Fail2ban SSH Filter IP Spoofing Vulnerability

Executive Summary

This vulnerability in fail2ban versions 0.10.5 through 1.1.1 allows an attacker to manipulate the sshd filter by injecting a malicious username containing an IP address followed by 'on'. Fail2ban then incorrectly associates failed login attempts with the injected IP address, leading to false bans of legitimate users.

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Detection Guidance

Check Fail2Ban logs for unusual bans or patterns where legitimate IPs are blocked. Use commands like 'sudo fail2ban-client status sshd' to inspect active bans and 'sudo tail -f /var/log/fail2ban.log' to monitor real-time log entries for injected usernames containing IP addresses or port numbers.

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Impact Analysis

An attacker could exploit this to ban your IP address from accessing a server by crafting a username like 'x YOUR-IP on'. This could lock you out of services protected by fail2ban, disrupting legitimate access.

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Compliance Impact

This vulnerability could indirectly impact compliance with standards like GDPR or HIPAA by enabling denial-of-service conditions or unauthorized IP bans, potentially disrupting legitimate access to systems handling sensitive data. However, the primary impact is on security controls rather than direct compliance violations.

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Mitigation Strategies

Update Fail2Ban to the latest version after commit ddd879d. Review and adjust the sshd.conf filter to ensure it rejects usernames with suspicious suffixes like 'on' or port/IP patterns. Test the updated filter with sample logs to confirm it blocks malicious usernames.

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