Description

OS command injection in the example SSM document provisioning script in Trusted Remote Execution (REX) before 2026-07-28 might allow an authenticated remote user who is permitted to invoke the Rex-RO or Rex-RW SSM document to execute arbitrary commands as root on the managed instance, bypassing Cedar policy enforcement. To remediate this issue, users should delete any Rex-RO and Rex-RW SSM documents created from the example script, in each account and Region where it was run.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "9.9",
  "pubDate": "2026-11-06T17:17:20.313Z",
  "pubdate": "2026-11-06T17:17:20.313Z",
  "executiveSummary": "A critical OS command injection vulnerability exists within the example SSM document provisioning script used in Trusted Remote Execution (REX). This vulnerability affects deployments occurring prior to 2026-07-28.\nThe flaw permits an authenticated remote user, possessing permissions to invoke the Rex-RO or Rex-RW SSM documents, to execute arbitrary commands with root privileges on the target managed instance.\nBy injecting malicious input into the provisioning script, an attacker can bypass Cedar policy enforcement, effectively overriding security controls meant to restrict document execution.\nThe vulnerability poses a severe risk to system integrity and confidentiality, as it grants unauthorized elevated access. Exploitation is contingent upon the attacker already having the necessary permissions to invoke the specified SSM documents.",
  "technicalDetails": "The vulnerability originates from improper neutralization of special elements used in an OS command within the REX provisioning script. The script fails to adequately sanitize inputs before passing them to the underlying shell environment during the SSM document execution process.\nBecause the script executes with elevated privileges on the managed instance, specifically as the root user, the command injection vulnerability allows an attacker to inherit these privileges.\nThe attack flow commences when an authenticated user, who is authorized to invoke either the Rex-RO or Rex-RW SSM documents, crafts a malicious payload that includes OS command separators or subshell syntax. When the vulnerable SSM document processes this input, the payload is parsed and executed by the system shell.\nThis execution successfully bypasses Cedar policy enforcement because the injection happens at the script execution layer, which occurs after the initial authorization check but before the command logic is processed by the managed instance.\nBy escaping the intended execution context, an attacker can execute arbitrary binaries, modify system configurations, exfiltrate sensitive data, or install persistent backdoors on the managed instance. The impact is a total compromise of the affected instance's root security domain.\nThe scope of this vulnerability includes all Rex-RO and Rex-RW SSM documents generated via the provided example script prior to the 2026-07-28 threshold. The vulnerability is restricted to environments where the aforementioned scripts were utilized to provision SSM documents."
}