CVE-2023-20269 is a vulnerability identified in the remote access VPN functionality of Cisco Adaptive Security Appliance (ASA) Software and Cisco Firepower Threat Defense (FTD) Software. The root cause is an improper separation of authentication, authorization, and accounting (AAA) mechanisms between the remote access VPN feature and other features such as HTTPS management and site-to-site VPN. This flaw allows an unauthenticated remote attacker to conduct brute force attacks targeting default connection profiles or tunnel groups to enumerate valid username and password combinations. Additionally, an authenticated attacker can exploit this vulnerability to establish a clientless SSL VPN session using valid credentials, specifically on ASA Software releases 9.16 or earlier. However, the vulnerability does not allow bypassing authentication entirely; valid credentials are still required, including multi-factor authentication if enabled. The vulnerability affects a broad range of ASA software versions from 9.8.1 through 9.19.1, encompassing many minor releases. Exploitation could lead to unauthorized remote VPN access or credential disclosure, potentially enabling lateral movement or data exfiltration. Cisco has not reported any known exploits in the wild but has indicated that software updates and workarounds will be provided to address the issue. The CVSS v3.1 base score is 5.0 (medium), reflecting network attack vector, low complexity, requiring privileges, no user interaction, and impact limited to integrity loss without confidentiality or availability impact.

For European organizations, this vulnerability poses a risk of unauthorized access to corporate networks via the remote access VPN infrastructure, which is critical for secure remote work and inter-office connectivity. Successful brute force attacks could expose valid credentials, increasing the risk of credential stuffing or further compromise. Unauthorized clientless SSL VPN sessions could allow attackers to access internal resources without proper authorization, potentially leading to data integrity issues or lateral movement within the network. Given the widespread use of Cisco ASA and FTD devices in Europe for perimeter security and VPN services, exploitation could disrupt secure remote access and expose sensitive data or systems. Organizations relying on multi-factor authentication are somewhat protected, but credential enumeration still presents a significant threat. The vulnerability could also undermine trust in VPN security, impacting compliance with European data protection regulations such as GDPR if unauthorized access leads to data breaches.

European organizations should immediately audit their Cisco ASA and FTD devices to identify affected versions and prioritize upgrading to patched software releases once Cisco publishes updates. Until patches are available, organizations should implement the recommended workarounds from Cisco, such as disabling default connection profiles or tunnel groups that are vulnerable to brute force attacks. Enforcing strong password policies and account lockout mechanisms can reduce the risk of successful brute force attempts. Enabling and enforcing multi-factor authentication (MFA) for all VPN users is critical to mitigate unauthorized access even if credentials are compromised. Network segmentation and strict access controls should limit the exposure of VPN management interfaces to trusted networks only. Monitoring VPN logs for unusual authentication attempts or session anomalies can help detect exploitation attempts early. Regularly updating and hardening VPN configurations, including disabling unused features and profiles, will reduce the attack surface. Finally, organizations should conduct penetration testing and vulnerability assessments focused on VPN infrastructure to validate the effectiveness of mitigations.