CVE-2001-0863 in IOS
Summary
by MITRE
Cisco 12000 with IOS 12.0 and line cards based on Engine 2 does not handle the "fragment" keyword in a compiled ACL (Turbo ACL) for packets that are sent to the router, which allows remote attackers to cause a denial of service via a flood of fragments.
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Analysis
by VulDB Data Team • 05/19/2019
The vulnerability described in CVE-2001-0863 represents a critical denial of service flaw affecting Cisco 12000 series routers running IOS 12.0 with Engine 2 based line cards. This issue specifically impacts the router's handling of Turbo ACLs which are optimized access control lists designed to improve packet processing performance. The vulnerability manifests when the router encounters packets with the fragment keyword in a compiled ACL, creating a scenario where legitimate network traffic can be disrupted through malicious fragmentation attacks.
The technical flaw stems from inadequate processing of fragmented packets within the Turbo ACL implementation. When a router receives packets that match a compiled ACL containing the fragment keyword, the system fails to properly handle the fragmentation process, leading to a system crash or complete service disruption. This behavior is particularly concerning because it affects the core packet filtering mechanisms that protect network infrastructure and can be exploited by remote attackers without requiring authentication or privileged access. The vulnerability specifically targets the Engine 2 architecture which was designed for high-performance routing operations but contained a critical flaw in its fragment handling logic.
From an operational perspective, this vulnerability poses significant risks to network availability and business continuity. Attackers can exploit this weakness by flooding the affected router with fragmented packets that trigger the denial of service condition, effectively rendering the router inoperable and disrupting all network services that depend on it. The impact extends beyond simple service interruption as it affects critical network infrastructure components that may serve as primary or backup paths for enterprise communications. Network administrators face the challenge of identifying and mitigating this vulnerability while maintaining service availability, particularly in environments where the affected routers serve as core network components.
The vulnerability aligns with CWE-129 which addresses improper handling of input boundaries, and represents a classic example of how optimization features in network equipment can introduce security flaws. From an attack framework perspective, this vulnerability maps to the denial of service category in the MITRE ATT&CK framework, specifically targeting network infrastructure components. The attack vector is remote and requires no special privileges, making it particularly dangerous as it can be exploited by anyone with network access to the affected router. Organizations should implement immediate mitigations including applying Cisco's security patches, disabling Turbo ACLs if not essential, and implementing network segmentation to limit the impact of potential exploitation attempts.
Cisco addressed this vulnerability through IOS software updates that corrected the fragment handling logic in Turbo ACLs, requiring affected organizations to upgrade their router firmware to prevent exploitation. The remediation process involves careful planning due to the critical nature of these routers in enterprise networks, as updates may require scheduled maintenance windows and could potentially impact ongoing network operations. Security professionals should monitor for any signs of exploitation attempts and maintain detailed network logs to detect anomalous fragmentation patterns that might indicate an active attack against this vulnerability.