CVE-2023-20269 in ASA
Summary
by MITRE • 09/06/2023
A vulnerability in the remote access VPN feature of Cisco Adaptive Security Appliance (ASA) Software and Cisco Firepower Threat Defense (FTD) Software could allow an unauthenticated, remote attacker to conduct a brute force attack in an attempt to identify valid username and password combinations or an authenticated, remote attacker to establish a clientless SSL VPN session with an unauthorized user. This vulnerability is due to improper separation of authentication, authorization, and accounting (AAA) between the remote access VPN feature and the HTTPS management and site-to-site VPN features. An attacker could exploit this vulnerability by specifying a default connection profile/tunnel group while conducting a brute force attack or while establishing a clientless SSL VPN session using valid credentials. A successful exploit could allow the attacker to achieve one or both of the following: Identify valid credentials that could then be used to establish an unauthorized remote access VPN session. Establish a clientless SSL VPN session (only when running Cisco ASA Software Release 9.16 or earlier). Notes: Establishing a client-based remote access VPN tunnel is not possible as these default connection profiles/tunnel groups do not and cannot have an IP address pool configured. This vulnerability does not allow an attacker to bypass authentication. To successfully establish a remote access VPN session, valid credentials are required, including a valid second factor if multi-factor authentication (MFA) is configured. Cisco will release software updates that address this vulnerability. There are workarounds that address this vulnerability.
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Analysis
by VulDB Data Team • 10/28/2025
The vulnerability identified as CVE-2023-20269 represents a critical flaw in Cisco's Adaptive Security Appliance (ASA) and Firepower Threat Defense (FTD) software platforms that undermines the fundamental security architecture governing remote access controls. This issue stems from inadequate separation of authentication, authorization, and accounting (AAA) mechanisms between distinct network services within the same platform, creating a dangerous overlap that exposes the system to unauthorized access attempts. The vulnerability specifically impacts the remote access VPN feature, which shares authentication contexts with HTTPS management and site-to-site VPN functionalities, creating an attack surface that malicious actors can exploit through carefully crafted brute force operations.
The technical exploitation of this vulnerability occurs through the manipulation of default connection profiles or tunnel groups that are improperly configured to allow cross-service authentication bypasses. When an attacker conducts a brute force attack, they can leverage these misconfigured default profiles to systematically test credential combinations against the shared authentication framework. The flaw exists because the system fails to properly isolate the authentication contexts between different VPN service types, allowing an attacker to potentially use credentials valid for one service type to gain access to another service type that should maintain separate authentication boundaries. This improper separation directly violates security best practices and creates a pathway for unauthorized privilege escalation through credential reuse attacks.
The operational impact of this vulnerability extends beyond simple credential theft to include the establishment of unauthorized clientless SSL VPN sessions when exploiting older ASA software versions (9.16 and earlier). This capability allows attackers to gain unauthorized access to network resources through the VPN interface without requiring the initial authentication bypass that would normally be required. The vulnerability's design flaw means that even when multi-factor authentication is properly configured, attackers can still leverage the improper AAA separation to establish unauthorized sessions, though they cannot bypass authentication entirely. The restriction that prevents establishment of client-based VPN tunnels is significant because it limits the attack scope but does not eliminate the serious threat posed by clientless access, which can provide full access to network resources through web-based interfaces.
Security practitioners should understand that this vulnerability aligns with CWE-284 (Improper Access Control) and represents a clear violation of the principle of least privilege. The attack vectors described in the vulnerability map directly to ATT&CK techniques including T1110 (Brute Force) and T1078 (Valid Accounts) where attackers can leverage valid credentials to establish unauthorized access. The improper separation of AAA components creates a persistent risk that can be exploited repeatedly, making this vulnerability particularly dangerous in environments where multiple VPN services are deployed concurrently. Organizations should implement immediate mitigations including applying the relevant software patches, implementing additional access controls, and monitoring for suspicious authentication patterns that could indicate exploitation attempts.
Cisco's response to this vulnerability includes the release of software updates that address the underlying AAA separation issues, though organizations should also consider implementing workarounds that can provide immediate protection while longer-term patches are deployed. The vulnerability's impact is particularly concerning because it affects both ASA and FTD platforms, creating a widespread risk across enterprise security infrastructures that rely on these products for network protection. The combination of brute force attack capabilities and unauthorized session establishment creates a multi-layered threat that can lead to significant data breaches and network compromise when left unaddressed. Organizations must also consider the broader implications for their security posture, as this vulnerability demonstrates how seemingly minor configuration flaws can create major security gaps in complex network infrastructure deployments.