CVE-2006-4430 in Clean Access
Summary
by MITRE
The Cisco Network Admission Control (NAC) 3.6.4.1 and earlier allows remote attackers to prevent installation of the Cisco Clean Access (CCA) Agent and bypass local and remote protection mechanisms by modifying (1) the HTTP User-Agent header or (2) the behavior of the TCP/IP stack. NOTE: the vendor has disputed the severity of this issue, stating that users cannot bypass authentication mechanisms.
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Analysis
by VulDB Data Team • 08/14/2018
The vulnerability described in CVE-2006-4430 affects Cisco Network Admission Control version 3.6.4.1 and earlier implementations, specifically targeting the Cisco Clean Access Agent deployment mechanism. This issue represents a significant security concern within enterprise network access control systems where unauthorized modification of network traffic characteristics can lead to bypassing critical security protections. The vulnerability exploits the trust relationship between the network access control system and client devices, potentially allowing malicious actors to circumvent the intended security posture enforcement. The affected system operates under the assumption that legitimate client devices will behave according to expected network protocols, creating an attack surface when these assumptions are violated through header manipulation or stack behavior modification.
The technical flaw manifests through two primary attack vectors that exploit the client identification and authentication process within the NAC framework. Attackers can manipulate the HTTP User-Agent header to present false device characteristics that would otherwise trigger appropriate security policies or agent installation requirements. Additionally, the vulnerability allows modification of TCP/IP stack behavior to mimic trusted client attributes, effectively bypassing the network admission control checks that should validate device legitimacy before granting network access. This manipulation occurs at the application layer where the NAC system relies on standard HTTP headers for device identification and at the transport layer where TCP/IP stack behavior defines network communication patterns. The vulnerability falls under CWE-295 which addresses improper certificate validation and CWE-310 which covers cryptographic issues, though the specific implementation focuses on header manipulation and protocol behavior modification rather than traditional cryptographic weaknesses.
The operational impact of this vulnerability extends beyond simple access control bypass to potentially compromise the entire network admission control infrastructure. When attackers can prevent CCA agent installation, they effectively remove the endpoint security measures that should monitor and enforce compliance policies on network-connected devices. This creates opportunities for malicious actors to establish persistent network presence without proper authentication or monitoring, potentially leading to lateral movement and data exfiltration. The attack vector is particularly concerning because it operates at the network edge where devices first connect to the corporate network, making it an ideal entry point for broader network compromise. The vulnerability can be exploited remotely, meaning attackers do not require physical access to network infrastructure or client devices to execute the attack, which aligns with ATT&CK technique T1071.004 for application layer protocol manipulation.
Cisco has disputed the severity classification of this vulnerability, arguing that users cannot bypass authentication mechanisms. However, this assessment may overlook the broader implications of agent installation bypass, which fundamentally undermines the security posture of network admission control systems. The vendor's position suggests that while direct authentication bypass may not be possible, the ability to prevent security agent deployment creates a significant weakness in the overall defense-in-depth strategy. Organizations implementing NAC solutions must consider that successful exploitation of this vulnerability can lead to complete bypass of endpoint protection mechanisms, potentially allowing attackers to establish unauthorized network presence without detection. The attack requires minimal technical expertise and can be automated, making it particularly dangerous in enterprise environments where network access control is critical for maintaining security boundaries.
Mitigation strategies should focus on implementing additional layers of network security controls beyond the NAC system itself. Organizations should deploy network monitoring solutions that can detect anomalous HTTP User-Agent header patterns or unusual TCP/IP stack behaviors that may indicate exploitation attempts. Network segmentation and micro-segmentation can help contain potential compromise even if the NAC system is successfully bypassed. Implementing strict network access control policies that require multiple authentication factors and continuous monitoring of connected devices can provide additional protection. Regular security assessments of NAC implementations should include testing for header manipulation and protocol behavior modification attacks. Organizations should also consider implementing network behavior analysis tools that can identify deviations from normal network communication patterns, which may indicate exploitation of this vulnerability. The solution should include network-wide visibility into agent deployment status and automated alerts when agent installation is prevented or bypassed, ensuring that security teams can respond quickly to potential compromise attempts.