In plain language
What this advisory means
A security flaw in AWS Systems Manager Agent's port forwarding feature lets an authorized user bypass security blocks and connect to restricted local network addresses. This could let them steal temporary credentials from the managed server and act with its permissions from outside the server.
Technical explanation
How the issue affects the environment
CVE-2026-89049 is a server-side request forgery (SSRF) vulnerability in the Session Manager's remote-host port forwarding functionality of AWS Systems Manager Agent (SSM Agent). The issue arises from improper validation of equivalent IP address representations, allowing a user with port-forwarding permissions to bypass a denylist intended to block access to link-local network endpoints. Successfully exploiting this flaw can enable an authenticated user to connect to link-local services on the managed instance, potentially retrieving temporary IAM role credentials assigned to that instance and using them externally with those permissions.
Operational impact
Why teams should care
An attacker with valid port-forwarding permissions could misuse this vulnerability to access sensitive local network resources and obtain temporary credentials from an AWS managed instance. This unauthorized access could allow the attacker to perform actions with the permissions of the instance's IAM role, leading to potential data breaches, privilege escalation, and unauthorized resource manipulation.
Immediate action
Upgrade the AWS Systems Manager Agent to version 3.3.4851.0 or later. Additionally, patch any forked or derivative versions of the agent to include the fix. Until upgraded, restrict IAM permissions for the AWS-StartPortForwardingSessionToRemoteHost document, limiting which principals can start port-forwarding sessions to remote hosts.
Affected and fixed releases
Temporary risk reduction
Until the agent is updated, limit use of the AWS-StartPortForwardingSessionToRemoteHost document by carefully scoping IAM permissions (ssm:StartSession) and SSM document permissions to prevent untrusted users from initiating remote-host port forwarding sessions.
Evidence and validation checklist
- Official AWS Security Bulletin describes CVE-2026-89049 as an SSRF in port forwarding functionality.
- Improper validation of equivalent IP address representations allows bypass of denylist.
- Authenticated users with port-forwarding permission can access link-local endpoints.
- Potential to obtain managed instance's temporary IAM role credentials and act externally with those permissions.
- Resolved in SSM Agent version 3.3.4851.0.
- Workaround involves restricting IAM permissions for port forwarding sessions.
Authoritative reference
AWS Security Bulletins
QCS detected and normalized this record from the official source. Vendor guidance remains authoritative.