CVE-2001-0864 in IOS
Summary
by MITRE
Cisco 12000 with IOS 12.0 and line cards based on Engine 2 does not properly handle the implicit "deny ip any any" rule in an outgoing ACL when the ACL contains exactly 448 entries, which can allow some outgoing packets to bypass access restrictions.
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Analysis
by VulDB Data Team • 05/19/2019
Cisco IOS software running on the Cisco 12000 series router with Engine 2 line cards contains a critical access control vulnerability that stems from improper handling of implicit deny rules in access control lists. This flaw specifically manifests when an outgoing ACL contains exactly 448 entries, creating a boundary condition that bypasses the expected security controls. The vulnerability operates at the packet processing level where the router's routing engine fails to properly evaluate the implicit deny ip any any rule that should normally be enforced at the end of any access control list. This represents a classic implementation flaw where the software does not correctly account for the boundary condition of exactly 448 ACL entries, allowing unauthorized packet forwarding that would otherwise be blocked by the security policy.
The technical implementation issue arises from the router's internal processing of ACL entries where the software calculates hash values or lookup table entries differently when the exact count of 448 entries is reached. This boundary condition affects the routing engine's ability to properly map and validate access control decisions, particularly in the context of outgoing packet filtering where the implicit deny rule should always apply as the final catch-all. The vulnerability exists at the software level within the Cisco IOS kernel and affects all Cisco 12000 series routers running IOS version 12.0 with Engine 2 line cards, making it a significant concern for network security administrators managing these platforms. The flaw operates at the network layer and affects packet forwarding decisions, potentially allowing malicious traffic to bypass security controls that should be enforced by the access control list.
The operational impact of this vulnerability extends beyond simple packet filtering to potentially compromise network security boundaries and allow unauthorized access to protected network segments. Attackers could exploit this condition to bypass outgoing access restrictions, potentially enabling data exfiltration, lateral movement, or other malicious activities that would normally be prevented by the configured ACL policies. The vulnerability affects the fundamental security model of the router where the implicit deny rule that should always apply fails to function properly, creating a security gap that could be exploited by threat actors. Network administrators may not immediately detect this vulnerability since the bypass occurs only under specific conditions with exactly 448 ACL entries, making it particularly dangerous as it could remain undetected for extended periods. This type of vulnerability aligns with CWE-119 which describes weaknesses in memory handling and improper handling of boundary conditions in software implementations.
Mitigation strategies for this vulnerability require immediate attention from network administrators and include several approaches to address the underlying issue. The most direct solution involves modifying ACL configurations to avoid exactly 448 entries in outgoing access control lists, either by adding or removing entries to shift the count away from the problematic boundary condition. Network administrators should also consider upgrading to newer IOS versions where this specific boundary condition has been addressed through software patches and code modifications. The vulnerability demonstrates the importance of proper boundary condition testing in security-critical software implementations and highlights the need for thorough regression testing of access control mechanisms. Organizations should implement monitoring procedures to detect ACL configurations that approach this specific entry count and establish change management processes to prevent the deployment of vulnerable configurations. This vulnerability also emphasizes the need for continuous security assessment and validation of network device configurations against known security flaws, particularly those that manifest under specific boundary conditions as defined in the ATT&CK framework's network security testing methodologies.