CVE-2005-4357 in phpBB
Summary
by MITRE
Cross-site scripting (XSS) vulnerability in phpBB 2.0.18, when "Allowed HTML tags" is enabled, allows remote attackers to inject arbitrary Javascript via a permitted HTML tag with " (quote) characters and active attributes such as onmouseover.
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Analysis
by VulDB Data Team • 06/13/2019
The vulnerability described in CVE-2005-4357 represents a classic cross-site scripting flaw affecting phpBB version 2.0.18 where the application fails to properly sanitize user input containing HTML tags. This issue arises specifically when the "Allowed HTML tags" feature is enabled, creating a dangerous condition where malicious users can inject javascript code through seemingly benign HTML elements. The flaw demonstrates a critical weakness in the application's input validation and output encoding mechanisms, allowing attackers to bypass security controls that should restrict user-generated content to safe HTML elements only.
The technical implementation of this vulnerability exploits the improper handling of HTML attribute parsing within phpBB's content processing pipeline. When users submit content containing HTML tags that are permitted by the configuration, the application fails to adequately escape or remove potentially dangerous attributes such as onmouseover, onclick, or other event handlers that can execute javascript code in the victim's browser context. The presence of quote characters in the HTML attributes creates a parsing vulnerability where the javascript payload can be injected between the attribute delimiters, effectively executing malicious code when the content is rendered in a web browser. This represents a failure in the application's security model and demonstrates a lack of proper HTML sanitization.
The operational impact of this vulnerability is significant as it allows remote attackers to execute arbitrary javascript code in the context of any user's browser who views the malicious content. This creates a wide range of potential attack vectors including session hijacking, credential theft, defacement of forum content, and redirection to malicious websites. The vulnerability can be exploited through forum posts, user profiles, or any other area where user-generated content is displayed, potentially compromising all forum users who view the malicious content. The attack requires no special privileges or authentication, making it particularly dangerous as it can be exploited by anyone with access to the forum.
Security mitigations for this vulnerability should include immediate implementation of proper HTML sanitization and output encoding practices. The most effective approach involves configuring the application to strip or escape all potentially dangerous HTML attributes regardless of whether the tag is in the allowed list, implementing a whitelist-based approach for HTML processing, and ensuring that all user input is properly encoded before being rendered in the browser context. Organizations should also consider implementing content security policies and regular security audits of their web applications to prevent similar vulnerabilities from being introduced through improper input validation. This vulnerability aligns with CWE-79 which specifically addresses cross-site scripting flaws and represents a clear violation of the principle of least privilege in web application security. The attack pattern follows typical XSS exploitation techniques documented in the MITRE ATT&CK framework under the T1059.007 sub-technique for script injection, emphasizing the need for comprehensive input validation and output encoding as core security controls.