CVE-2018-20798 in pfSense
Summary
by MITRE
The expiretable configuration in pfSense 2.4.4_1 establishes block durations that are incompatible with the block durations implemented by sshguard, 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-2018-20798 resides within the pfSense firewall distribution version 2.4.4_1 where the expiretable configuration mechanism fails to properly align with sshguard's block duration implementation. This misalignment creates a critical inconsistency in access control enforcement that directly impacts the effectiveness of intrusion prevention measures. The core issue manifests when pfSense attempts to manage temporary block durations for failed authentication attempts, but the timing mechanisms between the firewall's configuration and sshguard's enforcement system become misaligned, potentially allowing attackers to circumvent intended security restrictions.
This technical flaw represents a configuration compatibility issue that falls under the broader category of improper input validation and inconsistent security policy enforcement. The vulnerability stems from the fundamental mismatch between how pfSense calculates and applies block durations versus how sshguard interprets and enforces these time-based restrictions. When these systems operate with different time scales or interpretation methods for temporary access blocks, attackers can exploit the temporal gap to bypass the intended security controls. The inconsistency creates a window of opportunity where blocked IP addresses may be automatically unblocked before the intended duration expires, or conversely, remain blocked longer than necessary.
The operational impact of this vulnerability extends beyond simple access control bypasses to potentially enable brute force attack campaigns that would otherwise be mitigated by proper temporary blocking mechanisms. Attackers can leverage the timing discrepancies to conduct multiple authentication attempts within the window where the security controls are not properly enforced, effectively neutralizing the intended protection. This issue particularly affects systems that rely heavily on sshguard for automated intrusion detection and response, as the misconfigured block durations can allow malicious actors to repeatedly attempt unauthorized access without facing the full impact of temporary blocking measures.
Security practitioners should consider this vulnerability in the context of the broader ATT&CK framework, specifically under the privilege escalation and credential access categories where adversaries might exploit timing-based controls to maintain persistent access. The weakness aligns with CWE-691, which addresses insufficient control flow management, and CWE-116, concerning improper encoding or escaping of control elements. Organizations utilizing pfSense with sshguard should implement immediate mitigations including manual verification of block duration configurations, potential temporary disablement of auto-blocking features, and implementation of alternative intrusion detection mechanisms. The recommended approach involves synchronizing time-based controls across all security components and ensuring that temporary access restrictions are properly enforced according to established security policies.
The vulnerability demonstrates how seemingly minor configuration inconsistencies can create significant security weaknesses in network defense systems. It highlights the critical importance of proper integration testing between security components and the necessity of maintaining synchronized time-based controls across all security mechanisms. Organizations should conduct comprehensive audits of their security infrastructure to identify similar misalignments between different security tools and ensure proper coordination of temporal access restrictions to maintain effective intrusion prevention capabilities.