AWS Cross Account Activity From Previously Unseen Account

SPLUNK detection rule by Rico Valdez, Splunk · Apache-2.0

The following analytic identifies AssumeRole events where an IAM role in a different AWS account is accessed for the first time. It detects this activity by analyzing authentication logs and comparing the requesting and requested account IDs, flagging new cross-account activities. This behavior is significant because unauthorized cross-account access can indicate potential lateral movement or privilege escalation attempts. If confirmed malicious, an attacker could gain unauthorized access to resources in another account, potentially leading to data exfiltration, service disruption, or further compromise of the AWS environment.

name: AWS Cross Account Activity From Previously Unseen Account
id: 21193641-cb96-4a2c-a707-d9b9a7f7792b
version: 5
creation_date: '2020-04-29'
modification_date: '2026-05-13'
author: Rico Valdez, Splunk
status: removed
type: Anomaly
description: The following analytic identifies AssumeRole events where an IAM role in a different AWS account is accessed for the first time. It detects this activity by analyzing authentication logs and comparing the requesting and requested account IDs, flagging new cross-account activities. This behavior is significant because unauthorized cross-account access can indicate potential lateral movement or privilege escalation attempts. If confirmed malicious, an attacker could gain unauthorized access to resources in another account, potentially leading to data exfiltration, service disruption, or further compromise of the AWS environment.
data_source:
    - AWS CloudTrail
search: '| tstats min(_time) as firstTime max(_time) as lastTime from datamodel=Authentication where Authentication.signature=AssumeRole by Authentication.vendor_account Authentication.user Authentication.src Authentication.user_role | `drop_dm_object_name(Authentication)` | rex field=user_role "arn:aws:sts:*:(?<dest_account>.*):" | where vendor_account != dest_account | rename vendor_account as requestingAccountId dest_account as requestedAccountId | lookup previously_seen_aws_cross_account_activity requestingAccountId, requestedAccountId, OUTPUTNEW firstTime | eval status = if(firstTime > relative_time(now(), "-24h@h"),"New Cross Account Activity","Previously Seen") |  where status = "New Cross Account Activity" | `security_content_ctime(firstTime)` | `security_content_ctime(lastTime)`| `aws_cross_account_activity_from_previously_unseen_account_filter`'
how_to_implement: You must be ingesting your cloud infrastructure logs from your cloud provider. You should run the baseline search `Previously Seen AWS Cross Account Activity - Initial` to build the initial table of source IP address, geographic locations, and times. You must also enable the second baseline search `Previously Seen AWS Cross Account Activity - Update` to keep this table up to date and to age out old data. You can also provide additional filtering for this search by customizing the `aws_cross_account_activity_from_previously_unseen_account_filter` macro.
known_false_positives: Using multiple AWS accounts and roles is perfectly valid behavior. It's suspicious when an account requests privileges of an account it hasn't before. You should validate with the account owner that this is a legitimate request.
references: []
rba:
    message: AWS account $requestingAccountId$ is trying to access resource from some other account $requestedAccountId$, for the first time.
    risk_objects:
        - field: user
          type: user
          score: 15
    threat_objects: []
tags:
    analytic_story:
        - Suspicious Cloud Authentication Activities
    asset_type: AWS Instance
    product:
        - Splunk Enterprise
        - Splunk Enterprise Security
        - Splunk Cloud
    security_domain: network
tests:
    - name: True Positive Test
      attack_data:
          - data: https://media.githubusercontent.com/media/splunk/attack_data/master/datasets/suspicious_behaviour/abnormally_high_cloud_instances_launched/cloudtrail_behavioural_detections.json
            sourcetype: aws:cloudtrail
            source: aws_cloudtrail
deprecation_info:
    reason: Detection deprecated as it no longer effectively identifies the intended malicious activity
    removed_in_version: 5.4.0
    replacement_content: []

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