CVE-2006-4758 in phpBB
Summary
by MITRE
phpBB 2.0.21 does not properly handle pathnames ending in %00, which allows remote authenticated administrative users to upload arbitrary files, as demonstrated by a query to admin/admin_board.php with an avatar_path parameter ending in .php%00.
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Analysis
by VulDB Data Team • 07/09/2019
The vulnerability described in CVE-2006-4758 represents a critical file upload security flaw within phpBB version 2.0.21 that exploits improper handling of null byte sequences in file pathnames. This issue specifically affects authenticated administrative users who can leverage the vulnerability to execute arbitrary code on the target system through malicious file uploads. The flaw manifests when the application processes pathnames containing the null byte sequence %00, which is a standard representation of a null terminator in URL encoding. This particular vulnerability resides in the administration panel functionality, specifically within the admin_board.php script that manages board configuration settings including avatar path specifications.
The technical exploitation occurs through the avatar_path parameter manipulation where an attacker can append .php%00 to the end of a filename, effectively bypassing normal file extension validation mechanisms. When phpBB processes this malformed pathname, the null byte terminates the string processing, causing the application to interpret the filename as ending with .php while actually storing it with the null terminator, which allows the file to be saved with an executable extension. This behavior stems from inadequate input sanitization and string handling routines that fail to properly validate or sanitize user-supplied pathnames before processing them in the file system context. The vulnerability directly relates to CWE-174, which describes the weakness of insufficient input sanitization of null bytes in file paths, and can be classified under ATT&CK technique T1505.003 for server-side include injection through file upload mechanisms.
The operational impact of this vulnerability is severe as it provides authenticated administrative users with the capability to upload malicious files that can execute arbitrary code on the web server. Once an attacker gains administrative access through legitimate credentials, they can upload a PHP shell or other malicious payloads that persist on the server and allow for continued unauthorized access. The vulnerability enables a wide range of malicious activities including data exfiltration, system compromise, and potential lateral movement within the network. The attack vector requires only an authenticated administrative account, making it particularly dangerous as it leverages legitimate administrative privileges rather than requiring additional exploitation techniques. This vulnerability can be exploited to establish persistent backdoors, escalate privileges, or compromise the entire web application infrastructure.
Mitigation strategies for CVE-2006-4758 should focus on implementing robust input validation and sanitization mechanisms within the phpBB application. The most effective approach involves updating to phpBB version 2.0.22 or later, which contains the necessary patches to address the null byte handling issue. Administrators should also implement strict file extension validation that rejects any filenames containing null bytes or other potentially malicious sequences. Additional security measures include configuring the web server to reject files with executable extensions in upload directories, implementing proper file permissions, and monitoring for suspicious file upload activities. Network-level protections such as web application firewalls can help detect and block malicious upload attempts, while regular security audits should verify that no vulnerable versions remain in production environments. The vulnerability demonstrates the critical importance of proper input validation and string handling in web applications, particularly when dealing with file system operations and user-supplied data that could influence application behavior.