CVE-1999-1306 in IOS
Summary
by MITRE
Cisco IOS 9.1 and earlier does not properly handle extended IP access lists when the IP route cache is enabled and the "established" keyword is set, which could allow attackers to bypass filters.
Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.
Analysis
by VulDB Data Team • 10/26/2025
The vulnerability described in CVE-1999-1306 represents a critical flaw in Cisco IOS software version 9.1 and earlier where the routing infrastructure fails to properly process extended IP access control lists under specific conditions. This issue occurs when the IP route cache is enabled and the established keyword is configured within access control lists, creating a scenario where packet filtering mechanisms can be circumvented. The vulnerability stems from an improper interaction between the routing cache mechanism and access list processing logic, specifically when the established keyword is used in conjunction with route caching.
The technical implementation of this vulnerability involves the interaction between two fundamental networking components within Cisco IOS. When IP route caching is enabled, the router maintains a cache of previously computed routing decisions to improve performance. However, the established keyword in access lists is designed to match packets that are part of an existing connection, typically used for allowing return traffic from established sessions. The flaw manifests when these two features combine, causing the router to bypass the access list filtering mechanism entirely. This occurs because the route cache does not properly validate the access list rules when packets are matched against cached routes, allowing malicious traffic to pass through filters that should have blocked it.
The operational impact of this vulnerability is significant for network security and can be categorized under the CWE-284 weakness type, which deals with improper access control mechanisms. Attackers can exploit this vulnerability to bypass network security policies that rely on extended access lists for filtering traffic. The attack vector requires an understanding of the specific configuration parameters and timing, but once exploited, it allows unauthorized traffic to traverse network boundaries that should have been protected. This represents a fundamental failure in the router's security model where the expected behavior of access control lists is subverted by the interaction with route caching. The vulnerability can be leveraged to establish unauthorized connections, potentially leading to data exfiltration, network infiltration, or other malicious activities that bypass network security controls.
Mitigation strategies for this vulnerability should focus on immediate configuration changes and long-term architectural considerations. The most direct approach involves disabling IP route caching when extended access lists with the established keyword are in use, though this may impact network performance. Network administrators should also consider implementing additional security layers such as ingress and egress filtering, or using more recent versions of Cisco IOS software where this issue has been addressed. The remediation process aligns with ATT&CK technique T1071.004 for application layer protocol and T1566.001 for phishing, as attackers may exploit this vulnerability to establish unauthorized network connections. Organizations should also implement comprehensive monitoring to detect unusual traffic patterns that might indicate exploitation attempts, and regularly update their network infrastructure to avoid known vulnerabilities in older software versions. This vulnerability demonstrates the complexity of modern network security where seemingly independent features can interact in unexpected ways to create security gaps.