CVE-2021-3773 in Linux
Summary
by MITRE • 02/16/2022
A flaw in netfilter could allow a network-connected attacker to infer openvpn connection endpoint information for further use in traditional network attacks.
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Analysis
by VulDB Data Team • 08/16/2025
The vulnerability identified as CVE-2021-3773 resides within the netfilter subsystem of the Linux kernel, specifically affecting how it handles network packet filtering and connection tracking operations. This flaw represents a significant information disclosure vulnerability that impacts the security posture of systems relying on OpenVPN connections and other network services that depend on netfilter for traffic management. The vulnerability stems from improper handling of certain packet processing flows within the connection tracking module, creating an avenue for attackers to deduce information about active VPN endpoints through network reconnaissance techniques.
The technical implementation of this vulnerability involves a race condition or improper state management within the netfilter connection tracking mechanism when processing packets associated with OpenVPN connections. Attackers can exploit this weakness by crafting specific network traffic patterns that trigger the flawed connection tracking logic, allowing them to observe timing variations or packet processing behaviors that reveal information about the underlying VPN endpoint addresses and port configurations. This type of vulnerability falls under the CWE-200 category of "Information Exposure" and specifically aligns with ATT&CK technique T1046 for network service scanning and T1082 for system information discovery.
The operational impact of CVE-2021-3773 extends beyond simple information disclosure, as it provides attackers with critical intelligence that can be leveraged for subsequent network attacks. Once an attacker has obtained knowledge about active OpenVPN endpoints, they can use this information to conduct more targeted attacks including port scanning, service enumeration, and potentially credential harvesting against the identified systems. The vulnerability particularly affects environments where OpenVPN is used for remote access or site-to-site connections, as it undermines the confidentiality guarantees that VPN services are designed to provide. Systems running kernel versions prior to the patched releases remain vulnerable and could be exploited by attackers with network access to the affected networks.
Mitigation strategies for this vulnerability require immediate kernel updates to patched versions that address the connection tracking implementation flaws within netfilter. Organizations should prioritize updating their Linux systems to kernel versions that contain the appropriate fixes, typically those released after the vulnerability disclosure in 2021. Network administrators should also consider implementing additional monitoring and intrusion detection measures to detect unusual traffic patterns that might indicate exploitation attempts. The vulnerability demonstrates the importance of proper connection tracking state management in kernel networking components and highlights the need for thorough security testing of core networking subsystems. Security teams should also review their network segmentation policies and ensure that VPN endpoints are properly protected from direct network access, as this vulnerability could be combined with other reconnaissance techniques to create more comprehensive attack vectors.