CVE-2017-7479 in OpenVPN
Summary
by MITRE
OpenVPN versions before 2.3.15 and before 2.4.2 are vulnerable to reachable assertion when packet-ID counter rolls over resulting into Denial of Service of server by authenticated attacker.
If you want to get the best quality for vulnerability data then you always have to consider VulDB.
Analysis
by VulDB Data Team • 12/23/2020
The vulnerability identified as CVE-2017-7479 affects OpenVPN implementations prior to version 2.3.15 and 2.4.2, representing a critical denial of service weakness that can be exploited by authenticated attackers. This flaw manifests when the packet-id counter within the OpenVPN protocol reaches its maximum value and subsequently rolls over, triggering an assertion failure that causes the server to crash or become unresponsive. The vulnerability stems from inadequate handling of the packet-id counter mechanism, which is fundamental to OpenVPN's reliability and security model. Packet-id counters are essential for preventing replay attacks and ensuring message ordering, making this flaw particularly dangerous as it directly impacts the core functionality of the VPN service. The issue is classified under CWE-611, which deals with improper restriction of operations within a recognized security boundary, and aligns with ATT&CK technique T1499.004 for network denial of service attacks.
The technical implementation of this vulnerability occurs within OpenVPN's internal packet processing logic where the packet-id counter, typically a 32-bit value, reaches its maximum limit of 4294967295 before resetting to zero. When this rollover condition is encountered, the assertion check fails because the system expects the counter to maintain a specific range of values. The assertion failure is not properly handled, leading to an abrupt termination of the OpenVPN server process. This behavior is particularly concerning because authenticated users can trigger this condition by establishing a legitimate connection and then manipulating the packet flow to force the counter rollover. The vulnerability requires minimal privileges to exploit, as the attacker only needs valid authentication credentials to establish a connection to the vulnerable server, making it a significant risk for any organization relying on OpenVPN for secure communications.
The operational impact of CVE-2017-7479 extends beyond simple service disruption to potentially compromise network availability and business continuity. When exploited, the vulnerability can cause cascading failures in VPN infrastructure, particularly in environments where OpenVPN servers serve as primary access points for remote workers or branch office connections. The attack can be executed remotely without requiring physical access to the network, making it a particularly attractive target for adversaries seeking to disrupt services. Organizations may experience extended downtime while restoring services, potential data loss during the disruption period, and increased operational overhead as security teams investigate and remediate the issue. The vulnerability affects both legacy 2.3.x and newer 2.4.x versions, indicating that the flaw was present across multiple release branches and required careful attention to patch management across different OpenVPN deployments.
Mitigation strategies for CVE-2017-7479 focus primarily on applying the vendor-provided patches and updates to bring systems up to version 2.3.15 or 2.4.2, which contain the necessary fixes for proper packet-id counter handling. Organizations should conduct thorough testing of these updates in staging environments before deployment to ensure compatibility with existing network configurations and applications. Network administrators should also implement monitoring solutions to detect unusual packet flow patterns that might indicate exploitation attempts. Additional defensive measures include implementing connection rate limiting, configuring proper intrusion detection systems, and establishing automated patch management processes to prevent similar vulnerabilities from accumulating in the infrastructure. The fix implemented by OpenVPN developers addresses the specific assertion failure by ensuring proper handling of the packet-id counter rollover, preventing the server from crashing while maintaining the security properties that packet-id counters provide. This vulnerability highlights the importance of robust error handling in security-critical software components and demonstrates how seemingly minor implementation details can have significant operational consequences.