CVE-2006-6301 in DenyHosts
Summary
by MITRE
DenyHosts 2.5 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 login name containing certain strings with an IP address, which is not properly handled by a regular expression.
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Analysis
by VulDB Data Team • 07/13/2019
The vulnerability identified as CVE-2006-6301 affects DenyHosts version 2.5, a security tool designed to protect sshd servers from brute force attacks by automatically blocking IP addresses that attempt unauthorized logins. This flaw represents a critical input validation issue that undermines the security mechanisms intended to protect the system. The vulnerability stems from improper parsing of sshd log files, creating a condition where malicious actors can manipulate the system's behavior through crafted log entries. The flaw specifically manifests in the regular expression handling used to process login attempts, allowing attackers to inject arbitrary IP addresses that will be incorrectly interpreted and added to the system's hosts.deny file.
The technical implementation of this vulnerability involves a regex parsing error that occurs when DenyHosts processes sshd log entries containing specially crafted login names. When users attempt to log in with login names that contain IP address patterns, the regular expression fails to properly sanitize or validate the input before processing it for inclusion in the hosts.deny file. This parsing failure creates a path for arbitrary code execution through legitimate system interfaces, as the tool's defensive mechanism becomes compromised and turns into an attack vector. The vulnerability operates at the application level, specifically targeting the log file parsing functionality that is fundamental to DenyHosts' operation.
From an operational perspective, this vulnerability creates a severe denial of service condition where remote attackers can effectively disable legitimate access to the sshd service. The attacker can add arbitrary IP addresses to the hosts.deny file, which will then block legitimate users from accessing the system while simultaneously allowing the attacker to bypass normal access controls. This creates a situation where the security tool becomes a weapon rather than a protective mechanism, undermining the trust model that system administrators rely upon when deploying such tools. The impact extends beyond simple service disruption to include potential unauthorized access and complete compromise of the system's authentication controls.
The vulnerability aligns with CWE-20, which describes improper input validation in software systems, and demonstrates characteristics consistent with CWE-77, which covers command and buffer injection flaws. From an ATT&CK framework perspective, this vulnerability maps to T1133 - External Remote Services and T1078 - Valid Accounts, as it allows attackers to manipulate system access controls and gain persistent access through legitimate authentication mechanisms. The attack surface is particularly concerning as it requires no special privileges beyond basic network access to execute the exploit, making it highly accessible to malicious actors. Organizations deploying DenyHosts should immediately implement mitigations including updating to patched versions, implementing additional input validation, and monitoring log file integrity to prevent exploitation of this vulnerability.
This vulnerability highlights the critical importance of proper input sanitization in security tools, particularly those that operate on log file data. The flaw demonstrates how defensive mechanisms can be subverted when basic security principles are not properly applied to input processing. System administrators should consider implementing additional monitoring and validation controls around log file parsing operations, as well as ensuring that security tools themselves are regularly updated and audited for similar vulnerabilities. The incident underscores the need for comprehensive security testing that includes adversarial input validation scenarios to prevent exactly these types of bypass vulnerabilities.