CVE-2005-3420 in phpBB
Summary
by MITRE
usercp_register.php in phpBB 2.0.17 allows remote attackers to modify regular expressions and execute PHP code via the signature_bbcode_uid parameter, as demonstrated by injecting an "e" modifier into a preg_replace statement.
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Analysis
by VulDB Data Team • 06/11/2019
The vulnerability identified as CVE-2005-3420 represents a critical security flaw in phpBB version 2.0.17 that enables remote code execution through improper input validation in the usercp_register.php script. This issue falls under the category of code injection vulnerabilities and specifically demonstrates how unsafe handling of user-supplied data can lead to complete system compromise. The vulnerability is particularly dangerous because it allows attackers to execute arbitrary PHP code on the target server, effectively granting them full control over the web application and potentially the underlying system.
The technical exploitation occurs through the signature_bbcode_uid parameter which is processed without adequate sanitization or validation. When an attacker submits malicious input containing a regular expression with the "e" modifier, the phpBB application passes this unfiltered data directly to a preg_replace function call. This particular modifier in PHP's regular expression engine executes the replacement string as PHP code, creating a classic code injection vector. The vulnerability is classified as CWE-94 - Improper Control of Generation of Code and aligns with ATT&CK technique T1059.007 - Command and Scripting Interpreter: Unix Shell, as it enables arbitrary code execution through the web application interface.
The operational impact of this vulnerability is severe and far-reaching, as it allows attackers to perform multiple malicious activities including data exfiltration, privilege escalation, and system compromise. An attacker can use this vulnerability to upload additional malicious files, create backdoors, modify existing user accounts, or even establish persistent access to the compromised system. The attack surface is particularly broad since user registration is typically an open process that requires minimal authentication, making this vulnerability exploitable by anyone with access to the phpBB application. The vulnerability also demonstrates poor input validation practices and highlights the importance of proper sanitization of user-supplied data in web applications.
Mitigation strategies for CVE-2005-3420 require immediate action including upgrading to phpBB version 2.0.18 or later where this vulnerability has been patched. Organizations should implement proper input validation and sanitization mechanisms that filter out dangerous characters and modifiers from user inputs, particularly those related to regular expression processing. The use of parameterized queries and input whitelisting should be enforced, along with proper output encoding to prevent code injection attacks. Additionally, implementing web application firewalls and intrusion detection systems can provide additional layers of protection against such exploits. Security teams should also conduct regular vulnerability assessments and code reviews to identify similar patterns in other applications that might be susceptible to the same type of injection attacks, as this vulnerability represents a common weakness in web application security practices that has been documented in numerous other systems over the years.