CVE-2022-40617 is a high-severity vulnerability affecting strongSwan VPN software versions prior to 5.9.8. The issue resides in the revocation plugin, which is responsible for validating certificates by checking their revocation status using CRL (Certificate Revocation List) or OCSP (Online Certificate Status Protocol) URLs embedded within end-entity or intermediate CA certificates. An attacker can craft a certificate containing a CRL/OCSP URL that points to a malicious server under their control. This server may either fail to respond after the initial TCP handshake or flood the client with excessive application data. Because strongSwan's revocation plugin does not properly handle such scenarios, this can lead to a denial of service (DoS) condition. The vulnerability is categorized under CWE-400 (Uncontrolled Resource Consumption), indicating that the attack exploits resource exhaustion to disrupt service availability. The CVSS 3.1 base score is 7.5, reflecting a network attack vector with low attack complexity, no privileges or user interaction required, and resulting in high impact on availability but no impact on confidentiality or integrity. There are no known exploits in the wild as of the publication date, and no official patches are linked in the provided data, though the fixed version is 5.9.8 or later. This vulnerability can be triggered remotely by sending a specially crafted certificate during the VPN authentication or certificate validation process, causing strongSwan instances to become unresponsive or crash, thereby disrupting VPN connectivity and potentially impacting dependent network services.

For European organizations relying on strongSwan VPN solutions for secure remote access or site-to-site VPN connectivity, this vulnerability poses a significant risk to network availability. A successful exploitation can cause denial of service, interrupting secure communications, potentially halting business operations that depend on VPN connectivity. This is particularly critical for sectors such as finance, healthcare, government, and critical infrastructure where VPNs are integral to secure data transmission. The disruption could lead to operational downtime, loss of productivity, and increased risk exposure if fallback or alternative secure channels are not available. Since the vulnerability does not affect confidentiality or integrity directly, data breaches are less likely; however, the loss of availability can indirectly impact security posture by forcing organizations to use less secure communication methods or delaying critical updates and incident responses. Additionally, the ease of exploitation without authentication or user interaction increases the threat level, as attackers can remotely target exposed VPN endpoints without prior access.

European organizations should promptly upgrade strongSwan installations to version 5.9.8 or later, where this vulnerability is addressed. In environments where immediate upgrade is not feasible, administrators should consider disabling the revocation plugin temporarily if it is not essential for their certificate validation process, thereby mitigating the attack vector. Network-level mitigations include implementing strict firewall rules to restrict access to VPN endpoints only to trusted IP addresses and monitoring for unusual traffic patterns indicative of exploitation attempts, such as repeated connections with malformed certificates or abnormal data flows from CRL/OCSP servers. Additionally, organizations should audit their certificate infrastructure to ensure that CRL/OCSP URLs embedded in certificates point to reliable and responsive servers. Employing rate limiting and connection timeouts on the VPN server can also reduce the impact of resource exhaustion attacks. Finally, maintaining comprehensive logging and alerting on VPN server behavior will aid in early detection and response to potential exploitation attempts.