CVE-2001-0427 in VPN Concentrator
Summary
by MITRE
Cisco VPN 3000 series concentrators before 2.5.2(F) allow remote attackers to cause a denial of service via a flood of invalid login requests to (1) the SSL service, or (2) the telnet service, which do not properly disconnect the user after several failed login attempts.
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Analysis
by VulDB Data Team • 05/16/2019
The vulnerability identified as CVE-2001-0427 affects Cisco VPN 3000 series concentrators running firmware versions prior to 2.5.2(F). This issue represents a classic denial of service weakness that exploits the improper handling of authentication failures within the device's security services. The vulnerability specifically targets two critical service endpoints: the Secure Sockets Layer service responsible for encrypted communications and the telnet service used for administrative access. Both services exhibit identical behavior in their failure to properly terminate connections after repeated unsuccessful authentication attempts, creating a persistent resource exhaustion condition that can be exploited by remote attackers.
The technical flaw manifests in the concentrator's session management mechanism where failed login attempts are not adequately handled to prevent resource accumulation. When attackers flood the system with invalid login requests, the device fails to implement proper connection termination protocols after multiple authentication failures. This behavior creates a condition where connection slots remain occupied indefinitely, consuming system resources such as memory and processing power. The vulnerability falls under the category of improper resource management as defined by CWE-400, specifically CWE-402 which addresses improper resource shutdown or release. The root cause lies in the absence of proper rate limiting and connection cleanup mechanisms within the authentication service implementations.
The operational impact of this vulnerability extends beyond simple service disruption to potentially compromise the entire network infrastructure. Attackers can leverage this weakness to exhaust available connection slots on the VPN concentrator, effectively preventing legitimate users from establishing secure connections. This creates a situation where authorized personnel cannot access network resources while the device remains operational but functionally impaired. The vulnerability is particularly concerning in enterprise environments where VPN concentrators serve as primary gateways for remote access, making them attractive targets for attackers seeking to disrupt business operations. The attack vector requires only remote access to the targeted services, making exploitation straightforward and accessible to threat actors with minimal technical expertise.
Mitigation strategies for this vulnerability should focus on immediate firmware upgrades to version 2.5.2(F) or later, which contain the necessary patches to address the improper connection handling. Network administrators should also implement additional security controls such as rate limiting on authentication attempts and connection throttling to prevent abuse of the authentication services. The implementation of intrusion detection systems capable of identifying patterns of repeated authentication failures can provide early warning of potential exploitation attempts. Organizations should also consider configuring access control lists to restrict access to the vulnerable services from specific IP ranges and implementing logging mechanisms to monitor for abnormal authentication patterns. From an ATT&CK framework perspective, this vulnerability aligns with techniques such as T1499.004 (Endpoint Denial of Service) and T1566.001 (Phishing via Social Engineering) as attackers may use this weakness as part of broader attack campaigns to disrupt network availability and compromise business continuity. Regular security assessments and vulnerability scanning should be conducted to ensure that similar issues are not present in other network infrastructure components, as this represents a pattern of inadequate resource management that could affect other systems within the enterprise environment.