CVE-2007-4321 in fail2ban
Summary
by MITRE
fail2ban 0.8 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 client protocol version identification containing an IP address string, a different vector than CVE-2006-6302.
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Analysis
by VulDB Data Team • 12/15/2024
The vulnerability described in CVE-2007-4321 represents a critical flaw in the fail2ban intrusion prevention software version 0.8 and earlier, specifically targeting the sshd log file parsing mechanism. This vulnerability operates at the intersection of log analysis and access control, creating a pathway for remote attackers to manipulate the system's security posture through carefully crafted log entries. The issue stems from insufficient input validation and sanitization within the fail2ban daemon's processing of sshd authentication logs, allowing malicious actors to inject arbitrary IP addresses into the system's access control mechanisms. The vulnerability is particularly concerning because it directly impacts the integrity of the hosts.deny file, which serves as a fundamental access control mechanism in Unix-like systems for denying connections based on IP addresses.
The technical exploitation of this vulnerability occurs through the manipulation of sshd log file entries, specifically targeting the client protocol version identification string that contains IP address information. When fail2ban processes these log entries, it fails to properly validate or sanitize the input data, allowing attacker-controlled IP addresses to be parsed and subsequently added to the /etc/hosts.deny file. This parsing flaw enables an attacker to inject malicious IP addresses into the system's access control configuration, effectively creating a denial of service condition by either blocking legitimate users or creating a scenario where the hosts.deny file becomes overwhelmed with malicious entries. The vulnerability operates as a privilege escalation vector because the attacker does not need direct system access to manipulate the hosts.deny file through the fail2ban process, instead leveraging the legitimate log processing functionality to achieve their objective.
The operational impact of this vulnerability extends beyond simple denial of service, as it fundamentally compromises the integrity of the system's access control mechanisms. Attackers can exploit this weakness to block legitimate users from accessing the system, create confusion in network security monitoring, or potentially use the compromised hosts.deny file as a staging ground for further attacks. The vulnerability demonstrates a classic case of improper input validation where fail2ban's log parser fails to distinguish between legitimate authentication attempts and maliciously crafted log entries. This flaw creates a persistent threat vector that can be exploited repeatedly, as each successful injection of an IP address into the hosts.deny file maintains its effect until manually removed by system administrators. The vulnerability's impact is amplified by the fact that fail2ban is commonly deployed in production environments as a security hardening measure, making the exploitation of its parsing flaws particularly dangerous.
From a security standards perspective, this vulnerability aligns with CWE-20, which describes improper input validation, and demonstrates characteristics consistent with ATT&CK technique T1562.001, specifically "Disable or Modify Tools", where an attacker modifies system-level security controls. The vulnerability also relates to CWE-770, which addresses resource exhaustion, as the hosts.deny file can become overwhelmed with malicious entries. The exploitation pattern shows similarities to privilege escalation techniques where an attacker leverages legitimate system functionality to gain unauthorized control over access control mechanisms. Organizations implementing fail2ban as part of their security infrastructure should consider this vulnerability when assessing their overall security posture, as it represents a fundamental flaw in the software's ability to process log data securely. The vulnerability's classification as a remote attack vector means that it can be exploited from outside the network perimeter, making it particularly dangerous in environments where fail2ban is deployed to protect publicly accessible services.
The recommended mitigations for this vulnerability include immediate upgrade to fail2ban version 0.9 or later, which contains the necessary patches to properly validate log file entries. System administrators should also implement additional monitoring of the hosts.deny file for unauthorized modifications and establish automated alerting mechanisms when unexpected entries are detected. Network segmentation and additional log monitoring solutions can provide defense-in-depth measures to detect and prevent exploitation attempts. Organizations should also review their fail2ban configuration to ensure that log file parsing rules are appropriately restrictive and that the software is running with minimal necessary privileges to limit the impact of potential exploitation. The vulnerability underscores the importance of proper input validation in security-critical software components and highlights the need for regular security updates to address known vulnerabilities in intrusion prevention systems.