CVE-2005-2533 in OpenVPN
Summary
by MITRE
OpenVPN before 2.0.1, when running in "dev tap" Ethernet bridging mode, allows remote authenticated clients to cause a denial of service (memory exhaustion) via a flood of packets with a large number of spoofed MAC addresses.
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Analysis
by VulDB Data Team • 06/09/2019
The vulnerability described in CVE-2005-2533 represents a critical denial of service flaw in OpenVPN versions prior to 2.0.1 that operates within Ethernet bridging mode using the "dev tap" configuration. This vulnerability specifically targets the packet processing mechanism of the VPN gateway when it encounters malicious traffic patterns from authenticated remote clients. The flaw manifests when the system receives a flood of packets containing numerous spoofed MAC addresses, leading to excessive memory consumption that ultimately results in system resource exhaustion and service disruption.
The technical implementation of this vulnerability stems from inadequate input validation and memory management within OpenVPN's bridging functionality. When operating in "dev tap" mode, the system creates a virtual ethernet interface that processes incoming packets and maintains association tables for MAC addresses. The vulnerability occurs because the system fails to properly limit or rate-limit the number of unique MAC addresses that can be processed within a given timeframe. Attackers can exploit this by sending packets with hundreds or thousands of different spoofed MAC addresses, causing the system to allocate memory for each unique address while maintaining associated tracking structures. This memory allocation continues until system resources are exhausted, rendering the VPN service unavailable to legitimate users.
The operational impact of this vulnerability extends beyond simple service disruption to potentially compromise the entire network infrastructure that relies on the affected OpenVPN service. In enterprise environments where VPN gateways serve as critical access points for remote workers, this vulnerability could be exploited to create sustained denial of service conditions that persist until the system is manually restarted or the malicious traffic is filtered at the network perimeter. The authenticated nature of the attack means that only users with valid credentials can exploit this vulnerability, but this still represents a significant risk as it allows for privilege escalation attacks where compromised accounts can be used to launch resource exhaustion attacks against network services.
This vulnerability maps directly to CWE-400, which categorizes "Uncontrolled Resource Consumption" as a fundamental weakness in software design that leads to denial of service conditions. The flaw demonstrates poor resource management practices where the system does not implement adequate bounds checking or rate limiting for MAC address processing. From an ATT&CK framework perspective, this vulnerability aligns with techniques in the T1499 category related to network denial of service attacks, specifically targeting the T1499.004 sub-technique for "OS Exhaustion Flood" where attackers consume system resources to prevent legitimate usage. The attack vector requires network access and authentication, placing it in the T1078.004 sub-technique for "Valid Accounts" within the privilege escalation category, as the attacker must possess legitimate credentials to execute the exploit.
Mitigation strategies for this vulnerability include immediate patching of OpenVPN installations to version 2.0.1 or later, where the developers implemented proper MAC address rate limiting and memory allocation controls. Network administrators should also implement traffic filtering rules at the perimeter to limit the number of unique MAC addresses that can be processed within a given time window, effectively creating a first line of defense against this specific attack pattern. Additional protective measures include implementing monitoring systems that can detect unusual MAC address proliferation patterns and automatic alerts when memory usage exceeds predetermined thresholds. Organizations should also consider implementing network segmentation strategies that isolate VPN gateways from critical infrastructure and establish automated failover mechanisms to maintain service availability during attack conditions. The vulnerability highlights the importance of proper resource management in network security systems and demonstrates how seemingly benign functionality can be exploited to create significant service disruptions when adequate safeguards are not implemented.