CVE-2017-6766 in Firepower System Software
Summary
by MITRE
A vulnerability in the Secure Sockets Layer (SSL) Decryption and Inspection feature of Cisco Firepower System Software 5.4.0, 5.4.1, 6.0.0, 6.1.0, 6.2.0, 6.2.1, and 6.2.2 could allow an unauthenticated, remote attacker to bypass the SSL policy for decrypting and inspecting traffic on an affected system. The vulnerability is due to unexpected interaction with Known Key and Decrypt and Resign configuration settings of SSL policies when the affected software receives unexpected SSL packet headers. An attacker could exploit this vulnerability by sending a crafted SSL packet through an affected device in a valid SSL session. A successful exploit could allow the attacker to bypass the SSL decryption and inspection policy for the affected system, which could allow traffic to flow through the system without being inspected. Cisco Bug IDs: CSCve12652.
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Analysis
by VulDB Data Team • 11/04/2019
The vulnerability described in CVE-2017-6766 represents a critical security flaw within Cisco Firepower System Software that specifically targets the Secure Sockets Layer decryption and inspection functionality. This weakness exists in multiple versions including 5.4.0 through 6.2.2, making it a widespread concern for organizations utilizing Cisco's next-generation firewall solutions. The vulnerability operates at the intersection of SSL/TLS protocol handling and policy enforcement mechanisms, creating a potential pathway for attackers to circumvent security controls that are specifically designed to monitor and inspect encrypted traffic. The affected system's inability to properly validate SSL packet headers when processing certain configurations creates an exploitable condition that undermines the fundamental security posture of the firewall.
The technical implementation of this vulnerability stems from the improper handling of SSL packet headers within the context of specific SSL policy configurations. When the affected Cisco Firepower systems encounter unexpected SSL packet structures, the interaction between Known Key and Decrypt and Resign configuration settings creates an unexpected state where the system fails to properly enforce the configured SSL decryption policies. This behavior manifests when an attacker sends specifically crafted SSL packets through an active SSL session, exploiting the software's failure to validate header integrity and configuration consistency. The flaw essentially allows the system to transition into a state where it ignores the configured inspection policies, effectively creating a bypass mechanism that operates without authentication or authorization requirements. This type of vulnerability falls under CWE-248, which addresses "Uncaught Exception" conditions in software systems, specifically when the exception handling fails to maintain system integrity during protocol processing.
The operational impact of this vulnerability extends beyond simple policy bypass, potentially allowing attackers to establish persistent communication channels through the firewall without detection. Organizations relying on SSL inspection for threat prevention and compliance monitoring face significant risk as this vulnerability enables malicious traffic to flow through the network infrastructure undetected. The implications are particularly severe for environments where SSL decryption is mandatory for security policy enforcement, as the bypass mechanism could allow sophisticated attacks to remain hidden from security monitoring systems. Attackers could leverage this vulnerability to establish command and control channels, exfiltrate sensitive data, or deploy malware while evading detection mechanisms that depend on SSL inspection for security posture enforcement. This scenario directly relates to ATT&CK technique T1071.004, which covers application layer protocol: DNS, where attackers might utilize bypassed inspection capabilities to establish covert communication channels.
Mitigation strategies for CVE-2017-6766 require immediate implementation of software updates from Cisco, specifically addressing the identified bug CSCve12652. Organizations should also consider implementing additional network monitoring measures to detect anomalous SSL traffic patterns that might indicate exploitation attempts. Configuration reviews should focus on ensuring proper SSL policy enforcement and implementing strict header validation mechanisms. Network segmentation strategies can help limit the potential impact of successful exploitation attempts, while enhanced logging and alerting systems should be deployed to monitor for unusual SSL session behavior. The vulnerability highlights the importance of maintaining current security patches and conducting regular vulnerability assessments of network security infrastructure, particularly for critical systems that handle SSL/TLS traffic inspection. Security teams should also implement network behavior analysis tools that can identify deviations from normal SSL traffic patterns, as these may indicate exploitation attempts targeting this specific vulnerability.