CVE-1999-0445 in IOS
Summary
by MITRE
In Cisco routers under some versions of IOS 12.0 running NAT, some packets may not be filtered by input access list filters.
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Analysis
by VulDB Data Team • 04/19/2026
This vulnerability exists within Cisco routers operating specific versions of IOS 12.0 that implement Network Address Translation functionality. The flaw represents a significant security weakness in the router's packet processing pipeline where input access list filters fail to properly inspect certain packets that traverse the NAT translation process. The vulnerability stems from a design oversight in how the router handles packet filtering when packets are being translated from private to public addresses or vice versa, creating a potential bypass mechanism for unauthorized network traffic.
The technical implementation issue occurs at the intersection of NAT processing and access control list enforcement within the IOS 12.0 software architecture. When packets enter the router and undergo NAT translation, the system fails to properly validate whether these packets should be subject to input access list filtering rules. This creates a scenario where packets that would normally be rejected by access control lists can successfully pass through the router, potentially allowing malicious actors to bypass network security controls. The vulnerability is particularly concerning because it affects routers that are commonly deployed in enterprise and corporate environments where network segmentation and access control are critical security measures.
The operational impact of this vulnerability extends beyond simple packet filtering failures and represents a fundamental breach in network security boundaries. Attackers could potentially exploit this weakness to gain unauthorized access to internal network resources by crafting packets that bypass input access lists while still traversing the NAT translation process. This could enable lateral movement within networks, data exfiltration attempts, or other malicious activities that would normally be prevented by properly configured access control lists. The vulnerability affects the integrity of network security policies and could lead to significant compromise of network infrastructure if not addressed promptly.
Mitigation strategies for this vulnerability should focus on immediate software updates and configuration adjustments. Cisco released IOS 12.0 and subsequent versions that address this specific flaw by correcting the packet filtering logic during NAT processing. Network administrators should prioritize applying the appropriate IOS patches and updates to all affected routers. Additionally, implementing redundant security controls such as firewall rules, intrusion detection systems, and monitoring solutions can provide additional layers of protection while awaiting patch deployment. Organizations should also conduct thorough network assessments to identify all affected devices and implement temporary workarounds including stricter access control configurations and enhanced network monitoring. This vulnerability aligns with CWE-284, which addresses improper access control, and could be leveraged by threat actors following ATT&CK techniques related to privilege escalation and lateral movement within compromised networks.