An authenticated remote user can exploit a Server-Side Request Forgery vulnerability in the Amazon SSM Agent to potentially capture sensitive IAM role credentials and escalate privileges.

The vulnerability exists in the port forwarding functionality of the SSM Agent, where improper validation of destination addresses allows an authenticated user to bypass denylists. This enables the agent to reach link-local services, such as the instance metadata service, to exfiltrate temporary IAM credentials.

The exploitation of this flaw poses a critical risk to cloud infrastructure security. By obtaining temporary IAM credentials, an attacker could move laterally through the AWS environment, access sensitive data, or perform unauthorized administrative actions, justifying its 9.9 CVSS score. This represents a significant threat to the confidentiality and integrity of the affected managed instances.

Immediate Action: Upgrade the Amazon SSM Agent to version 3.3.4851.0 or later across all managed instances immediately.

Proactive Monitoring: Review VPC flow logs and SSM Agent access logs for anomalous requests directed toward link-local addresses (e.g., 169.254.169.254) originating from non-standard processes.

Compensating Controls: Implement strict IAM policies that follow the principle of least privilege, ensuring that even if credentials are compromised, the blast radius is minimized.

Public Exploit Available: Unknown

Given the critical severity of this vulnerability and its direct impact on IAM credential security, organizations should prioritize the deployment of the 3.3.4851.0 update. Failure to remediate this issue leaves managed instances vulnerable to significant privilege escalation and unauthorized access by any user with authenticated access to the agent.