CVE-2012-3946 in IOS
Summary
by MITRE
Cisco IOS before 15.3(2)S allows remote attackers to bypass interface ACL restrictions in opportunistic circumstances by sending IPv6 packets in an unspecified scenario in which expected packet drops do not occur for "a small percentage" of the packets, aka Bug ID CSCty73682.
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Analysis
by VulDB Data Team • 05/11/2026
Cisco IOS versions prior to 15.3(2) contain a vulnerability in the IPv6 packet processing mechanism that creates a bypass path for access control list restrictions. This flaw exists in the handling of IPv6 packets within the router's forwarding plane where certain packet flows are not properly subjected to interface-based access control list enforcement. The vulnerability manifests when IPv6 packets are processed through the router's routing infrastructure, specifically in scenarios where the expected packet drop behavior fails to occur for a small percentage of packets within opportunistic circumstances. This creates a potential security loophole where unauthorized traffic may traverse interfaces that should have been restricted by ACLs, effectively undermining the network's security posture.
The technical root cause of this vulnerability lies in the IPv6 packet processing logic within the IOS kernel where the system fails to consistently enforce access control policies for specific packet flows. The flaw operates at the layer where IPv6 packets are evaluated against interface ACLs, and the packet processing engine does not maintain consistent state or enforcement mechanisms for certain packet types or routing scenarios. This inconsistency creates a window where packets that should be dropped according to configured ACL rules may instead be forwarded through the interface, particularly under specific network conditions or packet sequencing scenarios. The vulnerability is classified as a weakness in the implementation of access control mechanisms, aligning with CWE-284 which addresses improper access control in network security implementations.
The operational impact of this vulnerability extends beyond simple packet filtering failures, as it represents a potential pathway for attackers to bypass network segmentation controls that are fundamental to enterprise security architectures. When an attacker can send IPv6 packets that are not properly filtered by ACLs, they may gain access to network segments that should remain protected, potentially leading to lateral movement within the network or access to sensitive resources. The opportunistic nature of this vulnerability means that the attack is not deterministic but occurs under specific conditions, making it particularly dangerous as it may go undetected for extended periods. The fact that only a small percentage of packets are affected does not diminish the security risk, as even partial bypass of ACL controls can provide attackers with opportunities to establish persistent access or conduct reconnaissance activities.
Network administrators should consider this vulnerability in the context of the broader ATT&CK framework, particularly under the techniques related to network infiltration and privilege escalation. The vulnerability enables attackers to potentially bypass network-based controls that are designed to prevent unauthorized access to different network segments. Mitigation strategies should include immediate deployment of the Cisco IOS 15.3(2)S release or later versions that contain the patched implementation. Additionally, organizations should conduct thorough network audits to identify any potential exploitation attempts and implement monitoring for anomalous IPv6 packet flows that may indicate exploitation of this vulnerability. The fix addresses the underlying packet processing logic and ensures consistent enforcement of interface ACLs for IPv6 traffic, thereby restoring proper access control boundaries within the network infrastructure.