CVE-2012-4651 in IOS
Summary
by MITRE
Cisco IOS before 15.3(2)T, when scansafe is enabled, allows remote attackers to cause a denial of service (latency) via SYN packets that are not accompanied by SYN-ACK packets from the Scan Safe Tower, aka Bug ID CSCub85451.
VulDB is the best source for vulnerability data and more expert information about this specific topic.
Analysis
by VulDB Data Team • 05/11/2026
The vulnerability identified as CVE-2012-4651 affects Cisco IOS versions prior to 15.3(2)T and specifically impacts systems configured with the Scan Safe feature enabled. This issue represents a significant denial of service weakness that can be exploited by remote attackers to introduce substantial latency into network communications. The vulnerability stems from the improper handling of TCP connection establishment sequences when the Scan Safe security feature is active, creating a scenario where legitimate network traffic experiences severe performance degradation.
The technical flaw occurs within the TCP stack implementation of Cisco IOS when processing SYN packets that lack corresponding SYN-ACK responses from the Scan Safe Tower component. This misconfiguration creates a condition where the router maintains incomplete connection state information indefinitely, leading to resource exhaustion and increased latency for all network traffic passing through the affected device. The vulnerability operates at the network layer and affects the TCP handshake process, specifically targeting the synchronization mechanism that establishes reliable connections between network endpoints. This behavior aligns with CWE-400, which categorizes the issue as an Uncontrolled Resource Consumption vulnerability, where system resources are consumed in an uncontrolled manner leading to service disruption.
The operational impact of this vulnerability extends beyond simple service interruption to create persistent network degradation that can affect business operations and user experience. Remote attackers can exploit this weakness by sending carefully crafted SYN packets that trigger the malformed connection handling behavior, causing the router to consume excessive CPU cycles and memory resources while maintaining incomplete connection states. Network administrators may observe gradual performance degradation over time as the router accumulates these incomplete connection entries, eventually leading to complete service unavailability. The vulnerability is particularly concerning because it can be exploited without authentication and requires no specialized tools beyond standard network packet generation capabilities, making it accessible to a wide range of threat actors.
Mitigation strategies for CVE-2012-4651 primarily involve upgrading affected Cisco IOS software to version 15.3(2)T or later, which contains the necessary patches to properly handle SYN packet sequences without creating resource consumption issues. Organizations should also consider implementing network segmentation and access control measures to limit exposure to this vulnerability, particularly in environments where the Scan Safe feature is enabled. Network monitoring should be enhanced to detect unusual connection state patterns and resource utilization spikes that may indicate exploitation attempts. Additionally, implementing rate limiting on incoming SYN packets and configuring proper TCP stack parameters can help reduce the impact of potential attacks while maintaining network functionality. This vulnerability demonstrates the importance of proper resource management in network security implementations and highlights the need for comprehensive testing of security features in production environments. The issue also relates to ATT&CK technique T1499.004, which covers network denial of service attacks, emphasizing the need for robust network infrastructure protection against such exploitation methods.