CVE-2023-27100: Netgate pfSense Plus Auth Bypass Flaw

CVE-2023-27100 Overview

CVE-2023-27100 is an Authentication Bypass vulnerability affecting the SSHGuard component of Netgate pfSense Plus software v22.05.1 and pfSense CE software v2.6.0. This vulnerability allows attackers to bypass brute force protection mechanisms via crafted web requests, effectively defeating a critical security control designed to prevent credential-stuffing and password-guessing attacks.

Critical Impact

Attackers can bypass brute force protection mechanisms, enabling unrestricted authentication attempts against pfSense systems without triggering lockouts or rate limiting.

Affected Products

• Netgate pfSense Plus v22.05.1
• pfSense CE v2.6.0
• SSHGuard component integrated within affected pfSense versions

Discovery Timeline

• 2023-03-22 - CVE CVE-2023-27100 published to NVD
• 2025-02-25 - Last updated in NVD database

Technical Details for CVE-2023-27100

Vulnerability Analysis

This vulnerability is classified under CWE-307 (Improper Restriction of Excessive Authentication Attempts). The SSHGuard component in pfSense is designed to monitor authentication logs and automatically block IP addresses that exhibit suspicious behavior, such as repeated failed login attempts. However, a flaw in the implementation allows attackers to craft specific web requests that circumvent these protective measures entirely.

The vulnerability enables network-based attacks without requiring any privileges or user interaction. When successfully exploited, attackers gain the ability to conduct unlimited brute force attacks against authentication endpoints, potentially compromising confidentiality, integrity, and availability of the affected system.

Root Cause

The root cause lies in the improper restriction of excessive authentication attempts within the SSHGuard component. The protection mechanism fails to properly account for certain crafted web requests, creating a gap in the rate-limiting logic. This design flaw allows malicious actors to structure their authentication attempts in a way that evades detection and blocking by the SSHGuard service.

Attack Vector

The attack vector is network-based, meaning attackers can exploit this vulnerability remotely without physical access to the target system. The attack requires low complexity to execute and does not require any prior authentication or privileges on the target system. By crafting specific HTTP requests, attackers can repeatedly attempt authentication without triggering the brute force protection that would normally block their IP address after multiple failed attempts.

The exploitation technique involves sending authentication requests in a manner that the SSHGuard component does not properly track or correlate, effectively making each attempt appear as a new, isolated event rather than part of a sustained attack pattern.

Detection Methods for CVE-2023-27100

Indicators of Compromise

• Unusually high volumes of authentication attempts from single or multiple IP addresses without corresponding SSHGuard blocks
• Authentication log entries showing repeated failed logins that do not trigger expected lockout behavior
• Web server logs containing crafted request patterns targeting authentication endpoints

Detection Strategies

• Monitor authentication logs for anomalous patterns where failed login counts exceed SSHGuard threshold without blocks being applied
• Implement additional logging at the web server level to capture request details that may indicate bypass attempts
• Deploy network intrusion detection systems (IDS) with rules to identify brute force patterns against pfSense web interfaces

Monitoring Recommendations

• Enable verbose logging on both SSHGuard and the pfSense web authentication service
• Correlate authentication failure events with SSHGuard blocking actions to identify discrepancies
• Set up alerts for authentication attempt volumes that exceed normal baseline metrics

How to Mitigate CVE-2023-27100

Immediate Actions Required

• Update pfSense Plus to a version newer than v22.05.1 that contains the security fix
• Update pfSense CE to a version newer than v2.6.0 that addresses this vulnerability
• Review authentication logs for signs of prior exploitation attempts
• Consider implementing additional network-level access controls for management interfaces

Patch Information

Netgate has released security patches addressing this vulnerability. Administrators should consult the Netgate pfSense Security Advisory for detailed patch information and upgrade instructions. Additional technical discussion is available on the pfSense Issue Tracker Entry.

Workarounds

• Restrict access to pfSense management interfaces to trusted IP addresses only using firewall rules
• Implement VPN requirements for administrative access to prevent direct exposure of authentication endpoints
• Deploy an external Web Application Firewall (WAF) with brute force protection capabilities in front of pfSense management interfaces
• Consider disabling web-based management access entirely and relying on console or SSH access with key-based authentication

# Example: Restrict web GUI access to trusted management network only
# Add this rule before any allow rules for the web interface
# In pfSense Firewall > Rules > WAN (or appropriate interface)
# Action: Block
# Interface: WAN
# Protocol: TCP
# Destination Port: 443 (or your web GUI port)
# Source: NOT your trusted management IP/subnet