CVE-2020-26693 in pfSense
Summary
by MITRE • 06/02/2021
A stored cross-site scripting (XSS) vulnerability was discovered in pfSense 2.4.5-p1 which allows an authenticated attacker to execute arbitrary web scripts via exploitation of the load_balancer_monitor.php function.
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Analysis
by VulDB Data Team • 06/03/2021
The vulnerability CVE-2020-26693 represents a critical stored cross-site scripting flaw in pfSense version 2.4.5-p1 that fundamentally compromises the security of network infrastructure devices. This vulnerability exists within the load_balancer_monitor.php component of the pfSense web interface, which is part of the widely deployed open-source firewall and router platform used by thousands of organizations worldwide. The flaw enables authenticated attackers with valid credentials to inject malicious scripts that persist within the application's data storage, making it particularly dangerous as the malicious code executes automatically whenever affected pages are accessed by other users. The vulnerability directly violates security principle of input validation and output encoding, creating a persistent threat vector that can be exploited across multiple user sessions.
The technical exploitation of this vulnerability occurs through the load_balancer_monitor.php function which fails to properly sanitize user-supplied input before storing and rendering it within the web interface. An attacker with valid login credentials can craft malicious payloads that get stored in the system's monitoring configuration, specifically within load balancer monitoring parameters. When other authenticated users access the affected monitoring pages, their browsers execute the stored malicious scripts, potentially leading to session hijacking, credential theft, or redirection to malicious sites. This stored XSS vulnerability maps directly to CWE-79 which defines the weakness of insufficient input validation in web applications, and the attack surface is significantly amplified due to the privileged nature of pfSense administrators who typically possess broad network access permissions.
The operational impact of CVE-2020-26693 extends far beyond simple script execution, as pfSense administrators often maintain elevated privileges within network environments and may have access to sensitive network monitoring data. An attacker who successfully exploits this vulnerability can establish persistent access to network monitoring functions, potentially gaining insights into network traffic patterns, system configurations, and security event logs. The attack vector aligns with ATT&CK technique T1059.007 for command and scripting interpreter and T1566.001 for spearphishing attachment, as attackers can use the stored XSS to deliver additional malicious payloads or establish command and control channels. Organizations using pfSense for critical network infrastructure protection face significant risk of lateral movement within their networks, as the compromised monitoring system could provide attackers with information about network topology and security controls.
Mitigation strategies for this vulnerability require immediate patching of pfSense installations to version 2.4.5-p2 or later, which includes proper input sanitization and output encoding fixes. Network administrators should implement additional security controls such as monitoring for suspicious configuration changes, enabling web application firewalls to detect XSS patterns, and conducting regular security assessments of network infrastructure management interfaces. The remediation process should include comprehensive user access reviews to ensure only necessary personnel maintain administrative privileges, as well as implementing multi-factor authentication for all administrative accounts. Security teams must also establish monitoring procedures to detect unauthorized modifications to load balancer configurations and other system monitoring components, as these changes may indicate exploitation attempts. Organizations should consider implementing network segmentation to limit the scope of potential compromise and ensure that monitoring systems are regularly audited for configuration integrity.