CVE-1999-1466 in IOS
Summary
by MITRE
Vulnerability in Cisco routers versions 8.2 through 9.1 allows remote attackers to bypass access control lists when extended IP access lists are used on certain interfaces, the IP route cache is enabled, and the access list uses the "established" keyword.
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Analysis
by VulDB Data Team • 10/26/2025
This vulnerability exists in Cisco routers running software versions 8.2 through 9.1 and represents a significant bypass of access control mechanisms that violates fundamental network security principles. The flaw specifically affects routers where extended IP access lists are implemented on certain interfaces while the IP route cache functionality is enabled, creating a dangerous intersection of routing and access control behaviors. The vulnerability is particularly concerning because it allows remote attackers to circumvent security controls that are designed to protect network resources from unauthorized access, effectively undermining the integrity of the router's security model.
The technical root cause of this vulnerability stems from how Cisco routers handle the interaction between IP route caching and access control list processing when the "established" keyword is used in extended IP access lists. The "established" keyword in access control lists is designed to match packets that are part of an existing connection, but the router's implementation fails to properly validate the access control decision when route caching is active. This creates a scenario where packets that should be filtered based on access control rules can bypass these restrictions when they are part of cached routes, effectively allowing unauthorized traffic to traverse the network. The vulnerability is classified under CWE-284 Access Control Bypass, which specifically addresses situations where access control mechanisms are circumvented or bypassed.
The operational impact of this vulnerability is severe and can result in unauthorized network access and potential data compromise. Attackers can exploit this weakness to gain access to network resources that should be protected by access control lists, potentially leading to full network infiltration, data exfiltration, or disruption of services. The vulnerability is particularly dangerous because it operates at the core routing functionality of the network infrastructure, meaning that successful exploitation can affect all traffic flowing through the affected interfaces. This aligns with ATT&CK technique T1046 Network Service Scanning, where attackers can leverage such bypasses to expand their reconnaissance and access capabilities within the network environment.
The exploitation of this vulnerability requires specific conditions to be met including the use of extended IP access lists with the "established" keyword, enabled IP route caching, and proper interface configuration. This makes the vulnerability somewhat situational but still highly dangerous when present in network environments. Organizations with Cisco routers running affected software versions are at risk regardless of their network size or complexity, as the vulnerability exists within the fundamental routing and access control mechanisms of the device. The attack vector is remote, meaning that adversaries do not need physical access to the device to exploit the vulnerability, making it particularly attractive to cybercriminals and nation-state actors seeking to compromise network infrastructure.
Mitigation strategies for this vulnerability include immediate software upgrades to Cisco IOS versions that address the specific access control bypass issue, which would resolve the underlying implementation flaw. Network administrators should also consider disabling IP route caching on interfaces where extended access lists with the "established" keyword are implemented, though this may impact performance. Additionally, implementing redundant access control mechanisms such as firewall rules at multiple network layers, monitoring for anomalous traffic patterns that might indicate exploitation attempts, and conducting regular security assessments of network infrastructure can help detect and prevent exploitation attempts. The vulnerability demonstrates the critical importance of proper access control implementation and the need for comprehensive security testing of network infrastructure components. Organizations should also maintain updated security baselines and regularly review their network access control policies to ensure that all potential bypass scenarios are addressed.