CVE-2017-7521 in OpenVPN
Summary
by MITRE
OpenVPN versions before 2.4.3 and before 2.3.17 are vulnerable to remote denial-of-service due to memory exhaustion caused by memory leaks and double-free issue in extract_x509_extension().
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Analysis
by VulDB Data Team • 12/29/2020
OpenVPN is a widely deployed open-source virtual private network solution that facilitates secure remote access to networks through encrypted tunnels. The vulnerability identified as CVE-2017-7521 affects versions prior to 2.4.3 and 2.3.17, presenting a critical remote denial-of-service risk that can be exploited by attackers to exhaust system memory resources. This vulnerability specifically resides within the extract_x509_extension() function which processes X.509 certificate extensions during the SSL/TLS handshake process. The flaw manifests through improper memory management practices that create both memory leaks and double-free conditions, allowing an attacker to craft malicious X.509 certificates that trigger these memory handling errors when processed by the vulnerable OpenVPN server or client components.
The technical implementation of this vulnerability stems from insufficient input validation and memory deallocation logic within the certificate processing pipeline. When OpenVPN encounters X.509 extensions in certificates, the extract_x509_extension() function attempts to parse and extract specific extension data while maintaining memory allocations for various processing structures. However, the function fails to properly manage memory references, leading to scenarios where allocated memory blocks are freed multiple times or where memory allocations are not properly released upon function exit. This memory management failure creates a predictable pattern of resource exhaustion that can be systematically exploited by sending specially crafted certificates to the target system. The vulnerability is particularly dangerous because it operates at the SSL/TLS layer where certificate validation occurs, making it possible for an attacker to trigger the condition without requiring authentication or prior access to the system.
From an operational perspective, this vulnerability presents significant risk to organizations relying on OpenVPN for remote access and site-to-site connectivity. Attackers can exploit this weakness by establishing connections to OpenVPN servers using maliciously constructed certificates, causing the service to consume increasing amounts of memory until system resources are exhausted and the service becomes unavailable. The impact extends beyond simple service disruption as memory exhaustion can potentially lead to system crashes, affecting not only the OpenVPN service but also underlying operating system stability and other applications running on the same host. Network administrators may observe gradual performance degradation before complete service outages, making this vulnerability particularly challenging to detect and mitigate in production environments where multiple concurrent connections are common.
The vulnerability aligns with CWE-415: Double Free and CWE-416: Use After Free categories from the Common Weakness Enumeration catalog, representing classic memory safety issues that have plagued network security applications for decades. From the MITRE ATT&CK framework perspective, this vulnerability maps to T1499.004: Endpoint Denial of Service, specifically targeting the availability of network services through resource exhaustion techniques. Organizations should immediately implement patches to upgrade to OpenVPN versions 2.4.3 or 2.3.17 and later, as these releases contain proper memory management fixes for the extract_x509_extension() function. Additional mitigations include implementing certificate validation policies that limit the complexity of X.509 extensions accepted, deploying intrusion detection systems to monitor for unusual certificate processing patterns, and establishing network segmentation to limit the attack surface of vulnerable OpenVPN installations. Security monitoring should focus on tracking memory usage patterns and connection failures that may indicate exploitation attempts, while regular vulnerability assessments should verify that all OpenVPN components have been properly updated to address this memory exhaustion threat.