Detection: AWS IAM Assume Role Policy Brute Force

Updated Date: 2025-05-02 ID: f19e09b0-9308-11eb-b7ec-acde48001122 Author: Michael Haag, Splunk Type: TTP Product: Splunk Enterprise Security

Description

The following analytic detects multiple failed attempts to assume an AWS IAM role, indicating a potential brute force attack. It leverages AWS CloudTrail logs to identify MalformedPolicyDocumentException errors with a status of failure and filters out legitimate AWS services. This activity is significant as repeated failures to assume roles can indicate an adversary attempting to guess role names, which is a precursor to unauthorized access. If confirmed malicious, this could lead to unauthorized access to AWS resources, potentially compromising sensitive data and services.

Search

1`cloudtrail` (errorCode=MalformedPolicyDocumentException) status=failure (userAgent!=*.amazonaws.com) 
2| rename user_name as user 
3| stats count min(_time) as firstTime max(_time) as lastTime values(requestParameters.policyName) as policy_name by src, user, vendor_account vendor_region, vendor_product, signature, dest, errorCode 
4| where count >= 2 
5| `security_content_ctime(firstTime)` 
6| `security_content_ctime(lastTime)` 
7| `aws_iam_assume_role_policy_brute_force_filter`

Data Source

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

Macros Used

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

aws_iam_assume_role_policy_brute_force_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
T1580	Cloud Infrastructure Discovery	Discovery
T1110	Brute Force	Credential Access

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

The Splunk AWS Add-on and Splunk App for AWS is required to utilize this data. The search requires AWS CloudTrail logs. Set the where count greater than a value to identify suspicious activity in your environment.

Known False Positives

This detection will require tuning to provide high fidelity detection capabilties. Tune based on src addresses (corporate offices, VPN terminations) or by groups of users.

Associated Analytic Story

AWS IAM Privilege Escalation

Risk Based Analytics (RBA)

Risk Message:

User $user$ has caused multiple failures with errorCode $errorCode$, which potentially means adversary is attempting to identify a role name.

Risk Object	Risk Object Type	Risk Score	Threat Objects
user	user	28	src

References

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: 6