CVE-2015-6366 in IOS
Summary
by MITRE
Cisco IOS 15.2(04)M6 and 15.4(03)S lets physical-interface ACLs supersede tunnel-interface ACLs, which allows remote attackers to bypass intended network-traffic restrictions in opportunistic circumstances by using a tunnel, aka Bug ID CSCur01042.
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Analysis
by VulDB Data Team • 11/18/2024
Cisco IOS vulnerability CVE-2015-6366 represents a critical access control flaw in the routing and switching software that affects versions 15.2(04)M6 and 15.4(03)S. This vulnerability stems from improper handling of Access Control List (ACL) processing where physical interface ACLs can override tunnel interface ACLs, creating an unexpected privilege escalation path. The flaw manifests when network traffic traverses tunnel interfaces, allowing malicious actors to exploit the ACL hierarchy to bypass intended security restrictions.
The technical implementation of this vulnerability resides in the packet processing logic within Cisco IOS where the system evaluates ACLs in an incorrect order during packet inspection. When a packet enters through a physical interface and is destined for a tunnel interface, the system applies the physical interface ACL before the tunnel interface ACL, which creates an opportunity for bypassing security controls. This behavior violates the expected security model where tunnel interface policies should take precedence over physical interface policies. The vulnerability is classified as a weakness in the authorization mechanism and aligns with CWE-284, which addresses improper access control.
From an operational perspective, this vulnerability allows remote attackers to circumvent network traffic restrictions that are typically enforced through tunnel interfaces. The security impact is particularly severe because it enables attackers to exploit opportunistic circumstances where tunnel interfaces are used for secure communication paths, such as VPN connections or site-to-site tunnels. Attackers can leverage this flaw to gain unauthorized access to network resources that should be protected by tunnel-based ACLs, potentially leading to data exfiltration, lateral movement, or complete network compromise.
The exploitability of this vulnerability requires that an attacker have access to a network segment where the affected Cisco IOS device operates and can establish a tunnel connection. The conditionality of the exploit means it is not always present but occurs when specific traffic patterns align with the ACL processing order. This makes the vulnerability particularly challenging to detect and remediate since it only manifests under specific operational circumstances. Organizations should consider this vulnerability in the context of ATT&CK technique T1071.004 for Application Layer Protocol: DNS, as attackers may use tunnel interfaces to bypass network controls that would otherwise restrict communication.
Mitigation strategies for CVE-2015-6366 require immediate deployment of Cisco IOS software updates that correct the ACL processing order. Network administrators should implement comprehensive monitoring of tunnel interface traffic to detect anomalous patterns that may indicate exploitation attempts. Additionally, organizations should review their network architecture to minimize reliance on tunnel interfaces for critical security controls and implement redundant access control mechanisms. The vulnerability highlights the importance of proper security configuration management and demonstrates the necessity of thorough testing of network security policies before deployment. Organizations should also consider implementing network segmentation strategies that reduce the attack surface and limit the potential impact of such access control bypasses.