AWS SNS Topic Message Publish by Rare User

This detection rule identifies when a user or role publishes a message to an Amazon Simple Notification Service (SNS) topic for the first time. Attackers may abuse SNS topics for various malicious purposes, including internal spearphishing, data exfiltration, or lateral movement within the AWS environment. SNS topics are used to send notifications and messages to subscribed endpoints such as applications, mobile devices, or email addresses. A successful attack could lead to sensitive data being exposed or malicious content being distributed to a wide range of recipients. This rule leverages a "new terms" approach, focusing on identifying previously unseen behavior to surface potentially suspicious activity. It helps security teams detect unusual SNS activity and investigate potential security breaches in AWS environments.

Attack Chain

1. An attacker gains initial access to an AWS environment, potentially through compromised credentials or exploiting a misconfigured IAM role.
2. The attacker discovers accessible SNS topics within the AWS environment using AWS CLI or SDKs.
3. The attacker identifies an SNS topic with a broad subscription base, such as one connected to an internal communications channel.
4. The attacker publishes a malicious message to the SNS topic using the AWS CLI, SDK, or AWS Management Console, utilizing a rare or new user account.
5. The SNS service delivers the message to all subscribed endpoints, including applications, mobile devices, or email addresses.
6. Recipients of the malicious message may be tricked into clicking on a phishing link or downloading malicious content, leading to further compromise of their systems.
7. The attacker may leverage the compromised systems for lateral movement, data exfiltration, or other malicious activities.
8. The attacker achieves impact by spreading malware, stealing sensitive information, or disrupting business operations through the compromised systems and data.

Impact

A successful attack exploiting SNS topics can lead to several negative consequences. Internal spearphishing attacks could compromise employees and sensitive data. Data exfiltration through SNS messages may lead to regulatory violations and reputational damage. Resource hijacking could allow attackers to leverage AWS resources for malicious purposes, increasing costs and potentially disrupting services. Organizations in any sector that rely on AWS for their infrastructure are vulnerable, and the number of potential victims depends on the scope and reach of the SNS topic subscriptions.

Recommendation

• Enable AWS CloudTrail logging for SNS data events to ensure the necessary logs are captured (reference: setup section).
• Deploy the provided Sigma rule to your SIEM to detect rare users publishing SNS messages (reference: rules section). Tune the history_window_start parameter in the rule to match your environment's baseline.
• Investigate any alerts generated by the Sigma rule, focusing on the aws.cloudtrail.user_identity.arn and aws.cloudtrail.resources.arn to identify the actor and SNS topic involved (reference: rule.investigation_fields).
• Review IAM policies associated with users and roles publishing to SNS topics to ensure appropriate permissions are enforced and minimize potential misuse (reference: T1534, T1496).
• Monitor for unusual API calls related to SNS, such as AssumeRole or CreateAccessKey, associated with the rare user (reference: T1102).