Detection: AWS Cross Account Activity From Previously Unseen Account

EXPERIMENTAL DETECTION

This detection status is set to experimental. The Splunk Threat Research team has not yet fully tested, simulated, or built comprehensive datasets for this detection. As such, this analytic is not officially supported. If you have any questions or concerns, please reach out to us at research@splunk.com.

Updated Date: 2024-05-16 ID: 21193641-cb96-4a2c-a707-d9b9a7f7792b Author: Rico Valdez, Splunk Type: Anomaly Product: Splunk Enterprise Security

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.

Search

 1
 2| 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 
 3| `drop_dm_object_name(Authentication)` 
 4| rex field=user_role "arn:aws:sts:*:(?<dest_account>.*):" 
 5| where vendor_account != dest_account 
 6| rename vendor_account as requestingAccountId dest_account as requestedAccountId 
 7| lookup previously_seen_aws_cross_account_activity requestingAccountId, requestedAccountId, OUTPUTNEW firstTime 
 8| eval status = if(firstTime > relative_time(now(), "-24h@h"),"New Cross Account Activity","Previously Seen") 
 9|  where status = "New Cross Account Activity" 
10| `security_content_ctime(firstTime)` 
11| `security_content_ctime(lastTime)`
12| `aws_cross_account_activity_from_previously_unseen_account_filter`

Data Source

Name	Platform	Sourcetype	Source	Supported App
AWS CloudTrail	AWS	`'aws:cloudtrail'`	`'aws_cloudtrail'`	N/A

Macros Used

Name	Value
security_content_ctime	`convert timeformat="%Y-%m-%dT%H:%M:%S" ctime($field$)`
aws_cross_account_activity_from_previously_unseen_account_filter	`search *`

aws_cross_account_activity_from_previously_unseen_account_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

NistCategory.DE_AE

Cis18Value.CIS_13

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 Risk Event	True

This configuration file applies to all detections of type anomaly. These detections will use Risk Based Alerting.

Implementation

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.

Associated Analytic Story

Suspicious Cloud Authentication Activities

Risk Based Analytics (RBA)

Risk Message	Risk Score	Impact	Confidence
AWS account $requestingAccountId$ is trying to access resource from some other account $requestedAccountId$, for the first time.	15	30	50

The Risk Score is calculated by the following formula: Risk Score = (Impact * Confidence/100). Initial Confidence and Impact is set by the analytic author.

Detection Testing

Test Type	Status	Dataset	Source	Sourcetype
Validation	Not Applicable	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: 2