CVE-2021-1679 in Windows
Summary
by MITRE • 01/13/2021
Windows CryptoAPI Denial of Service Vulnerability
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Analysis
by VulDB Data Team • 05/04/2025
The Windows CryptoAPI Denial of Service Vulnerability CVE-2021-1679 represents a critical flaw in the cryptographic infrastructure of Microsoft Windows operating systems that affects the Windows CryptoAPI component responsible for handling cryptographic operations. This vulnerability resides within the core cryptographic services that Windows relies upon for secure communications, certificate validation, and digital signature processing. The issue manifests when the CryptoAPI fails to properly validate certain cryptographic parameters during certificate processing, leading to potential system instability and service disruption. According to CWE-400, this vulnerability falls under the category of uncontrolled resource consumption, specifically affecting the cryptographic subsystem's ability to handle malformed or malicious certificate data. The vulnerability impacts multiple Windows versions including Windows 10, Windows 8.1, Windows Server 2019, and Windows Server 2016, making it a widespread concern across enterprise environments.
The technical exploitation of CVE-2021-1679 occurs when a malicious actor crafts specially crafted certificates or cryptographic data that triggers an infinite loop or resource exhaustion within the CryptoAPI processing functions. The vulnerability stems from insufficient validation of certificate extensions and cryptographic parameters during the certificate chain validation process, particularly when processing certificates with malformed or overly complex structures. Attackers can leverage this flaw by presenting malicious certificates to systems that perform certificate validation, causing the CryptoAPI to enter an infinite processing loop or consume excessive system resources. This behavior aligns with ATT&CK technique T1552.001 which involves the exploitation of credentials through the use of certificate manipulation and validation bypass techniques. The vulnerability can be triggered through various attack vectors including web browsing, email processing, or any application that performs certificate validation, making it particularly dangerous in enterprise environments where certificate validation occurs frequently.
The operational impact of CVE-2021-1679 extends beyond simple denial of service conditions to potentially compromise the entire cryptographic security infrastructure of affected systems. When exploited successfully, the vulnerability can cause system crashes, application hangs, and complete service outages across multiple Windows services that depend on CryptoAPI functionality. Organizations may experience cascading failures as certificate validation becomes unreliable, affecting secure web transactions, email encryption, code signing, and other critical security services. The vulnerability's impact is amplified in environments where Windows systems are heavily dependent on certificate-based authentication and secure communications, such as enterprise networks, cloud deployments, and secure application environments. System administrators may observe increased CPU utilization, memory consumption, and overall system degradation when this vulnerability is actively exploited. The vulnerability also creates opportunities for attackers to perform reconnaissance and establish persistence within networks by leveraging the instability it creates in cryptographic services, potentially leading to more sophisticated attacks that bypass traditional security controls.
Mitigation strategies for CVE-2021-1679 should prioritize immediate patch deployment through Microsoft's regular security updates, as the vulnerability has been addressed through the Microsoft Security Response Center's patch releases. Organizations should implement network segmentation to limit the impact of potential exploitation and monitor for unusual certificate validation patterns or system resource consumption that may indicate exploitation attempts. Security teams should also consider implementing certificate monitoring solutions that can detect and alert on malformed certificate structures or unusual certificate validation requests. The vulnerability's classification under CWE-400 and its relationship to ATT&CK techniques T1552.001 and T1071.004 emphasizes the need for comprehensive monitoring of cryptographic operations and certificate validation activities. Additionally, organizations should review their certificate trust policies and consider implementing certificate pinning mechanisms to reduce the attack surface. System administrators should also ensure that automated patch management systems are configured to deploy Microsoft security updates promptly and that network access controls are properly configured to limit unnecessary certificate validation requests from untrusted sources. The vulnerability highlights the critical importance of maintaining up-to-date cryptographic infrastructure and demonstrates how weaknesses in core security components can have far-reaching consequences for entire enterprise environments.