
data_source = "o365"
severity = "Medium"
priority = "Medium"
events:
$file.metadata.event_type = "USER_RESOURCE_UPDATE_CONTENT"
$file.metadata.product_event_type = "FileDownloaded"
$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)
$target_application = array_distinct($file.target.application)
$principal_user_userid = array_distinct($file.principal.user.userid)
$src_file_full_path = array_distinct($file.src.file.full_path)
$src_url = array_distinct($file.src.url)
condition:
$file
}
Detection logic
Fires when at least one $file event in the 5m window.
Events
$file
metadata.product_event_type = "FileDownloaded"principal.user.userid matches "^urn:spo:anon#"principal.user.userid = "anonymous"
Correlation
Outcome
Fields the detection emits on a match. $risk_score drives alerting; Chronicle surfaces the rest on the detection.
| Field | Expression |
|---|---|
risk_score | 35 |
event_count | count_distinct($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) |
src_file_full_path | array_distinct($file.src.file.full_path) |
src_url | array_distinct($file.src.url) |