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AWS STS Role Assumption by Service

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AWS STS Role Assumption by Service

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Identifies when a service has assumed a role in AWS Security Token Service (STS). Services can assume a role to obtain temporary credentials and access AWS resources. Adversaries can use this technique for credential access and privilege escalation. This is a [New Terms](https://www.elastic.co/guide/en/security/current/rules-ui-create.html#create-new-terms-rule) rule that identifies when a service assumes a role in AWS Security Token Service (STS) to obtain temporary credentials and access AWS resources. While often legitimate, adversaries may use this technique for unauthorized access, privilege escalation, or lateral movement within an AWS environment.

Rule type: new_terms

Rule indices:

  • filebeat-*
  • logs-aws.cloudtrail-*

Severity: low

Risk score: 21

Runs every: 5m

Searches indices from: now-9m (Date Math format, see also Additional look-back time)

Maximum alerts per execution: 100

References:

Tags:

  • Domain: Cloud
  • Data Source: AWS
  • Data Source: Amazon Web Services
  • Data Source: AWS STS
  • Resources: Investigation Guide
  • Use Case: Identity and Access Audit
  • Tactic: Privilege Escalation

Version: 209

Rule authors:

  • Elastic
  • Austin Songer

Rule license: Elastic License v2

Investigation guide

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Triage and Analysis

Investigating AWS STS Role Assumption by Service

This rule identifies instances where AWS STS (Security Token Service) is used to assume a role, granting temporary credentials for AWS resource access. While this action is often legitimate, it can be exploited by adversaries to obtain unauthorized access, escalate privileges, or move laterally within an AWS environment.

Possible Investigation Steps

  • Identify the Actor and Assumed Role:
  • User Identity: Review the aws.cloudtrail.user_identity.arn and aws.cloudtrail.user_identity.type fields to determine who initiated the AssumeRole action.
  • Role Assumed: Check the aws.cloudtrail.flattened.request_parameters.roleArn field to confirm the assumed role and ensure it aligns with expected responsibilities.
  • Session Name: Observe the aws.cloudtrail.flattened.request_parameters.roleSessionName for context on the session’s intended purpose, if available.
  • Analyze the Role Session and Duration:
  • Session Context: Look at the aws.cloudtrail.user_identity.session_context.creation_date to understand when the session began and check if multi-factor authentication (MFA) was used, indicated by the aws.cloudtrail.user_identity.session_context.mfa_authenticated field.
  • Credential Validity: Examine the aws.cloudtrail.flattened.request_parameters.durationSeconds for the credential’s validity period.
  • Expiration Time: Verify aws.cloudtrail.flattened.response_elements.credentials.expiration to determine when the credentials expire or expired.
  • Inspect the User Agent for Tooling Identification:
  • User Agent Details: Review the user_agent.original field to identify the tool or SDK used for the role assumption. Indicators include:
  • AWS SDKs (e.g., Boto3): Often used in automated workflows or scripts.
  • AWS CLI: Suggests command-line access, potentially indicating direct user interaction.
  • Custom Tooling: Unusual user agents may signify custom or suspicious tools.
  • Source IP and Location: Evaluate the source.address and source.geo fields to confirm if the access source aligns with typical access locations for your environment.
  • Contextualize with Related Events:
  • Review Event Patterns: Check surrounding CloudTrail events to see if other actions coincide with this AssumeRole activity, such as attempts to access sensitive resources.
  • Identify High-Volume Exceptions: Due to the potential volume of AssumeRole events, determine common, legitimate roleArn values or user_agent patterns, and consider adding these as exceptions to reduce noise.
  • Evaluate the Privilege Level of the Assumed Role:
  • Permissions: Inspect permissions associated with the assumed role to understand its access level.
  • Authorized Usage: Confirm whether the role is typically used for administrative purposes and if the assuming entity frequently accesses it as part of regular responsibilities.

False Positive Analysis

  • Automated Workflows and Applications: Many applications or scheduled tasks may assume roles for standard operations. Check user agents and ARNs for consistency with known workflows.
  • Routine IAM Policy Actions: Historical data may reveal if the same user or application assumes this specific role regularly as part of authorized operations.

Response and Remediation

  • Revoke Unauthorized Sessions: If unauthorized, consider revoking the session by adjusting IAM policies or permissions associated with the assumed role.
  • Enhance Monitoring and Alerts: Set up enhanced monitoring for high-risk roles, especially those with elevated privileges.
  • Manage Exceptions: Regularly review and manage high-frequency roles and user agent patterns, adding trusted ARNs and user agents to exception lists to minimize alert fatigue.
  • Incident Response: If malicious behavior is identified, follow incident response protocols, including containment, investigation, and remediation.

Additional Information

For more information on managing and securing AWS STS, refer to the AWS STS documentation and AWS security best practices.

Rule query

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event.dataset: "aws.cloudtrail"
    and event.provider: "sts.amazonaws.com"
    and event.action: "AssumeRole"
    and event.outcome: "success"
    and aws.cloudtrail.user_identity.type: "AWSService"
    and not aws.cloudtrail.user_identity.invoked_by: (
              "config.amazonaws.com" or
              "securityhub.amazonaws.com" or
              "sso.amazonaws.com"
            )

Framework: MITRE ATT&CKTM

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