Attack Chain

1. An attacker gains initial access to an AWS account, potentially through compromised credentials or an exposed API key.
2. The attacker attempts to upload an SSH public key to an EC2 instance using the SendSSHPublicKey or SendSerialConsoleSSHPublicKey API calls.
3. The ec2-instance-connect.amazonaws.com service logs the SendSSHPublicKey or SendSerialConsoleSSHPublicKey API action in CloudTrail if successful.
4. The attacker leverages the uploaded SSH key to gain SSH access to the targeted EC2 instance.
5. Once inside the instance, the attacker performs reconnaissance, gathering information about the system and network configuration.
6. The attacker moves laterally to other EC2 instances or AWS services using the compromised instance as a pivot point.
7. If SendSerialConsoleSSHPublicKey was used, the attacker attempts privilege escalation via serial console access, potentially gaining root access.
8. The attacker achieves persistence by maintaining SSH access via the uploaded key, allowing continued access to the AWS environment.

Impact

Successful exploitation can lead to unauthorized access to sensitive data, lateral movement within the AWS environment, and privilege escalation. This can result in data breaches, service disruption, and significant financial loss. The impact is amplified if the targeted EC2 instance hosts critical applications or data. The number of potential victims depends on the scope of the attacker's access and the sensitivity of the data stored within the compromised AWS environment.

Recommendation

• Review CloudTrail logs for events matching the SendSSHPublicKey or SendSerialConsoleSSHPublicKey API actions to identify potential unauthorized SSH key uploads to EC2 instances (log source: aws.cloudtrail).
• Investigate the source IP addresses (source.ip) and user identities (aws.cloudtrail.user_identity.arn) associated with these events to determine the legitimacy of the actions.
• Deploy the provided Sigma rule "AWS EC2 Instance Connect SSH Public Key Uploaded" to your SIEM and tune for your environment to detect potentially malicious SSH key uploads (rule: AWS EC2 Instance Connect SSH Public Key Uploaded).
• Audit EC2 instance policies and permissions to ensure adherence to the principle of least privilege and restrict unauthorized SSH key uploads.
• Monitor for the ec2:EnableSerialConsoleAccess permission usage in conjunction with SendSerialConsoleSSHPublicKey to identify potential privilege escalation attempts (log source: aws.cloudtrail).