CVE-2018-4020 in pfSense
Summary
by MITRE
An exploitable command injection vulnerability exists in the way Netgate pfSense CE 2.4.4-RELEASE processes the parameters of a specific POST request. The attacker can exploit this and gain the ability to execute arbitrary commands on the system. An attacker needs to be able to send authenticated POST requests to the administration web interface. Command injection is possible in the `powerd_ac_mode` POST parameter parameter.
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Analysis
by VulDB Data Team • 06/12/2023
The vulnerability described in CVE-2018-4020 represents a critical command injection flaw within the pfSense community edition version 2.4.4-RELEASE, specifically affecting the web administration interface. This issue stems from inadequate input validation and sanitization mechanisms within the application's processing of HTTP POST requests. The vulnerability is particularly concerning as it allows authenticated attackers to execute arbitrary commands on the underlying system, effectively granting them full administrative control over the network security appliance. The flaw manifests in the `powerd_ac_mode` parameter handling, which demonstrates a classic lack of proper parameter sanitization that enables malicious command injection attacks.
The technical implementation of this vulnerability occurs when the pfSense web interface processes the `powerd_ac_mode` POST parameter without adequate validation or escaping of special characters. This parameter is intended to control power management settings for the system, but due to insufficient input filtering, an attacker can inject malicious shell commands that get executed within the context of the web server process. The vulnerability is classified under CWE-77 as a command injection flaw, which is a well-known weakness that has been consistently exploited in various network appliances and web applications. The attack vector requires authentication to the web administration interface, making it less trivial to exploit but still highly dangerous when successful.
The operational impact of this vulnerability extends far beyond simple privilege escalation, as it provides attackers with complete system compromise capabilities. Once an attacker gains access to the web administration interface, they can execute commands with the privileges of the web server process, typically running as root or with elevated system permissions. This allows for comprehensive system reconnaissance, data exfiltration, persistent backdoor installation, and network traffic interception. The vulnerability is particularly dangerous in network security contexts where pfSense appliances serve as firewalls, routers, and intrusion detection systems, as compromising such devices can lead to complete network infiltration and lateral movement within the organization's infrastructure. The ATT&CK framework categorizes this as a privilege escalation and command execution technique, with potential for persistent access and defense evasion.
Mitigation strategies for CVE-2018-4020 should focus on immediate patching of the pfSense appliance to version 2.4.4-RELEASE-p1 or later, which contains the necessary input validation fixes. Organizations should also implement network segmentation and access controls to limit the attack surface, ensuring that only authorized personnel have access to the web administration interface. Additional defensive measures include monitoring for unusual POST requests containing shell metacharacters, implementing web application firewalls, and conducting regular security audits of network infrastructure components. The vulnerability underscores the critical importance of input validation and the principle of least privilege in securing network appliances, as well as the necessity of maintaining current security patches across all systems. Organizations should also consider implementing multi-factor authentication for administrative access and regular security training for personnel managing such critical infrastructure components.