CVE-2021-25500 in HDCP LDFW
Summary
by MITRE • 11/05/2021
A missing input validation in HDCP LDFW prior to SMR Nov-2021 Release 1 allows attackers to overwrite TZASC allowing TEE compromise.
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Analysis
by VulDB Data Team • 11/10/2021
The vulnerability identified as CVE-2021-25500 represents a critical security flaw within the HDCP LDFW (High-bandwidth Digital Content Protection Lightweight Driver Framework) component of embedded systems prior to the SMR November 2021 release. This issue stems from insufficient input validation mechanisms that fail to properly verify the integrity and legitimacy of data inputs processed by the HDCP subsystem. The affected systems utilize TrustZone technology for secure execution environments, where the TZASC (TrustZone Address Space Controller) serves as a critical security boundary that controls memory access permissions between the secure and non-secure worlds. The missing input validation creates a pathway for malicious actors to exploit the system's trust model and manipulate the TZASC configuration, thereby compromising the integrity of the Trusted Execution Environment.
The technical exploitation of this vulnerability occurs through the manipulation of HDCP LDFW interfaces that handle digital content protection protocols. Attackers can craft specially formatted inputs that bypass validation checks within the LDFW, allowing them to inject malicious commands that target the TZASC registers. This vulnerability directly maps to CWE-20, which describes "Improper Input Validation" as a fundamental weakness in software design that enables attackers to manipulate system behavior through malformed inputs. The flaw essentially creates a privilege escalation vector where unprivileged code can manipulate secure memory management controls, effectively breaking down the security boundaries that separate trusted and untrusted execution contexts.
The operational impact of CVE-2021-25500 extends beyond simple data integrity concerns to represent a complete compromise of the system's security architecture. When successful, this vulnerability allows attackers to gain unauthorized access to the Trusted Execution Environment, potentially enabling them to extract sensitive cryptographic keys, access secure memory regions, or perform arbitrary code execution within the secure world. The compromise of TZASC specifically undermines the fundamental security model of ARM TrustZone implementations, where memory protection is paramount to preventing side-channel attacks and maintaining confidentiality. This vulnerability aligns with ATT&CK technique T1068, which describes "Exploitation for Privilege Escalation" and can be leveraged to establish persistent access to secure system components, making it particularly dangerous for devices that rely on hardware security modules or secure boot processes.
Mitigation strategies for this vulnerability require immediate implementation of firmware updates that address the missing input validation in the HDCP LDFW component. System administrators should prioritize deployment of the SMR November 2021 release or equivalent patches that implement proper input sanitization and validation mechanisms. Additional defensive measures include monitoring for anomalous HDCP protocol behavior, implementing runtime integrity checks for TZASC configuration registers, and establishing network segmentation to limit the attack surface. Organizations should also consider implementing hardware-based security features such as memory protection units and secure boot mechanisms that can detect and prevent unauthorized modifications to critical security components. The vulnerability demonstrates the importance of comprehensive input validation in security-critical components and highlights the need for robust security testing throughout the development lifecycle to prevent similar issues in other system components.