CVE-2006-6302 in fail2ban
Summary
by MITRE
fail2ban 0.7.4 and earlier does not properly parse sshd log files, which allows remote attackers to add arbitrary hosts to the /etc/hosts.deny file and cause a denial of service by adding arbitrary IP addresses to the sshd log file, as demonstrated by logging in via ssh with a login name containing certain strings with an IP address.
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Analysis
by VulDB Data Team • 07/14/2019
The vulnerability identified as CVE-2006-6302 affects fail2ban versions 0.7.4 and earlier, presenting a critical security flaw in the log parsing mechanism for sshd services. This vulnerability stems from improper handling of sshd log file entries, creating a scenario where remote attackers can manipulate the system's intrusion detection and prevention capabilities. The flaw specifically targets the parsing logic that processes authentication attempts and subsequent blocking actions within the fail2ban framework, which is designed to automatically block malicious IP addresses attempting unauthorized access to ssh services.
The technical exploitation of this vulnerability occurs through carefully crafted login attempts that contain specific string patterns including embedded IP addresses within the login name parameter. When sshd processes these malformed login attempts, it generates log entries that fail2ban's parser incorrectly interprets, leading to arbitrary IP addresses being added to the /etc/hosts.deny file without proper validation or authentication. This misinterpretation represents a classic input validation flaw that falls under the CWE-20 category of "Improper Input Validation" and demonstrates weaknesses in the security configuration management process. The vulnerability operates at the intersection of log parsing and access control mechanisms, allowing attackers to leverage legitimate authentication logging processes to manipulate system security policies.
The operational impact of this vulnerability extends beyond simple denial of service to encompass potential privilege escalation and unauthorized access to system resources. By adding arbitrary IP addresses to the hosts.deny file, attackers can effectively create a backdoor or bypass mechanism that allows them to establish persistent access to the system while simultaneously disrupting legitimate user access. This vulnerability affects the fundamental integrity of the fail2ban security framework, which is designed to protect against brute force attacks and unauthorized access attempts. The attack vector demonstrates sophisticated understanding of how security tools can be subverted through manipulation of their input processing mechanisms, representing a significant threat to system administrators who rely on automated security measures.
The mitigation strategies for this vulnerability require immediate patching of fail2ban installations to versions that properly validate log file entries and implement robust input sanitization techniques. System administrators should also implement additional monitoring of the /etc/hosts.deny file and log parsing processes to detect anomalous entries that may indicate exploitation attempts. Network security controls including intrusion detection systems and firewall rules should be configured to monitor for unusual patterns in authentication attempts and access control modifications. This vulnerability highlights the importance of proper input validation and the need for comprehensive security testing of automated security tools. Organizations should also consider implementing additional layers of security monitoring and log analysis to detect potential exploitation attempts before they can cause significant damage. The flaw demonstrates the critical importance of validating all inputs regardless of their source and implementing proper access control mechanisms to prevent unauthorized modification of security configuration files. This vulnerability serves as a reminder of the potential for security tools themselves to become attack vectors when not properly secured against malicious input manipulation.