CVE-2007-4323 in DenyHosts
Summary
by MITRE
DenyHosts 2.6 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-2006-6301.
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Analysis
by VulDB Data Team • 07/29/2019
The vulnerability described in CVE-2007-4323 affects DenyHosts version 2.6, a security tool designed to protect sshd servers from brute force attacks by automatically adding malicious IP addresses to the /etc/hosts.deny file. This flaw represents a significant security weakness in the tool's log parsing mechanism that enables remote attackers to manipulate the system's access control policies. The vulnerability operates through a sophisticated attack vector that leverages the structured format of sshd log entries to inject malicious IP addresses into the system's denial of service configuration. The flaw specifically manifests when DenyHosts processes sshd log files without proper validation of the log entry content, allowing attackers to craft malicious log entries that appear legitimate to the parsing logic.
The technical implementation of this vulnerability stems from improper input validation within the sshd log parsing component of DenyHosts. When an attacker establishes an ssh connection to a vulnerable system, they can manipulate the client protocol version identification string to include an IP address that matches the pattern expected by DenyHosts for parsing. This allows the attacker to inject arbitrary IP addresses into the system's access control mechanisms through what appears to be normal ssh connection logging. The vulnerability is particularly concerning because it operates at the intersection of log file processing and system security controls, where malformed input can be transformed into legitimate system modifications. This represents a classic case of insecure input handling that violates fundamental security principles and enables privilege escalation through indirect means.
The operational impact of this vulnerability extends beyond simple denial of service to encompass potential system compromise and unauthorized access control modifications. An attacker who successfully exploits this vulnerability can effectively bypass the intended security protections of DenyHosts by adding arbitrary IP addresses to the /etc/hosts.deny file, thereby creating a backdoor or disabling legitimate access for authorized users. The attack requires minimal privileges and can be executed through standard ssh connection attempts, making it particularly dangerous in production environments where ssh access is common. This vulnerability essentially transforms a security tool designed to protect against unauthorized access into a mechanism that can be exploited to undermine its own protective function, creating a paradoxical security risk that affects the integrity of the system's access control policies.
Mitigation strategies for this vulnerability require immediate patching of the DenyHosts software to implement proper input validation and sanitization of sshd log entries. System administrators should also consider implementing additional monitoring and logging controls to detect anomalous patterns in sshd log files that might indicate exploitation attempts. The fix should include comprehensive validation of log entry formats and content to ensure that only legitimate connection attempts are processed by the DenyHosts parsing logic. Organizations should also implement network segmentation and access control measures to limit the potential impact of such vulnerabilities, while maintaining regular security audits to identify similar parsing flaws in other security tools. This vulnerability aligns with CWE-20, which addresses improper input validation, and represents a typical attack pattern found in the ATT&CK framework under privilege escalation techniques that leverage insecure parsing mechanisms.