CVE-1999-0157 in IOS
Summary
by MITRE
Cisco PIX firewall and CBAC IP fragmentation attack results in a denial of service.
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Analysis
by VulDB Data Team • 04/17/2026
The vulnerability identified as CVE-1999-0157 represents a critical denial of service flaw affecting Cisco PIX firewalls and Context-Based Access Control implementations. This weakness stems from the improper handling of fragmented IP packets within the network security infrastructure, specifically targeting the packet inspection mechanisms that govern traffic flow through the firewall. The vulnerability manifests when the system encounters certain malformed or specially crafted IP fragments that trigger unexpected behavior in the packet processing pipeline, ultimately leading to complete service disruption for legitimate network traffic.
The technical root cause of this vulnerability lies in the insufficient validation and processing of IP fragmentation within the Cisco PIX firewall's packet inspection engine. When the firewall receives fragmented packets that do not conform to standard network protocols, the CBAC module fails to properly handle the fragmentation reassembly process. This flaw creates a condition where the firewall's memory management or processing threads become overwhelmed or enter an invalid state, causing the device to become unresponsive and unable to process subsequent network traffic. The issue specifically impacts the handling of overlapping or malformed fragments that violate standard IP protocol specifications, creating a scenario where normal network operations are completely interrupted.
The operational impact of CVE-1999-0157 extends beyond simple service disruption to encompass complete network availability compromise for organizations relying on Cisco PIX firewalls for their security infrastructure. Attackers exploiting this vulnerability can generate a denial of service condition with relatively simple packet crafting techniques, requiring minimal resources or specialized tools. The consequences include complete network outages for protected segments, potentially affecting business continuity and mission-critical operations. Organizations with extensive reliance on PIX firewalls for perimeter security face significant risk exposure, as the vulnerability can be exploited remotely without requiring authentication or privileged access to the network infrastructure.
Mitigation strategies for this vulnerability involve implementing immediate firmware updates from Cisco to address the packet processing flaw, alongside network segmentation and access control measures to limit exposure. Organizations should deploy packet filtering rules that prevent problematic fragmented traffic from reaching the affected firewalls, while also implementing monitoring systems to detect unusual traffic patterns that may indicate exploitation attempts. The remediation process requires careful planning due to the critical nature of firewall services, with organizations needing to coordinate updates across their entire network infrastructure to ensure consistent protection. Additionally, network administrators should consider implementing redundant security controls and failover mechanisms to maintain service availability during the patching process, as the vulnerability can be exploited to completely disable network security services.
This vulnerability aligns with CWE-129 and CWE-131 categories related to improper input validation and insufficient boundary checking in network security systems. The attack pattern corresponds to techniques described in the ATT&CK framework under network denial of service tactics, specifically targeting infrastructure components that govern network traffic flow. The flaw demonstrates the importance of robust input validation in security appliances and highlights the critical need for proper state management in network protocol implementations. Organizations should recognize this vulnerability as representative of broader security challenges in network infrastructure, particularly the need for comprehensive testing of protocol implementations under edge case conditions to prevent similar issues in other security systems.