CVE-2016-4911 in Identity
Summary
by MITRE
The Fernet Token Provider in OpenStack Identity (Keystone) 9.0.x before 9.0.1 (mitaka) allows remote authenticated users to prevent revocation of a chain of tokens and bypass intended access restrictions by rescoping a token.
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Analysis
by VulDB Data Team • 01/20/2019
The CVE-2016-4911 vulnerability resides within the Fernet Token Provider implementation of OpenStack Identity service known as Keystone version 9.0.x prior to 9.0.1 in the mitaka release cycle. This security flaw represents a critical authorization bypass issue that fundamentally undermines the token revocation mechanism designed to maintain secure access control within cloud environments. The vulnerability specifically affects the token scoping process and demonstrates how improper token handling can lead to persistent unauthorized access even after intended revocations have occurred.
The technical flaw manifests in the token rescoping mechanism where authenticated users can manipulate token chains to prevent proper revocation of subsequent tokens in the chain. When a user rescopes a token, the system fails to properly invalidate the entire chain of derived tokens that were created from the original token, creating a persistent access vector that bypasses the intended access control restrictions. This occurs because the token provider does not maintain proper tracking of token dependencies and their revocation status when tokens are rescoped, allowing attackers to leverage the original token's validity to maintain access to resources that should have been revoked.
The operational impact of this vulnerability extends far beyond simple access bypass, as it enables attackers to maintain unauthorized access to cloud resources for extended periods without detection. An authenticated attacker can exploit this flaw to create a persistent access channel that remains functional even after token revocation policies have been applied to the system. This vulnerability particularly affects cloud environments where token-based authentication is heavily relied upon, as it undermines the fundamental security principle that revoked tokens should not be usable for accessing protected resources. The flaw also impacts the integrity of audit trails and access logging since the system cannot properly track when unauthorized access occurred through token chain manipulation.
The vulnerability aligns with CWE-284, which addresses improper access control, and demonstrates how token management flaws can lead to privilege escalation and persistent access violations. From an ATT&CK framework perspective, this vulnerability maps to T1550.001 (Use of stolen credentials) and T1078.004 (Valid accounts) as attackers can maintain access using compromised tokens without detection. Organizations should implement immediate mitigations including upgrading to Keystone version 9.0.1 or later, implementing additional token monitoring mechanisms, and establishing more robust token lifecycle management policies. System administrators should also consider implementing token revocation auditing and regular token chain validation processes to detect potential exploitation attempts. The vulnerability underscores the importance of proper token dependency tracking and the need for comprehensive access control validation in distributed cloud environments where token-based authentication is prevalent.