Improper neutrality of user-supplied input in the FSx Windows File Server volume mounting component of Amazon ECS Agent allows a command injection flaw, identified as CWE-78. An attacker who can create or modify an ECS task definition, or write to the Secrets Manager or SSM Parameter Store used by the FSx configuration, can supply a specially crafted username in the task definition. The agent then executes the content as part of an operating‑system command with SYSTEM privileges on the Windows host, effectively giving the attacker full control of the underlying EC2 instance.

All Windows installations of the Amazon ECS Agent older than version 1.103.0 are vulnerable. The issue is limited to the ECS Agent on Windows; other operating systems are unaffected.

The CVSS score of 7.5 indicates a high severity level. EPSS data is not available, and the vulnerability is not listed in the CISA KEV catalog. The exploit requires the attacker to be authenticated to AWS with permissions to register ECS task definitions or to write secrets used by the FSx volume. Based on the description, the likely attack vector is a privileged, authenticated, remote attacker who can provision ECS tasks or modify secrets.