CVE-2018-0243 in Firepower System Software
Summary
by MITRE
A vulnerability in the detection engine of Cisco Firepower System Software could allow an unauthenticated, remote attacker to bypass a configured file action policy that is intended to drop the Server Message Block Version 2 (SMB2) and SMB Version 3 (SMB3) protocols if malware is detected. The vulnerability is due to incorrect detection of an SMB2 or SMB3 file based on the total file length. An attacker could exploit this vulnerability by sending a crafted SMB2 or SMB3 transfer request through the targeted device. An exploit could allow the attacker to pass SMB2 or SMB3 files that could be malware even though the device is configured to block them. This vulnerability does not exist for SMB Version 1 (SMB1) files. This vulnerability affects Cisco Firepower System Software when one or more file action policies are configured, on software releases prior to 6.2.3. Cisco Bug IDs: CSCvg68807.
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Analysis
by VulDB Data Team • 01/30/2020
The vulnerability identified as CVE-2018-0243 resides within the detection engine of Cisco Firepower System Software, representing a critical security flaw that undermines the integrity of network traffic filtering mechanisms. This weakness specifically targets the handling of Server Message Block protocol versions 2 and 3, which are commonly used for file sharing in enterprise environments. The vulnerability stems from an incorrect implementation in how the system evaluates file characteristics during protocol detection, creating a pathway for malicious actors to circumvent security policies that should prevent the transmission of potentially harmful SMB2 and SMB3 traffic. The flaw manifests when the system's detection engine fails to properly assess SMB2 or SMB3 files based on their total file length, effectively allowing unauthorized access to systems that should be protected by configured file action policies.
The technical exploitation of this vulnerability occurs through the manipulation of SMB2 or SMB3 transfer requests that are designed to trigger the bypass mechanism. Attackers can craft specific SMB2 or SMB3 protocol requests that exploit the flawed detection logic, enabling them to deliver malware or other malicious content through the targeted Cisco Firepower device. The system's inability to correctly identify malicious SMB2 or SMB3 files based on their length characteristics creates a persistent security gap that can be leveraged by unauthenticated remote attackers. This vulnerability specifically affects devices running Cisco Firepower System Software versions prior to 6.2.3, where the file action policies that should block SMB2 and SMB3 protocols are rendered ineffective due to the flawed detection engine implementation. The attack vector requires no authentication and can be executed remotely, making it particularly dangerous for network security infrastructure. This weakness directly relates to CWE-20, which addresses "Improper Input Validation," and demonstrates how flawed validation logic can result in complete bypass of security controls.
The operational impact of CVE-2018-0243 extends beyond simple protocol bypass, as it fundamentally compromises the security posture of organizations relying on Cisco Firepower systems for network protection. When malicious SMB2 or SMB3 traffic bypasses configured policies, it creates opportunities for lateral movement within networks, data exfiltration, and the deployment of additional malware or backdoors. The vulnerability is particularly concerning because it affects the core functionality of file action policies that are designed to protect against known threats in SMB protocol traffic, which represents a significant portion of enterprise file sharing communications. Organizations may experience unauthorized access to sensitive systems and data, as the device configured to block potentially malicious SMB traffic becomes ineffective. The fact that this vulnerability does not affect SMB1 protocols means that organizations may have a false sense of security regarding their SMB traffic protection, while simultaneously leaving their SMB2 and SMB3 communications exposed to exploitation. This vulnerability also aligns with ATT&CK technique T1071.004, which covers "Application Layer Protocol: SMB/Windows Admin Shares," demonstrating how protocol-level weaknesses can enable broader attack chains. The impact is compounded by the fact that the vulnerability affects systems where file action policies are actively configured, meaning that organizations with robust security policies in place are still vulnerable to this specific bypass mechanism.
Mitigation strategies for CVE-2018-0243 focus primarily on upgrading affected Cisco Firepower System Software to version 6.2.3 or later, which contains the necessary patches to address the flawed detection engine logic. Organizations should also implement additional network segmentation and monitoring controls to detect anomalous SMB2 and SMB3 traffic patterns that could indicate exploitation attempts. Network administrators should review and validate their existing file action policies to ensure they are properly configured and that monitoring systems are alerting on suspicious SMB traffic. The vulnerability's nature as a detection engine flaw makes it particularly important for organizations to maintain current software versions and security updates, as the patch addresses the root cause of the incorrect file length evaluation logic. Security teams should also consider implementing additional intrusion detection systems or network monitoring solutions that can identify malformed SMB2 or SMB3 requests that attempt to exploit this vulnerability. Organizations should conduct thorough vulnerability assessments to identify all instances of affected Firepower systems and prioritize remediation efforts based on the criticality of the devices and their role in network security infrastructure. The remediation process should include verification that the software upgrade has properly addressed the detection engine behavior and that file action policies are functioning correctly for SMB2 and SMB3 protocols.