CVE-2007-5715 in DenyHosts
Summary
by MITRE
DenyHosts 2.6 processes OpenSSH sshd "not listed in AllowUsers" log messages with an incorrect regular expression that does not match an IP address, which might allow remote attackers to avoid detection and blocking when making invalid login attempts with a username not present in AllowUsers, as demonstrated by the root username, a different vulnerability than CVE-2007-4323.
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Analysis
by VulDB Data Team • 09/10/2018
The vulnerability identified as CVE-2007-5715 affects DenyHosts version 2.6, a security tool designed to protect against brute force attacks on OpenSSH servers by automatically blocking IP addresses that attempt invalid login credentials. This flaw represents a critical weakness in the system's ability to detect and respond to unauthorized access attempts, specifically targeting the tool's regex pattern matching mechanism that processes sshd log messages. The vulnerability stems from an incorrect regular expression implementation that fails to properly extract IP addresses from log entries containing "not listed in AllowUsers" messages, which are generated when users attempt to log in with usernames not present in the AllowUsers configuration directive.
The technical implementation of this vulnerability occurs within the DenyHosts daemon's log message parsing functionality, where the tool attempts to identify malicious IP addresses by analyzing sshd log output. When OpenSSH generates a log message indicating that a login attempt failed because the username was not listed in AllowUsers, DenyHosts should extract the originating IP address to add to its blacklist. However, due to the flawed regular expression, the IP address extraction fails, causing the system to ignore these specific types of log entries. This misconfiguration allows attackers to bypass detection mechanisms that would normally flag repeated failed login attempts from the same IP address, enabling prolonged unauthorized access attempts without triggering the automatic blocking behavior.
The operational impact of this vulnerability extends beyond simple detection failure, as it creates a window of opportunity for attackers to conduct extended brute force campaigns against systems protected by DenyHosts. When an attacker attempts to log in with a username not present in AllowUsers, such as the root account, the system generates log messages that should trigger immediate blocking actions. However, because the IP address cannot be properly extracted from these messages, the blocking mechanism fails to activate, allowing attackers to continue their attempts without restriction. This vulnerability demonstrates a clear deviation from the expected behavior outlined in security best practices and represents a failure in the principle of least privilege enforcement, where unauthorized access attempts should be immediately detected and blocked.
The flaw directly relates to CWE-20, which addresses "Improper Input Validation," and specifically manifests as a regex injection vulnerability that undermines the integrity of log message processing within security tools. From an adversarial perspective, this vulnerability aligns with ATT&CK technique T1110.003, which covers "Brute Force: Password Guessing," as it enables attackers to conduct sustained password guessing campaigns without detection. The vulnerability also connects to ATT&CK technique T1078.002, "Valid Accounts: Default Accounts," as it allows attackers to exploit default account names like root without triggering protective measures. Organizations implementing DenyHosts as part of their security infrastructure may experience false security confidence, believing their systems are protected against brute force attacks when in reality, certain attack vectors remain undetected and unblocked.
Mitigation strategies for this vulnerability require immediate patching of DenyHosts to version 2.6 or later, where the regular expression has been corrected to properly extract IP addresses from sshd log messages. System administrators should also implement additional monitoring solutions to detect when DenyHosts fails to process specific log message patterns, as well as consider implementing alternative security measures such as fail2ban or custom iptables rules that can provide more robust protection against brute force attacks. Organizations should conduct thorough audits of their security tool configurations to ensure that all log message patterns are properly handled and that detection mechanisms are functioning as intended. The vulnerability highlights the critical importance of proper input validation and regular security tool updates, as even minor regex implementation flaws can have significant security implications for network protection systems.