CVE-2025-11955 is a vulnerability classified under CWE-299 (Improper Check for Certificate Revocation) affecting TheGreenBow VPN Client Windows Enterprise versions 7.5 and 7.6. The vulnerability arises from incorrect validation of OCSP responses during the IKEv2 authentication phase of VPN tunnel establishment. Normally, OCSP is used to verify that a certificate has not been revoked by checking its status with a trusted certificate authority. However, in this case, the VPN client proceeds to establish the VPN tunnel even if it does not receive an OCSP response or if the response's signature is invalid. This improper validation effectively bypasses the certificate revocation check, allowing an attacker to use revoked or malicious certificates to authenticate and create VPN tunnels. The vulnerability has a CVSS 4.0 score of 8.2, reflecting a network-based attack vector with high attack complexity and partial attack prerequisites, but no user interaction required. The flaw compromises the confidentiality and integrity of VPN sessions by potentially allowing unauthorized access. No public exploits or active exploitation have been reported yet. The vulnerability is significant because VPNs are critical for secure remote access, and improper certificate validation undermines the trust model of PKI-based authentication. TheGreenBow VPN is used in enterprise environments, including critical infrastructure sectors, making this vulnerability a serious concern for organizations relying on these versions. The lack of patches at the time of publication necessitates immediate risk mitigation through alternative controls and monitoring.

For European organizations, this vulnerability poses a substantial risk to secure remote access infrastructure. Exploitation could allow attackers to bypass certificate revocation checks and establish unauthorized VPN tunnels, potentially gaining access to internal networks and sensitive data. This undermines confidentiality and integrity of communications and could facilitate lateral movement within corporate networks. Sectors with high reliance on VPNs for remote work, such as finance, government, healthcare, and critical infrastructure, are particularly vulnerable. The flaw could also impact compliance with European data protection regulations (e.g., GDPR) if unauthorized access leads to data breaches. Given the high CVSS score and the critical role of VPNs, organizations face increased risk of espionage, data theft, or disruption of services. The absence of known exploits currently provides a window for preemptive mitigation, but the vulnerability’s nature means it could be targeted by sophisticated threat actors. The impact is amplified in environments where TheGreenBow VPN is widely deployed and where OCSP validation is a key security control.

1. Immediately inventory all TheGreenBow VPN Client installations and identify versions 7.5 and 7.6 in use. 2. Monitor vendor communications closely for patches or updates addressing CVE-2025-11955 and apply them promptly once available. 3. Until patches are released, implement compensating controls such as restricting VPN access to known, trusted endpoints and IP addresses to reduce attack surface. 4. Enhance logging and monitoring of VPN authentication events to detect anomalous or unauthorized tunnel establishments potentially exploiting this flaw. 5. Consider deploying network-based anomaly detection systems to identify unusual VPN traffic patterns. 6. Review and tighten certificate management policies, including the use of short-lived certificates and alternative revocation mechanisms where possible. 7. Educate security teams about the vulnerability to ensure rapid incident response if exploitation attempts are detected. 8. Evaluate the feasibility of temporarily disabling OCSP validation in the client if it allows strict fail-closed behavior or switching to alternative VPN solutions until the vulnerability is remediated. 9. Conduct penetration testing or red team exercises simulating exploitation to assess organizational exposure and response readiness.