MinIO is Vulnerable to Privilege Escalation via Session Policy Bypass in Service Accounts and STS

Summary

A privilege escalation vulnerability allows service accounts and STS (Security Token Service) accounts with restricted session policies to bypass their inline policy restrictions when performing "own" account operations, specifically when creating new service accounts for the same user.

Details

The vulnerability exists in the IAM policy validation logic in cmd/iam.go. When validating session policies for restricted accounts performing operations on their own account (such as creating service accounts), the code incorrectly relied on the DenyOnly argument.

The DenyOnly flag is used to allow accounts to perform actions related to their own account by only checking if the action is explicitly denied. However, when a session policy (sub-policy) is present, the system should validate that the action is actually allowed by the session policy, not just that it isn't denied.

Attack Scenario

1. An administrator creates a service account or STS account with a restricted inline policy (e.g., access only to bucket1 and bucket2)
2. The restricted account attempts to create a new service account for itself without specifying any policy restrictions
3. Due to the bypass, the new service account is created with full parent privileges instead of being restricted by the inline policy
4. The attacker now has escalated privileges beyond the intended restrictions

Impact

Attack Complexity: LOW - Exploitation requires only valid credentials for a restricted service/STS account

Confidentiality: HIGH - Attackers can access buckets and objects beyond their intended restrictions