Detect New Open S3 Buckets over AWS CLI

name: Detect New Open S3 Buckets over AWS CLI
id: 39c61d09-8b30-4154-922b-2d0a694ecc22
version: 11
creation_date: '2021-01-12'
modification_date: '2026-05-13'
author: Patrick Bareiss, Splunk, vshcherbakov
status: production
type: TTP
description: |-
    The following analytic detects the creation of open/public S3 buckets via the AWS CLI.
    It leverages AWS CloudTrail logs to identify events where a user has set bucket permissions to allow access to "AuthenticatedUsers" or "AllUsers." This activity is significant because open S3 buckets can expose sensitive data to unauthorized users, leading to data breaches.
    If confirmed malicious, an attacker could gain unauthorized access to potentially sensitive information stored in the S3 bucket, posing a significant security risk.
data_source:
    - AWS CloudTrail
search: |-
    `cloudtrail`
    eventSource="s3.amazonaws.com"
    eventName="PutBucketAcl"
    errorCode=success
    userAgent IN ("[aws-cli*", "aws-cli*")

    | rename requestParameters.bucketName AS bucketName
             requestParameters.accessControlList.x-amz-grant-read AS grantRead
             requestParameters.accessControlList.x-amz-grant-read-acp AS grantReadAcp
             requestParameters.accessControlList.x-amz-grant-write AS grantWrite
             requestParameters.accessControlList.x-amz-grant-write-acp AS grantWriteAcp
             requestParameters.accessControlList.x-amz-grant-full-control AS grantFullControl
             requestParameters.x-amz-acl AS cannedAcl,
             userIdentity.principalId AS principalId
             userIdentity.userName AS userName
             userIdentity.arn AS userArn

    | eval grantRead=coalesce(grantRead,"-"),
           grantReadAcp=coalesce(grantReadAcp,"-"),
           grantWrite=coalesce(grantWrite,"-"),
           grantWriteAcp=coalesce(grantWriteAcp,"-"),
           grantFullControl=coalesce(grantFullControl,"-"),
           cannedAcl=coalesce(cannedAcl,"-"),
           user=coalesce(userName,userArn)

    | eval publicGrant = if(
                            match(grantRead, "(?i)AllUsers|AuthenticatedUsers")
                            OR
                            match(grantReadAcp, "(?i)AllUsers|AuthenticatedUsers")
                            OR
                            match(grantWrite, "(?i)AllUsers|AuthenticatedUsers")
                            OR
                            match(grantWriteAcp, "(?i)AllUsers|AuthenticatedUsers")
                            OR
                            match(grantFullControl, "(?i)AllUsers|AuthenticatedUsers"),
                            1,
                            0
                        )

    | eval publicCanned = if(cannedAcl IN ("public-read", "public-read-write", "authenticated-read"), 1, 0)

    | where publicGrant=1 OR publicCanned=1

    | eval exposure=case(cannedAcl=="public-read-write","public-write", match(grantWrite."".grantWriteAcp."".grantFullControl,"(?i)AllUsers"),"public-write", cannedAcl=="public-read" OR match(grantRead,"(?i)AllUsers"),"public-read", cannedAcl=="authenticated-read" OR match(grantRead,"(?i)AuthenticatedUsers"),"authenticated-read", true(),"custom-grant")

    | stats count
            min(_time) AS firstTime
            max(_time) AS lastTime
            values(cannedAcl) AS cannedAcl
            values(grantRead) AS grantRead
            values(grantReadAcp) AS grantReadAcp
            values(grantWrite) AS grantWrite
            values(grantWriteAcp) AS grantWriteAcp
            values(grantFullControl) AS grantFullControl
            values(eventName) AS eventName

      BY user principalId userAgent bucketName exposure

    | `security_content_ctime(firstTime)`
    | `security_content_ctime(lastTime)`
    | `detect_new_open_s3_buckets_over_aws_cli_filter`
how_to_implement: |-
    The Splunk AWS Add-on and Splunk App for AWS is required to utilize this data. The search requires AWS Cloudtrail logs.
known_false_positives: |-
    It is possible that an AWS admin has legitimately created a public bucket for a specific purpose. AWS strongly advises against granting full control to the "All Users" group. Investigate and apply the necessary filters as needed.
references:
    - https://docs.aws.amazon.com/cli/latest/reference/s3api/create-bucket.html
drilldown_searches:
    - name: View the detection results for - "$user$"
      search: '%original_detection_search% | search  user = "$user$"'
      earliest_offset: $info_min_time$
      latest_offset: $info_max_time$
    - name: View risk events for the last 7 days for - "$user$"
      search: '| from datamodel Risk.All_Risk | search normalized_risk_object IN ("$user$") | 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: 7d
      latest_offset: "0"
finding:
    title: User [$user$] has created an open/public S3 Bucket [$bucketName$] using AWS CLI with the following permissions - [$grantRead$] [$grantReadAcp$] [$grantWrite$] [$grantWriteAcp$] [$grantFullControl$] [$cannedAcl$]
    entity:
        field: user
        type: user
        score: 50
threat_objects:
    - field: bucketName
      type: bucket_name
analytic_story:
    - Suspicious AWS S3 Activities
asset_type: S3 Bucket
mitre_attack_id:
    - T1530
product:
    - Splunk Enterprise
    - Splunk Enterprise Security
    - Splunk Cloud
category: 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
      test_type: unit