Detection: Detect New Open S3 buckets

Updated Date: 2026-02-25 ID: 2a9b80d3-6340-4345-b5ad-290bf3d0dac4 Author: Bhavin Patel, Patrick Bareiss, Splunk Type: TTP Product: Splunk Enterprise Security

Description

The following analytic identifies the creation of open/public S3 buckets in AWS. It detects this activity by analyzing AWS CloudTrail events for PutBucketAcl actions where the access control list (ACL) grants permissions to all users or authenticated users. This activity is significant because open S3 buckets can expose sensitive data to unauthorized access, leading to data breaches. If confirmed malicious, an attacker could read, write, or fully control the contents of the bucket, potentially leading to data exfiltration or tampering.

Search

 1`cloudtrail` eventSource=s3.amazonaws.com eventName=PutBucketAcl
 2  
 3| rex field=_raw "(?<json_field>{.+})"
 4  
 5| spath input=json_field output=grantees path=requestParameters.AccessControlPolicy.AccessControlList.Grant{}
 6  
 7| search grantees=*
 8  
 9| mvexpand grantees
10  
11| spath input=grantees output=uri path=Grantee.URI
12  
13| spath input=grantees output=permission path=Permission
14  
15| search uri IN ("http://acs.amazonaws.com/groups/global/AllUsers","http://acs.amazonaws.com/groups/global/AuthenticatedUsers")
16  
17| search permission IN ("READ","READ_ACP","WRITE","WRITE_ACP","FULL_CONTROL")
18  
19| rename requestParameters.bucketName AS bucketName
20  
21| stats count min(_time) as firstTime max(_time) as lastTime
22    BY user_arn userIdentity.principalId userAgent
23       uri permission bucketName
24  
25| `security_content_ctime(firstTime)`
26  
27| `security_content_ctime(lastTime)`
28  
29| `detect_new_open_s3_buckets_filter`

Data Source

Name	Platform	Sourcetype	Source
AWS CloudTrail	AWS	`'aws:cloudtrail'`	`'aws_cloudtrail'`

Macros Used

Name	Value
cloudtrail	`sourcetype=aws:cloudtrail`
detect_new_open_s3_buckets_filter	`search *`

detect_new_open_s3_buckets_filter is an empty macro by default. It allows the user to filter out any results (false positives) without editing the SPL.

Annotations

- MITRE ATT&CK

+ Kill Chain Phases

+ NIST

+ CIS

- Threat Actors

ID	Technique	Tactic
T1530	Data from Cloud Storage	Collection

Exploitation

DE.CM

CIS 10

Default Configuration

This detection is configured by default in Splunk Enterprise Security to run with the following settings:

Setting	Value
Disabled	true
Cron Schedule	`0 * * * *`
Earliest Time	`-70m@m`
Latest Time	`-10m@m`
Schedule Window	`auto`
Creates Notable	Yes
Rule Title	`%name%`
Rule Description	`%description%`
Notable Event Fields	user, dest
Creates Risk Event	True

This configuration file applies to all detections of type TTP. These detections will use Risk Based Alerting and generate Notable Events.

Implementation

You must install the AWS App for Splunk.

Known False Positives

While this search has no known false positives, it is possible that an AWS admin has legitimately created a public bucket for a specific purpose. That said, AWS strongly advises against granting full control to the "All Users" group.

Associated Analytic Story

Suspicious AWS S3 Activities

Risk Based Analytics (RBA)

Risk Message:

User $user_arn$ has created an open/public bucket $bucketName$ with the following permissions $permission$

Risk Object	Risk Object Type	Risk Score	Threat Objects
user_arn	user	48	No Threat Objects

Detection Testing

Test Type	Status	Dataset	Source	Sourcetype
Validation	✅ Passing	N/A	N/A	N/A
Unit	✅ Passing	Dataset	`aws_cloudtrail`	`aws:cloudtrail`
Integration	✅ Passing	Dataset	`aws_cloudtrail`	`aws:cloudtrail`

Replay any dataset to Splunk Enterprise by using our replay.py tool or the UI. Alternatively you can replay a dataset into a Splunk Attack Range

Source: GitHub | Version: 8