CVE-2006-6763 in Keep It Simple Guest Book
Summary
by MITRE
Multiple PHP remote file inclusion vulnerabilities in the Keep It Simple Guest Book (KISGB) allow remote attackers to execute arbitrary PHP code via a URL in the (1) path_to_themes parameter in (a) authenticate.php, and the (2) default_path_for_themes parameter in (b) admin.php and (c) upconfig.php.
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Analysis
by VulDB Data Team • 10/02/2017
The CVE-2006-6763 vulnerability represents a critical remote file inclusion flaw affecting the Keep It Simple Guest Book (KISGB) web application, which operates under the broader category of PHP application security weaknesses. This vulnerability resides in the application's handling of user-supplied input parameters that are directly incorporated into file inclusion operations without proper sanitization or validation. The flaw specifically manifests in three distinct locations within the application's codebase where external URLs are accepted as parameters, creating an attack vector that allows remote threat actors to inject malicious PHP code execution capabilities. The vulnerability affects version 0.9.4 and earlier releases of the KISGB application, making it a significant concern for organizations running outdated web applications that have not received security updates.
The technical exploitation of this vulnerability occurs through manipulation of two primary parameter names within the application's authentication and administrative functions. The first vulnerable parameter path_to_themes in authenticate.php accepts a URL value that gets directly processed by PHP's include or require functions, while the second parameter default_path_for_themes in admin.php and upconfig.php exhibits identical behavior. These parameters are typically used to define the location of theme files within the application's directory structure, but due to insufficient input validation, attackers can supply malicious URLs that point to remote servers hosting attacker-controlled PHP code. When the application processes these parameters, it executes the included code with the privileges of the web server, potentially allowing full system compromise or data exfiltration. This vulnerability directly maps to CWE-88, which describes improper neutralization of special elements used in an expression, and CWE-94, which addresses execution of arbitrary code, both of which are fundamental to the Remote File Inclusion (RFI) attack pattern.
The operational impact of CVE-2006-6763 extends far beyond simple code execution, as it provides attackers with complete control over the affected web server environment. Once successfully exploited, threat actors can upload backdoors, steal sensitive data, modify website content, or use the compromised server as a launch point for further attacks against internal networks. The vulnerability's persistence is particularly concerning since it affects the core authentication and administrative functions of the guest book application, meaning that even legitimate users who attempt to access these functions could inadvertently trigger the exploit. From an ATT&CK framework perspective, this vulnerability enables multiple techniques including T1190 for exploitation of remote services, T1059 for command and scripting interpreter usage, and T1041 for data compression and T1078 for valid accounts usage. The attack chain typically begins with reconnaissance to identify vulnerable applications, followed by crafting malicious URLs that include PHP web shells, and concluding with execution of arbitrary commands on the target system, making it a prime candidate for automated exploitation tools targeting legacy web applications.
Mitigation strategies for CVE-2006-6763 require immediate action to address the root cause through comprehensive input validation and parameter sanitization. Organizations should implement strict input filtering that rejects any URLs containing suspicious protocols or characters, and should disable the ability to pass external URLs to file inclusion functions entirely. The recommended approach involves using whitelisting mechanisms to validate theme paths against a predefined list of acceptable values rather than accepting arbitrary user input. Additionally, the application should be updated to a patched version that removes the vulnerable code paths or implements proper parameter validation. System administrators should also consider implementing web application firewalls to detect and block malicious requests containing suspicious URL patterns. The vulnerability highlights the importance of following secure coding practices such as those outlined in the OWASP Top Ten and the SANS Institute's Secure Coding guidelines, particularly emphasizing the principle of least privilege and input validation as fundamental security controls. Organizations should also conduct comprehensive vulnerability assessments to identify similar issues in other legacy applications that may be susceptible to the same class of vulnerabilities, as the underlying flaw represents a common pattern in poorly secured PHP applications from that era.