CVE-1999-1465 in IOS
Summary
by MITRE
Vulnerability in Cisco IOS 11.1 through 11.3 with distributed fast switching (DFS) enabled allows remote attackers to bypass certain access control lists when the router switches traffic from a DFS-enabled input interface to an output interface with a logical subinterface, as described by Cisco bug CSCdk43862.
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Analysis
by VulDB Data Team • 04/20/2026
This vulnerability exists within Cisco IOS versions 11.1 through 11.3 where distributed fast switching functionality creates a security gap in access control list enforcement. The flaw specifically manifests when traffic transitions from an input interface with DFS enabled to an output interface utilizing logical subinterfaces, creating a scenario where certain access control lists fail to properly filter traffic. This represents a critical design weakness in the router's packet processing architecture where the security controls implemented at the input interface are not consistently applied when traffic is switched to a different output interface with subinterface configuration.
The technical implementation of this vulnerability stems from how Cisco IOS handles packet switching between interfaces when distributed fast switching is active. When DFS is enabled on an input interface, the router optimizes packet forwarding by creating fast switching entries in the cache. However, the system fails to properly validate access control list rules when packets are switched from the DFS-enabled input interface to an output interface that utilizes logical subinterfaces. This creates a path where packets can bypass access control enforcement mechanisms that should normally be applied at the output interface level. The vulnerability is categorized under CWE-284 Access Control Bypass, which specifically addresses situations where insufficient access control checks allow unauthorized access to resources.
The operational impact of this vulnerability extends beyond simple network access control bypass. Attackers can exploit this weakness to gain unauthorized access to network segments that should be protected by access control lists configured on the output interfaces. This creates a potential for lateral movement within the network, data exfiltration, and unauthorized access to sensitive resources. The vulnerability is particularly concerning because it operates at the routing layer where network traffic is processed, making it difficult to detect through traditional network monitoring techniques. Network administrators may not immediately recognize that access control is being bypassed since the traffic appears to flow normally through the router's switching mechanisms.
This vulnerability aligns with ATT&CK technique T1046 Network Service Scanning and T1071.1 Application Layer Protocol Web Protocols, as attackers could leverage this bypass to access restricted network services or web resources that should be protected by access control lists. The distributed fast switching feature was designed for performance optimization but inadvertently created a security boundary that could be exploited. Organizations implementing Cisco IOS versions 11.1 through 11.3 with DFS enabled should consider disabling distributed fast switching on interfaces that handle sensitive traffic or implement additional security controls to compensate for this bypass mechanism. The vulnerability demonstrates the inherent risk in optimizing network performance without maintaining proper security boundaries, particularly when logical subinterfaces are involved in the switching path.
Mitigation strategies should focus on disabling distributed fast switching on interfaces where access control bypass could occur, particularly when logical subinterfaces are configured on output interfaces. Network administrators should also implement redundant access control mechanisms such as zone-based firewalls or additional filtering at higher network layers to compensate for the bypass vulnerability. Regular security audits should verify that access control lists are properly enforced across all interface types and configurations. The vulnerability highlights the importance of maintaining security controls even when implementing performance optimization features, as the security implications of such optimizations can be significant. Organizations should also consider upgrading to newer Cisco IOS versions where this vulnerability has been addressed through improved access control enforcement mechanisms.