CVE-2009-0362 in Fail2ban
Summary
by MITRE
filter.d/wuftpd.conf in Fail2ban 0.8.3 uses an incorrect regular expression that allows remote attackers to cause a denial of service (forced authentication failures) via a crafted reverse-resolved DNS name (rhost) entry that contains a substring that is interpreted as an IP address, a different vulnerability than CVE-2007-4321.
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Analysis
by VulDB Data Team • 05/15/2025
The vulnerability identified as CVE-2009-0362 affects Fail2ban version 0.8.3 and specifically targets the filter.d/wuftpd.conf configuration file. This issue represents a significant security flaw that enables remote attackers to exploit a denial of service condition through manipulated DNS resolution data. The vulnerability stems from improper handling of reverse-resolved DNS names during the authentication process, creating an avenue for malicious actors to disrupt legitimate service operations.
The technical flaw manifests in the regular expression pattern used within the Fail2ban filter configuration. When Fail2ban processes log entries containing reverse-resolved hostnames, the flawed regex pattern incorrectly interprets certain string substrings as IP address formats. This misinterpretation occurs during the parsing of rhost entries from wuftpd log files, where the system attempts to extract and validate IP address information. The incorrect regex pattern fails to properly validate the format of the resolved hostname, allowing attackers to craft malicious DNS entries that appear to contain valid IP addresses.
This vulnerability operates under the broader context of DNS-based attack vectors and demonstrates how seemingly innocuous log parsing operations can become exploitation points. The flaw allows attackers to force authentication failures by submitting crafted reverse DNS entries that bypass normal validation mechanisms. The system's inability to properly distinguish between legitimate IP address formats and maliciously constructed substrings creates a persistent denial of service condition that affects the legitimate operation of the security monitoring system.
The operational impact of this vulnerability extends beyond simple service disruption, as it undermines the integrity of the Fail2ban protection mechanism itself. When exploited successfully, the vulnerability causes the system to generate excessive authentication failure records, potentially triggering false positive security alerts and consuming system resources. This creates a scenario where legitimate security monitoring becomes compromised, as the system's own defensive capabilities are subverted by the attack vector. The vulnerability is particularly concerning because it operates at the configuration level rather than requiring direct system compromise, making it accessible to attackers with limited privileges.
From a cybersecurity perspective, this vulnerability aligns with common attack patterns documented in the MITRE ATT&CK framework, specifically relating to service disruption and resource exhaustion techniques. The flaw represents a type of regex injection or pattern matching vulnerability that can be categorized under CWE-134, which deals with format string vulnerabilities. The attack vector exploits the trust placed in DNS resolution data and demonstrates how configuration-based security systems can be undermined through manipulation of expected data formats. Organizations implementing Fail2ban solutions should consider this vulnerability as part of their broader security posture assessment, particularly in environments where DNS-based authentication and monitoring are critical components of the security infrastructure.
The recommended mitigation strategy involves updating to a patched version of Fail2ban that corrects the regular expression pattern in the wuftpd.conf filter file. Administrators should also implement additional log validation mechanisms and consider deploying more robust input sanitization procedures for DNS resolution data. Network segmentation and monitoring of unusual authentication patterns can help detect exploitation attempts. The vulnerability serves as a reminder of the importance of proper input validation in security tools and the need for comprehensive testing of configuration files against potential attack vectors that manipulate expected data formats.