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العربيةHijack instance role via SSM session
Can open an SSM shell on an instance and steal its instance-profile role.
Grants escalation only in combination. All of these are needed together:
ssm:StartSessionec2:DescribeInstancesssm:StartSession can open an SSM shell on an instance and steal its instance-profile role, but only when it is held together with ec2:DescribeInstances. Held together they reach administrator access.
Why it is dangerous
A principal can call ssm:StartSession to open an interactive Session Manager shell on an SSM-managed instance that already carries the instance-profile role, then read the instance's live credentials from the instance metadata service (IMDS). No SSH key, no open security-group port, and no write to the instance, because the instance is already bound to the box a shell on it is enough to steal the role. If the instance is privileged this is full privilege escalation against an instance that was already trusted.
In MITRE ATT&CK terms this maps to T1578, Modify Cloud Compute Infrastructure, under the privilege escalation tactic. AWS records it in CloudTrail as StartSession, DescribeInstances. The call shows up in CloudTrail but blends into normal administration, so it needs a rule to stand out.
Detection
StartSessionDescribeInstancesPrerequisites
An existing SSM-managed instance carrying an instance profile; ssm:StartSession permission. ec2:DescribeInstances enumerates targets and is checked against a wildcard. The SSM agent must be registered on the instance.
MITRE ATT&CK
T1578 · Modify Cloud Compute InfrastructureTactic
Privilege Escalation
A control that would remove it: Restrict ssm:StartSession to break-glass principals; reduce the instance-profile role's permissions.
Whether to apply it depends on how the permission is used in your environment, so it is a business decision rather than an automatic fix. The same permission can be load-bearing in one account and pure risk in another.
Pathbreak confirms whether this path resolves in your account, ranks it against the others by how much it opens up, and uses your threat model and business context to show which paths are worth cutting.
Questions
What does ssm:StartSession allow?
Can open an SSM shell on an instance and steal its instance-profile role.
What else is needed for ssm:StartSession to be dangerous?
It becomes an escalation only when held together with ec2:DescribeInstances. On its own it is not enough, which is what makes the combination easy to miss.
How is hijack instance role via SSM session detected?
Watch for StartSession, DescribeInstances in CloudTrail. The call shows up in CloudTrail but blends into normal administration, so it needs a rule to stand out.
How do you break this path?
A control that would remove it is: Restrict ssm:StartSession to break-glass principals; reduce the instance-profile role's permissions. Whether to apply it depends on how the permission is used in your account, so it is a business decision rather than an automatic fix.
Which MITRE ATT&CK technique is this?
T1578, Modify Cloud Compute Infrastructure, under the privilege escalation tactic.
Related paths
References
Find this in your own account
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