CVE-2022-20928: Cisco ASA VPN Auth Bypass Vulnerability

CVE-2022-20928 Overview

A vulnerability in the authentication and authorization flows for VPN connections in Cisco Adaptive Security Appliance (ASA) Software and Firepower Threat Defense (FTD) Software could allow an unauthenticated, remote attacker to establish a connection as a different user. This vulnerability is due to a flaw in the authorization verifications during the VPN authentication flow. An attacker could exploit this vulnerability by sending a crafted packet during a VPN authentication. The attacker must have valid credentials to establish a VPN connection. A successful exploit could allow the attacker to establish a VPN connection with access privileges from a different user.

Critical Impact

Attackers with valid VPN credentials can impersonate other users and gain their access privileges, potentially escalating to administrative access within the VPN environment.

Affected Products

• Cisco Adaptive Security Appliance (ASA) Software versions 9.6.x through 9.17.x
• Cisco Firepower Threat Defense (FTD) Software versions 6.1.x through 7.1.x
• Multiple point releases across affected version branches (see Cisco Security Advisory for complete list)

Discovery Timeline

• November 15, 2022 - CVE CVE-2022-20928 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2022-20928

Vulnerability Analysis

This authorization bypass vulnerability (CWE-863: Incorrect Authorization) affects the VPN authentication flow in Cisco ASA and FTD software. The flaw exists in how authorization verifications are performed during VPN connection establishment. While the vulnerability requires valid credentials for initial authentication, improper authorization checks allow an authenticated attacker to assume the identity and access privileges of a different user.

The vulnerability can be exploited remotely over the network without user interaction. Although initial authentication is required, the authorization bypass effectively allows horizontal or vertical privilege escalation depending on the target user's access level. The impact is primarily to integrity, as attackers can perform actions under another user's context, potentially accessing resources they would not otherwise be authorized to reach.

Root Cause

The root cause of CVE-2022-20928 is a flaw in the authorization verification logic during the VPN authentication flow. The affected software fails to properly validate that the authorization context matches the authenticated user identity, allowing session attributes or privileges from one user to be applied to another authenticated session. This represents a classic authorization bypass where authentication succeeds but authorization checks are insufficient.

Attack Vector

Exploitation of this vulnerability requires the attacker to possess valid VPN credentials to establish an initial connection. During the VPN authentication process, the attacker sends specially crafted packets that exploit the authorization verification flaw. If successful, the attacker's VPN session is established with the access privileges of a different user rather than their own.

The attack is network-based and does not require local access to the affected device. The crafted packets must be sent during the authentication handshake to manipulate the authorization context. This vulnerability is particularly dangerous in environments where different VPN users have varying levels of access, as a low-privileged user could potentially gain access to resources reserved for administrators or other privileged accounts.

Detection Methods for CVE-2022-20928

Indicators of Compromise

• VPN session logs showing users accessing resources outside their normal authorization scope
• Authentication events followed by access to resources inconsistent with the user's assigned group policy
• Unusual VPN connection patterns where the same user credentials appear to have different access levels across sessions
• Discrepancies between authenticated username and assigned group policy or access lists in VPN session records

Detection Strategies

• Monitor VPN authentication logs for anomalies where user identity and assigned privileges do not match expected configurations
• Implement correlation rules to detect when authenticated users access resources outside their normal access patterns
• Review Cisco ASA/FTD syslog events related to VPN authentication (class auth) and authorization decisions
• Deploy network behavior analysis to identify lateral movement following VPN connections

Monitoring Recommendations

• Enable detailed logging for VPN authentication and authorization events on affected ASA/FTD devices
• Configure syslog forwarding to a centralized SIEM for correlation and analysis
• Set up alerts for VPN connections where assigned group policies differ from expected values for specific user accounts
• Regularly audit VPN access logs to identify potential authorization bypass attempts

How to Mitigate CVE-2022-20928

Immediate Actions Required

• Consult the Cisco Security Advisory for specific patched versions and upgrade guidance
• Audit current VPN user access to ensure no unauthorized privilege escalation has occurred
• Review VPN authentication logs for any suspicious activity patterns
• Consider implementing additional network segmentation to limit the impact of potential authorization bypass

Patch Information

Cisco has released software updates that address this vulnerability. Organizations running affected versions of Cisco ASA Software (versions 9.6.x through 9.17.x) or Cisco FTD Software (versions 6.1.x through 7.1.x) should upgrade to a fixed release as specified in the Cisco Security Advisory. The advisory identifier is cisco-sa-asa-ftd-vp-authz-N2GckjN6.

Organizations should consult the Cisco Security Advisory for the specific fixed software versions applicable to their deployment.

Workarounds

• Implement strict VPN access policies with principle of least privilege to minimize impact if exploitation occurs
• Enable comprehensive logging and monitoring of VPN sessions to detect potential abuse
• Consider implementing certificate-based authentication as an additional authentication factor
• Review and restrict VPN group policies to ensure users can only access necessary resources

# Example: Enable detailed VPN logging on Cisco ASA
logging enable
logging timestamp
logging trap informational
logging class auth console informational
logging class vpn console informational
logging class vpnc console informational
# Send logs to external syslog server
logging host inside 10.1.1.100