Detect New Open S3 buckets

name: Detect New Open S3 buckets
id: 2a9b80d3-6340-4345-b5ad-290bf3d0dac4
version: 9
date: '2026-03-10'
author: Bhavin Patel, Patrick Bareiss, Splunk
status: production
type: TTP
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.
data_source:
    - AWS CloudTrail
search: |-
    `cloudtrail` eventSource=s3.amazonaws.com eventName=PutBucketAcl
      | rex field=_raw "(?<json_field>{.+})"
      | spath input=json_field output=grantees path=requestParameters.AccessControlPolicy.AccessControlList.Grant{}
      | search grantees=*
      | mvexpand grantees
      | spath input=grantees output=uri path=Grantee.URI
      | spath input=grantees output=permission path=Permission
      | search uri IN ("http://acs.amazonaws.com/groups/global/AllUsers","http://acs.amazonaws.com/groups/global/AuthenticatedUsers")
      | search permission IN ("READ","READ_ACP","WRITE","WRITE_ACP","FULL_CONTROL")
      | rename requestParameters.bucketName AS bucketName
      | stats count min(_time) as firstTime max(_time) as lastTime
        BY user_arn userIdentity.principalId userAgent
           uri permission bucketName
      | `security_content_ctime(firstTime)`
      | `security_content_ctime(lastTime)`
      | `detect_new_open_s3_buckets_filter`
how_to_implement: 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.
references: []
drilldown_searches:
    - name: View the detection results for - "$user_arn$" and "$bucketName$"
      search: '%original_detection_search% | search  user_arn = "$user_arn$" bucketName = "$bucketName$"'
      earliest_offset: $info_min_time$
      latest_offset: $info_max_time$
    - name: View risk events for the last 7 days for - "$user_arn$" and "$bucketName$"
      search: '| from datamodel Risk.All_Risk | search normalized_risk_object IN ("$user_arn$", "$bucketName$") starthoursago=168  | stats count min(_time) as firstTime max(_time) as lastTime values(search_name) as "Search Name" values(risk_message) as "Risk Message" values(analyticstories) as "Analytic Stories" values(annotations._all) as "Annotations" values(annotations.mitre_attack.mitre_tactic) as "ATT&CK Tactics" by normalized_risk_object | `security_content_ctime(firstTime)` | `security_content_ctime(lastTime)`'
      earliest_offset: $info_min_time$
      latest_offset: $info_max_time$
rba:
    message: User $user_arn$ has created an open/public bucket $bucketName$ with the following permissions $permission$
    risk_objects:
        - field: user_arn
          type: user
          score: 50
    threat_objects: []
tags:
    analytic_story:
        - Suspicious AWS S3 Activities
    asset_type: S3 Bucket
    mitre_attack_id:
        - T1530
    product:
        - Splunk Enterprise
        - Splunk Enterprise Security
        - Splunk Cloud
    security_domain: threat
tests:
    - name: True Positive Test
      attack_data:
        - data: https://media.githubusercontent.com/media/splunk/attack_data/master/datasets/attack_techniques/T1530/aws_s3_public_bucket/aws_cloudtrail_events.json
          sourcetype: aws:cloudtrail
          source: aws_cloudtrail