CVE-2005-4163 in Captcha PHP
Summary
by MITRE
Directory traversal vulnerability in captcha.php in Captcha PHP 0.9 allows remote attackers to read arbitrary files via the _tcf parameter.
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Analysis
by VulDB Data Team • 07/27/2017
The vulnerability identified as CVE-2005-4163 represents a critical directory traversal flaw within the Captcha PHP 0.9 library, specifically affecting the captcha.php script. This vulnerability stems from inadequate input validation and sanitization mechanisms that fail to properly restrict user-supplied data from being processed within file system operations. The flaw manifests through the _tcf parameter which is directly incorporated into file path resolution without appropriate filtering or normalization, creating an exploitable condition that enables malicious actors to navigate the file system beyond intended boundaries. The vulnerability is classified under CWE-22 as a directory traversal attack, where an attacker can manipulate input to access files outside the restricted directory structure.
The technical exploitation of this vulnerability occurs when a remote attacker crafts a malicious request containing a specially formatted _tcf parameter value that includes directory traversal sequences such as ../ or ..\ characters. When the captcha.php script processes this parameter, it directly uses the unvalidated input to construct file paths, allowing attackers to reference files anywhere within the web server's file system. This can lead to unauthorized access to sensitive system files, configuration data, source code, or other confidential information that should remain protected from external access. The vulnerability demonstrates poor input validation practices and lacks proper path normalization, making it particularly dangerous as it can be exploited from any remote location without requiring authentication or prior access to the system.
The operational impact of CVE-2005-4163 extends beyond simple information disclosure, as successful exploitation can provide attackers with complete access to the underlying file system. This could result in the exposure of database credentials, application configuration files, user data, or even system binaries that could be used for further exploitation. The vulnerability affects any system running Captcha PHP 0.9 where the captcha.php script is accessible via web requests, potentially compromising entire web applications that rely on this library for security verification. The attack vector is particularly concerning because it requires minimal privileges and can be executed through standard web browser interactions, making it accessible to attackers with basic technical knowledge. This vulnerability aligns with ATT&CK technique T1083 (File and Directory Discovery) and T1566 (Phishing) as attackers could use the information gathered to craft more sophisticated attacks or establish persistence within the compromised environment.
Mitigation strategies for this vulnerability must address both immediate remediation and long-term security improvements. The most effective immediate solution involves implementing proper input validation and sanitization for all user-supplied parameters, particularly those used in file system operations. This includes normalizing file paths, implementing whitelist validation for acceptable parameter values, and ensuring that directory traversal sequences are explicitly blocked or removed from input data. System administrators should update to patched versions of Captcha PHP or replace the vulnerable library with a more secure alternative. Additionally, implementing proper access controls and file permissions can limit the damage from successful exploitation attempts. The vulnerability highlights the importance of secure coding practices and input validation, which are fundamental requirements in the OWASP Top Ten and ISO/IEC 27001 security standards. Regular security audits and penetration testing should be conducted to identify similar vulnerabilities in other components of the web application stack, as directory traversal flaws often indicate broader security weaknesses in input handling mechanisms.