CVE-2001-0862 in IOS
Summary
by MITRE
Cisco 12000 with IOS 12.0 and line cards based on Engine 2 does not block non-initial packet fragments, which allows remote attackers to bypass the ACL.
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Analysis
by VulDB Data Team • 05/19/2019
The vulnerability identified as CVE-2001-0862 affects Cisco 12000 series routers operating with IOS version 12.0 and line cards utilizing Engine 2 architecture. This security flaw represents a significant weakness in the router's packet processing capabilities and access control implementation. The issue stems from the router's failure to properly inspect and filter fragmented IP packets that are not the initial fragment within a packet sequence. When network traffic arrives in multiple fragments, the router's access control lists fail to adequately validate subsequent fragments, creating a potential pathway for malicious actors to circumvent security policies.
The technical root cause of this vulnerability lies in the router's packet reassembly and access control list processing mechanisms. In standard network operations, when an IP packet exceeds the maximum transmission unit of a network segment, it gets fragmented into smaller pieces for transmission. The first fragment contains the complete IP header information while subsequent fragments carry only payload data with minimal header information. The Cisco 12000 series with Engine 2 line cards demonstrates a flaw in their packet handling logic where the router's ACL processing does not properly validate the contents of non-initial fragments, allowing potentially malicious data to pass through security controls that should have blocked it. This behavior directly violates the fundamental security principle that all packet components should be subject to the same access control policies regardless of their position in a fragmented transmission sequence.
The operational impact of this vulnerability extends beyond simple access control bypass to potentially enable sophisticated attack vectors against network infrastructure. Remote attackers can exploit this weakness to send malicious traffic that would normally be blocked by ACL rules, effectively creating a backdoor through which they can access restricted network segments or services. The vulnerability affects the router's ability to provide consistent security enforcement across all packet types, undermining the integrity of the network's perimeter protection. This weakness particularly impacts organizations relying on Cisco 12000 series routers for critical network security functions, as it allows attackers to potentially bypass multiple layers of network security controls. The attack surface expands significantly since any ACL-based filtering rules are rendered ineffective for non-initial packet fragments, potentially enabling data exfiltration, service disruption, or unauthorized network access.
Cisco's implementation of access control lists in the affected router models demonstrates a gap in their packet processing architecture that aligns with common weaknesses identified in the CWE database under category 117, which addresses inadequate output escaping or insufficient escaping of output. The vulnerability also reflects patterns commonly associated with ATT&CK technique T1071.004, which involves application layer protocol tunneling, as attackers can potentially use this flaw to establish covert communication channels through fragmented packets that bypass normal security monitoring. Organizations should implement immediate mitigations including applying the appropriate Cisco IOS patches, implementing additional network monitoring to detect unusual fragmented packet patterns, and reviewing existing ACL configurations to identify potential attack vectors. The remediation process should involve comprehensive testing of the updated IOS version to ensure that the patch does not introduce compatibility issues with existing network services while maintaining the integrity of the router's core security functions.