Source: cve@mitre.org
In Cisco IOS 10.3, with the tacacs-ds or tacacs keyword, an extended IP access control list could bypass filtering.
Cisco IOS versions 10.3 are vulnerable to an access control list (ACL) bypass when the tacacs-ds or tacacs keyword is enabled. This allows attackers to circumvent network security policies and potentially gain unauthorized access to sensitive resources. Successful exploitation could lead to data exfiltration, system compromise, and denial-of-service (DoS) conditions.
Step 1: Configuration: The target Cisco IOS device must be running version 10.3 and configured with either the tacacs-ds or tacacs keyword enabled, indicating the use of TACACS+ authentication.
Step 2: Authentication: An attacker attempts to authenticate to the device, potentially using valid or compromised credentials. The authentication process is handled by the TACACS+ server.
Step 3: ACL Bypass Trigger: After successful authentication (or potentially even during a failed authentication attempt), the attacker crafts network traffic that should be blocked by the configured extended IP ACL.
Step 4: Traffic Processing: Due to the vulnerability, the IOS device fails to properly apply the ACL rules to the attacker's traffic. The traffic is incorrectly allowed to pass through the network.
Step 5: Exploitation: The attacker leverages the ACL bypass to access resources or perform actions that would otherwise be prohibited, such as accessing internal servers, exfiltrating data, or launching attacks against other systems.
The vulnerability stems from a flaw in how Cisco IOS handles extended IP ACLs in conjunction with TACACS+ authentication. Specifically, when the tacacs-ds or tacacs keyword is configured, the system's ACL filtering logic fails to properly enforce the defined rules. The root cause is likely a logic error within the code responsible for processing network traffic after TACACS+ authentication. The system may be incorrectly associating a user's authenticated privileges with traffic that should have been blocked by the ACL, effectively bypassing the intended filtering. This could be due to a flaw in how the system handles the user's context after authentication, potentially leading to a privilege escalation scenario. The specific function responsible for this behavior is likely related to the interaction between the ACL processing and the TACACS+ authentication modules. The vulnerability allows for unauthorized network access by bypassing the intended ACL rules.
Due to the age of the vulnerability, it's difficult to attribute it to specific APT groups with high confidence. However, any threat actor targeting Cisco IOS devices could potentially exploit this vulnerability. The vulnerability's impact on network security makes it a likely target for both state-sponsored actors and financially motivated cybercriminals. CISA KEV status: Not listed.
Monitor network traffic for unusual patterns or traffic that should be blocked by configured ACLs, especially after TACACS+ authentication.
Analyze network logs for traffic originating from unauthorized sources or destined for restricted resources.
Review Cisco IOS configuration for the presence of the tacacs-ds or tacacs keyword and the configuration of extended IP ACLs.
Use intrusion detection systems (IDS) and intrusion prevention systems (IPS) with signatures that detect ACL bypass attempts.
Monitor for unauthorized changes to the ACL configuration itself.
Upgrade to a patched version of Cisco IOS. Since the vulnerability is specific to version 10.3, upgrading to a later version is the primary mitigation.
Disable the tacacs-ds or tacacs keyword if not required. If TACACS+ authentication is not essential, removing this configuration eliminates the vulnerability.
Review and harden the extended IP ACL configuration to ensure it accurately reflects the desired security policies.
Implement strong network segmentation to limit the impact of a potential ACL bypass.
Regularly audit the Cisco IOS configuration for any unauthorized changes.
Implement multi-factor authentication (MFA) for all network access.
Consider using more modern authentication methods like RADIUS or SSH with strong encryption.