CVE-2021-37909 in TSSServiSign
Summary
by MITRE • 09/16/2021
WriteRegistry function in TSSServiSign component does not filter and verify users’ input, remote attackers can rewrite to the registry without permissions thus perform hijack attacks to execute arbitrary code.
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Analysis
by VulDB Data Team • 09/19/2021
The vulnerability identified as CVE-2021-37909 resides within the TSSServiSign component of a software system, specifically targeting the WriteRegistry function that lacks proper input validation and verification mechanisms. This critical flaw enables remote attackers to manipulate registry entries without requiring appropriate authorization, fundamentally compromising system integrity and security posture. The vulnerability operates at the system level where registry modifications can serve as attack vectors for privilege escalation and persistent malware installation. According to CWE classification, this represents a weakness in input validation and verification processes that directly enables unauthorized system modifications, falling under CWE-20 for improper input validation. The attack surface is particularly concerning as registry manipulation can affect critical system operations and security controls.
The technical implementation of this vulnerability stems from insufficient sanitization of user inputs passed to the WriteRegistry function within the TSSServiSign component. When legitimate users or attackers provide malformed or malicious registry path specifications, the function fails to validate these inputs against acceptable parameters or security boundaries. This lack of input filtering creates an opportunity for attackers to specify arbitrary registry locations that should normally require elevated privileges or specific authorization. The vulnerability essentially bypasses access control mechanisms that should prevent unauthorized registry modifications, allowing attackers to write to protected registry keys or values that control system behavior. This flaw directly enables registry-based attack techniques that align with ATT&CK tactics such as T1112 for registry run keys and T1059 for command and scripting interpreter usage.
The operational impact of CVE-2021-37909 extends beyond simple unauthorized registry modifications to encompass potential privilege escalation and persistent threat execution. Attackers can leverage this vulnerability to install malicious code that executes automatically upon system boot or user login by writing to registry run keys or service configurations. The registry hijacking capabilities provide attackers with a stealthy method to maintain persistence while avoiding detection by traditional security controls that may not monitor registry modifications. Additionally, the vulnerability can facilitate lateral movement within networks by allowing attackers to modify system configurations that control network access or authentication mechanisms. This represents a significant concern for enterprise environments where registry-based security controls are commonly used to enforce system policies and maintain operational integrity.
Mitigation strategies for CVE-2021-37909 should prioritize immediate software updates from vendors that address the input validation deficiencies in the TSSServiSign component. Organizations must implement comprehensive registry monitoring solutions that can detect unauthorized modifications to critical registry keys and values, particularly those related to system startup, service configurations, and security policies. Network segmentation and access control measures should be strengthened to limit the attack surface and prevent unauthorized access to systems hosting vulnerable components. Security teams should conduct thorough registry audits to identify any existing malicious modifications that may have occurred due to this vulnerability. Additionally, implementing least privilege principles for registry access and establishing robust input validation controls within the affected component will help prevent similar issues from occurring in future deployments. The vulnerability demonstrates the critical importance of proper input validation and access control implementation in system components that handle sensitive operations such as registry modifications.