CVE-2023-27100 in pfSense Plus
Summary
by MITRE • 03/23/2023
Improper restriction of excessive authentication attempts in the SSHGuard component of Netgate pfSense Plus software v22.05.1 and pfSense CE software v2.6.0 allows attackers to bypass brute force protection mechanisms via crafted web requests.
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Analysis
by VulDB Data Team • 11/15/2025
The vulnerability identified as CVE-2023-27100 represents a critical weakness in the SSHGuard component of Netgate pfSense Plus and pfSense Community Edition software versions 22.05.1 and 2.6.0 respectively. This flaw resides within the authentication mechanism's ability to properly enforce rate limiting and brute force protection controls, creating an exploitable condition that undermines the security posture of systems relying on these firewall distributions. The vulnerability specifically targets the web-based administrative interface components that manage SSHGuard configurations and enforcement policies.
The technical implementation flaw stems from inadequate validation and restriction of authentication attempts within the SSHGuard module's web interface handling. When crafted HTTP requests are submitted to the affected pfSense versions, the system fails to properly enforce the established limits on failed authentication attempts that should normally trigger temporary IP blocking or account lockout mechanisms. This improper restriction allows attackers to repeatedly attempt authentication without triggering the expected protective measures, effectively nullifying the intended security controls. The vulnerability manifests through the web API endpoints that manage SSHGuard policies and monitoring, where insufficient input validation permits malicious actors to bypass the rate limiting controls that are fundamental to preventing automated brute force attacks.
The operational impact of this vulnerability extends beyond simple authentication bypass to potentially compromise entire network security infrastructures that depend on pfSense for firewall and intrusion prevention services. Attackers can leverage this weakness to conduct extended brute force campaigns against SSH services without encountering the normal protective measures that would typically block or slow down automated attack vectors. This creates opportunities for unauthorized access to network resources, potentially leading to full system compromise and lateral movement within the network. The vulnerability particularly affects organizations that rely on pfSense for their perimeter security, as it undermines the fundamental assumption that SSHGuard will properly protect against credential guessing attacks.
Security controls and mitigation strategies should focus on immediate patching of affected software versions to address the core implementation flaw in SSHGuard's web interface handling. Organizations should also implement additional monitoring of authentication attempts and unusual traffic patterns through the web interface to detect potential exploitation attempts. Network segmentation and additional authentication layers such as multi-factor authentication should be considered as compensating controls while awaiting official patches. This vulnerability aligns with CWE-305 authentication bypass weaknesses and maps to ATT&CK technique T1110.003 for Brute Force Attacks, highlighting the need for robust credential protection mechanisms and proper rate limiting enforcement in web-based administrative interfaces. The incident underscores the critical importance of validating authentication attempt limits in web applications and the potential consequences when these controls fail to properly enforce security policies.