CVE-2018-0254 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 configured file action policies if an Intelligent Application Bypass (IAB) with a drop percentage threshold is also configured. The vulnerability is due to incorrect counting of the percentage of dropped traffic. An attacker could exploit this vulnerability by sending network traffic to a targeted device. An exploit could allow the attacker to bypass configured file action policies, and traffic that should be dropped could be allowed into the network. Cisco Bug IDs: CSCvf86435.
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Analysis
by VulDB Data Team • 01/30/2020
The vulnerability described in CVE-2018-0254 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 affects the Intelligent Application Bypass (IAB) functionality, which is designed to optimize network performance by selectively dropping traffic based on predefined thresholds. The vulnerability stems from an incorrect calculation of drop percentages within the IAB system, creating a scenario where malicious traffic can evade security controls that should have blocked it. The flaw manifests when an IAB policy with a drop percentage threshold is configured alongside file action policies, creating a pathway for attackers to circumvent intended security measures. This issue affects Cisco Firepower devices running specific software versions where the IAB engine fails to properly count and enforce the configured drop percentages, potentially allowing unauthorized network access.
The technical implementation of this vulnerability involves a fundamental error in the percentage calculation logic within the Firepower detection engine's IAB subsystem. When network traffic flows through a device with both IAB and file action policies configured, the system incorrectly processes the drop percentage calculations, leading to a misapplication of security controls. The vulnerability specifically impacts the traffic counting mechanism that determines whether traffic should be dropped or allowed based on the configured threshold. This miscounting occurs at the policy enforcement level where the system fails to accurately track the percentage of traffic that should be dropped versus traffic that should be permitted. The flaw represents a classic case of incorrect arithmetic or logic implementation in security-critical systems, where a simple miscalculation can result in complete bypass of security controls. The vulnerability is particularly concerning because it operates at the network layer where traffic is processed and filtered, making it difficult to detect and trace back to its source.
The operational impact of CVE-2018-0254 extends beyond simple traffic bypass, creating significant risks to network security posture and potentially enabling more sophisticated attacks. An unauthenticated remote attacker can exploit this vulnerability to allow malicious traffic that should have been blocked by file action policies to enter the network undetected. This could result in data exfiltration, command and control communications, or other malicious activities that bypass the network's primary security controls. The vulnerability essentially creates a backdoor where attackers can selectively send traffic that meets the IAB drop percentage criteria but still manages to bypass the file action policies, effectively rendering the configured security measures ineffective. Organizations may experience false security confidence due to the presence of seemingly robust file action policies while unknowingly allowing malicious traffic to flow through their networks. The impact is particularly severe in environments where file inspection and blocking are critical for preventing malware delivery and other security threats, as the vulnerability directly undermines these protective mechanisms.
Mitigation strategies for CVE-2018-0254 require immediate attention and careful implementation to address the underlying IAB percentage calculation flaw. Organizations should prioritize applying the relevant Cisco security patches and updates that correct the percentage counting logic within the Firepower detection engine. Until patches are applied, administrators should consider disabling IAB policies that contain drop percentage thresholds or reconfiguring these policies to use alternative enforcement mechanisms. Network segmentation and additional security controls should be implemented as compensating measures to reduce the attack surface while the primary vulnerability is addressed. Security monitoring should be enhanced to detect unusual traffic patterns that might indicate exploitation attempts, particularly around the time when IAB policies are triggered. The vulnerability aligns with CWE-191, which addresses integer underflow and overflow conditions, as the percentage calculation error could stem from improper handling of numeric values in the IAB subsystem. From an ATT&CK perspective, this vulnerability maps to technique T1071.004 for application layer protocol usage and T1566 for phishing attacks, as bypassed file inspection could allow malicious attachments to reach endpoints. Organizations should also review their network traffic analysis capabilities to ensure they can detect anomalous behavior that might indicate exploitation attempts, given that this vulnerability allows traffic to bypass standard security controls without generating typical security alerts.