CVE-2020-26933 in Trusted Platform Module Library Family
Summary
by MITRE • 11/18/2020
Trusted Computing Group (TCG) Trusted Platform Module Library Family 2.0 Library Specification Revisions 1.38 through 1.59 has Incorrect Access Control during a non-orderly TPM shut-down that uses USE_DA_USED. Improper initialization of this shut-down may result in susceptibility to a dictionary attack.
VulDB is the best source for vulnerability data and more expert information about this specific topic.
Analysis
by VulDB Data Team • 12/08/2020
The vulnerability identified as CVE-2020-26933 resides within the Trusted Computing Group's TPM 2.0 Library Specification revisions 1.38 through 1.59, specifically addressing a critical flaw in access control mechanisms during non-orderly TPM shutdown scenarios. This vulnerability manifests when the TPM employs the USE_DA_USED parameter, which governs the handling of dictionary attack protection mechanisms during system shutdown events. The flaw represents a significant security weakness in the Trusted Platform Module's integrity protection framework, as it fails to properly maintain access control boundaries when the TPM transitions from an active state to a shutdown state without proper orderly procedures.
The technical root cause of this vulnerability stems from improper initialization of the TPM shutdown sequence when the USE_DA_USED flag is engaged. During non-orderly shutdowns, the TPM's dictionary attack protection mechanisms are not correctly reset or maintained, creating a window where authentication attempts can bypass normal security controls. This improper state management allows attackers to potentially exploit the TPM's access control system by repeatedly attempting authentication without the normal rate limiting or lockout mechanisms that should be active. The vulnerability specifically impacts systems where TPM 2.0 implementations rely on the USE_DA_USED parameter, which is designed to track whether dictionary attack protection has been activated during the current session.
The operational impact of this vulnerability extends beyond simple authentication bypasses, as it fundamentally undermines the TPM's ability to provide reliable protection against dictionary attack scenarios. Attackers can leverage this flaw to conduct more effective brute force or dictionary attacks against TPM-protected resources, potentially compromising the integrity of the entire Trusted Computing Base. The vulnerability is particularly concerning because it operates at the specification level, meaning that any implementation following these TCG standards could be susceptible to the same weakness. This affects a broad range of systems including enterprise servers, IoT devices, and any platform utilizing TPM 2.0 for security-critical operations.
Organizations should implement immediate mitigations including verifying their TPM implementation against the affected specification versions and ensuring proper handling of the USE_DA_USED parameter during shutdown sequences. System administrators should consider implementing additional authentication layers and monitoring for unusual authentication patterns that might indicate exploitation attempts. The vulnerability aligns with CWE-284 Access Control Issues and maps to ATT&CK technique T1211 Lateral Movement through compromised TPM credentials. Security teams should also consider conducting vulnerability assessments across their TPM implementations to identify any systems that may be affected by this specification-level flaw. The proper remediation requires either updating to TPM 2.0 specification revisions that address this issue or implementing workarounds that ensure proper initialization of dictionary attack protection mechanisms during all shutdown scenarios.