CVE-2005-2161 in phpBB
Summary
by MITRE
Cross-site scripting (XSS) vulnerability in phpBB 2.0.16 allows remote attackers to inject arbitrary web script or HTML via nested [url] tags.
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Analysis
by VulDB Data Team • 06/06/2019
The vulnerability described in CVE-2005-2161 represents a classic cross-site scripting flaw that affected the widely used phpBB forum software version 2.0.16. This security weakness resides in the message parsing functionality of the bulletin board system, where user input containing nested bbcode url tags is not properly sanitized before being rendered in web pages. The vulnerability specifically manifests when the phpBB application processes forum posts that contain maliciously crafted nested [url] tags, allowing attackers to inject arbitrary web script or HTML content into the forum's display. This type of vulnerability falls under the CWE-79 category of Cross-Site Scripting, which is one of the most prevalent and dangerous web application security flaws identified by the CWE project.
The technical exploitation of this vulnerability occurs through the improper handling of nested bbcode tags within phpBB's parsing engine. When users create posts containing nested [url] tags with malicious payloads, the forum software fails to adequately validate or escape these inputs before rendering them in HTML output. Attackers can leverage this weakness by crafting posts that contain JavaScript code or other malicious HTML within the nested url tag structure, which then executes in the browsers of other forum users who view the affected posts. This vulnerability operates at the application layer and requires no special privileges to exploit, making it particularly dangerous for public forums where multiple users can post content.
The operational impact of CVE-2005-2161 extends beyond simple script execution, as it can enable attackers to perform various malicious activities including session hijacking, credential theft, and redirection to malicious websites. When exploited successfully, this vulnerability allows attackers to inject persistent scripts that can steal cookies, modify forum content, or redirect users to phishing sites. The vulnerability affects all users who view affected posts, making it particularly dangerous in large community forums where thousands of users may be exposed. From an attacker perspective, this vulnerability maps to ATT&CK technique T1566.001 for initial access through malicious web content, and T1059.007 for command and control through script injection. The long-term impact includes potential data compromise and reputational damage to forum administrators who may face legal consequences for hosting malicious content.
Mitigation strategies for this vulnerability require immediate application of security patches provided by phpBB developers, as the fix involves proper input sanitization and output encoding of user-provided content. Organizations should implement strict input validation mechanisms that prevent nested bbcode tag structures from being processed without proper sanitization, and should employ Content Security Policy headers to limit script execution capabilities. The vulnerability also highlights the importance of regular security updates and input validation in web applications, as it demonstrates how seemingly benign parsing features can become attack vectors when not properly secured. Security monitoring should include detection of unusual post content patterns and regular vulnerability scanning of forum installations to prevent exploitation of similar flaws in other components of the web application stack.