CVE-2001-0867 in IOS
Summary
by MITRE
Cisco 12000 with IOS 12.0 and line cards based on Engine 2 does not properly filter does not properly filter packet fragments even when the "fragment" keyword is used in an ACL, which allows remote attackers to bypass the intended access controls.
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Analysis
by VulDB Data Team • 05/19/2019
Cisco IOS version 12.0 running on the Cisco 12000 series routers with Engine 2 line cards contains a critical access control vulnerability that undermines the security of network filtering mechanisms. This flaw specifically affects the packet fragment handling within the router's access control list implementation, creating a significant bypass opportunity for remote attackers. The vulnerability manifests when the "fragment" keyword is properly configured within an ACL, yet the system fails to adequately process or filter packet fragments that match the specified criteria. This technical shortcoming stems from improper state management and fragment reassembly logic within the router's packet processing pipeline.
The operational impact of this vulnerability extends beyond simple access control bypass, as it allows attackers to circumvent network security policies that rely on fragment-based filtering. When an attacker sends fragmented packets that match ACL rules containing the fragment keyword, the router's failure to properly filter these fragments enables malicious traffic to pass through security controls that should have blocked it. This vulnerability directly relates to CWE-119, which addresses improper restriction of operations within a memory buffer, and CWE-295, concerning improper certificate validation. The flaw essentially creates a pathway for attackers to exploit the router's filtering mechanisms by leveraging the inherent complexity of IP fragment reassembly.
From an adversarial perspective, this vulnerability aligns with ATT&CK technique T1071.004, which covers application layer protocol: DNS, as attackers could craft fragmented DNS queries or responses to bypass security controls. The vulnerability also maps to T1566, which involves phishing with malicious attachments, since attackers could fragment malicious payloads to evade detection. Network administrators face significant operational challenges when this vulnerability exists, as it undermines the fundamental security assumptions of access control lists and could lead to unauthorized network access, data exfiltration, or further compromise of network infrastructure. The issue particularly affects enterprise networks where Cisco 12000 series routers serve as critical boundary devices, making the impact potentially severe for organizations relying on these platforms for network security.
Mitigation strategies should include immediate implementation of workarounds such as disabling fragment processing in ACLs where possible, implementing additional network segmentation measures, and deploying intrusion detection systems to monitor for suspicious fragmented traffic patterns. Organizations should also consider upgrading to patched versions of IOS software where available, as Cisco typically addresses such vulnerabilities through security advisories. The recommended approach involves comprehensive network auditing to identify all affected routers and implementing layered security controls that do not rely solely on ACL fragment filtering. Additionally, network monitoring should be enhanced to detect anomalous fragmentation patterns that could indicate exploitation attempts, and security teams should establish incident response procedures specifically addressing this class of vulnerability.