CVE-2023-20269

What is Cisco ASA and Firepower Threat Defense?

Cisco Adaptive Security Appliance (ASA) and Firepower Threat Defense (FTD) are Cisco's enterprise network security platforms, widely deployed as VPN gateways, firewalls, and threat inspection appliances. Both platforms provide remote access VPN (RA-VPN) functionality allowing employees to connect to corporate networks via SSL VPN (AnyConnect/Secure Client) and IKEv2. As VPN gateways, ASA and FTD are internet-accessible endpoints that authenticate large numbers of users — making their authentication mechanisms a critical and frequently targeted attack surface. Authentication bypass or weakness vulnerabilities in VPN gateways provide direct unauthorized network access, which ransomware operators exploit as a reliable initial access vector.

Overview

CVE-2023-20269 is an authentication bypass/weakness vulnerability (CWE-288) in the remote access VPN feature of Cisco ASA and Firepower Threat Defense. An unauthenticated remote attacker can exploit a flaw in the SSL VPN authentication handling to conduct brute-force attacks against valid usernames to identify correct credentials, or in some configurations establish unauthorized clientless SSL VPN sessions with an unauthorized user identity. The S:C (scope changed) CVSS metric reflects that successful exploitation impacts the broader corporate network, not just the VPN appliance.

CISA added CVE-2023-20269 to the KEV catalog on September 13, 2023 — one week after Cisco's advisory — with ransomwareUse: true, confirming active exploitation by Akira and LockBit ransomware groups for initial corporate network access.

Affected Versions

Note: The vulnerability is present only on devices configured with SSL VPN (AnyConnect/Secure Client) remote access. Devices using only site-to-site VPN or not configured for remote access VPN are not affected.

Technical Details

The authentication weakness (CWE-288) in CVE-2023-20269 involves the VPN authentication mechanism incorrectly handling certain authentication states or failing to enforce lockout and rate-limiting effectively, enabling two exploitation paths:

Path 1 — Username enumeration and credential brute force:

• The SSL VPN authentication response provides information that allows distinguishing valid from invalid usernames
• An attacker can enumerate valid usernames via the authentication endpoint's differential responses
• Once valid usernames are identified, targeted credential stuffing or brute force attacks can identify working credentials
• The vulnerability may allow this enumeration and brute-force without triggering account lockout under certain conditions

Path 2 — Unauthorized clientless SSL VPN session establishment:

• In specific ASA/FTD configurations, the authentication bypass allows establishing a clientless (browser-based) SSL VPN session without providing valid credentials
• This grants limited web-browser-based VPN access to internal resources accessible via the clientless VPN portal

Cisco's PR:L (low privilege) CVSS assignment reflects that some prior knowledge or limited access is required to conduct the exploitation efficiently.

Discovery

Cisco published the advisory on September 6, 2023, noting the vulnerability in the context of observed attacks by threat actors targeting ASA and FTD remote access VPN infrastructure. The rapid CISA KEV addition (September 13) confirms that exploitation was actively underway at the time of disclosure.

Exploitation Context

Akira ransomware and LockBit ransomware groups were specifically identified as exploiting CVE-2023-20269 to gain initial access to corporate networks. VPN gateway compromise is a high-value initial access technique because:

• VPN access provides authenticated network presence from a trusted client IP, bypassing most network segmentation
• A compromised VPN session gives the attacker the same access as a legitimate remote employee — internal systems, file shares, and Active Directory
• VPN access is frequently less monitored than direct server access, with security teams expecting high volumes of normal authentication traffic that can mask malicious activity

Akira ransomware, in particular, was observed conducting brute-force attacks against Cisco ASA VPN endpoints en masse in the September 2023 timeframe, using CVE-2023-20269 to identify and authenticate with valid credentials before deploying ransomware across compromised networks.

Remediation

1. Apply Cisco software fix — upgrade ASA or FTD software to a version that addresses CVE-2023-20269 per Cisco's advisory; specific fixed versions are listed in cisco-sa-asaftd-ravpn-auth-8LyfCkeC.
2. Enable group-lock — configure group-lock on tunnel-group connection profiles to restrict users to their assigned VPN group, limiting the scope of unauthorized session establishment.
3. Configure vpn-simultaneous-logins limits — restrict the number of simultaneous VPN sessions per user to detect and limit brute-force credential stuffing.
4. Enable multi-factor authentication (MFA) — require TOTP, Duo, or hardware tokens for all SSL VPN connections; MFA defeats credential-based brute-force attacks even if valid passwords are obtained.
5. Monitor VPN authentication logs for high volumes of authentication failures from single source IPs (brute force) or unusual successful authentications from unexpected geolocations or devices.
6. Deploy Cisco Secure Client with certificate-based authentication — client certificates bound to managed devices provide a second factor that cannot be brute-forced.