CVE-2018-20799 in pfSense
Summary
by MITRE
In pfSense 2.4.4_1, blocking of source IP addresses on the basis of failed HTTPS authentication is inconsistent with blocking of source IP addresses on the basis of failed SSH authentication (the behavior does not match the sshguard documentation), which might make it easier for attackers to bypass intended access restrictions.
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Analysis
by VulDB Data Team • 07/26/2023
The vulnerability identified as CVE-2018-20799 affects pfSense version 2.4.4_1 and relates to inconsistent behavior in source IP address blocking mechanisms for different authentication protocols. This inconsistency creates a security gap that could potentially allow attackers to bypass intended access controls by exploiting the differing blocking behaviors between HTTPS and SSH authentication failures. The issue stems from the implementation not aligning with established sshguard documentation standards, which govern how failed authentication attempts should be handled to prevent unauthorized access.
The technical flaw manifests in the differential handling of failed authentication attempts across protocols within the pfSense firewall system. When HTTPS authentication fails, the system applies source IP blocking in a manner that differs from the SSH authentication failure blocking mechanism. This inconsistency creates a potential attack surface where malicious actors could systematically target one protocol while exploiting the weaker blocking behavior of the other. The vulnerability essentially allows for protocol-specific bypass techniques that would not be possible if both protocols followed identical blocking patterns as specified in sshguard documentation.
From an operational impact perspective, this vulnerability undermines the integrity of the firewall's access control mechanisms by creating inconsistent security postures across different authentication protocols. Administrators who rely on pfSense's built-in protection against brute force attacks may find their security measures weakened when attackers exploit the protocol-specific inconsistencies. The vulnerability could enable attackers to perform successful authentication attempts against one protocol while being blocked from another, potentially allowing for extended attack windows or more sophisticated exploitation techniques that leverage the inconsistent blocking behavior.
The security implications extend beyond simple access control bypasses, as this inconsistency could facilitate credential stuffing attacks or more complex multi-stage attacks that take advantage of the differing blocking mechanisms. Attackers could potentially determine which protocol is more effectively protected by observing blocking behavior patterns, thereby optimizing their attack strategies. This issue represents a deviation from established security best practices and could lead to increased vulnerability to automated attack tools that specifically target such inconsistencies in firewall implementations.
Mitigation strategies should focus on ensuring consistent implementation of authentication blocking across all protocols within the pfSense system. Administrators should consider implementing additional monitoring and logging mechanisms to detect and respond to suspicious authentication patterns that might exploit this inconsistency. The recommended approach involves aligning the blocking behavior with established sshguard standards and potentially implementing additional layers of protection such as rate limiting, IP reputation filtering, or more robust centralized authentication management systems. Organizations should also consider updating to newer pfSense versions where this inconsistency has been addressed and validate that their current security policies account for protocol-specific variations in blocking behavior. This vulnerability highlights the importance of maintaining consistent security controls across all network services and demonstrates how seemingly minor implementation differences can create significant security weaknesses that adversaries can exploit to bypass intended protections.