CVE-2006-5504 in Simple Machines
Summary
by MITRE
Cross-site scripting (XSS) vulnerability in index.php in Simple Machines Forum (SMF) allows remote attackers to inject arbitrary web script or HTML via a base64 encoded params value in the action parameter.
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Analysis
by VulDB Data Team • 04/25/2026
The CVE-2006-5504 vulnerability represents a critical cross-site scripting flaw within the Simple Machines Forum (SMF) platform, specifically targeting the index.php script. This vulnerability falls under the CWE-79 category of Cross-Site Scripting, which is one of the most prevalent and dangerous web application security issues. The flaw manifests when the application fails to properly validate or sanitize user input passed through the action parameter, particularly when this parameter contains base64 encoded data. Attackers can exploit this weakness by crafting malicious payloads that, when processed by the vulnerable forum software, execute unintended scripts within the context of other users' browsers.
The technical mechanism of this vulnerability involves the improper handling of base64 encoded parameters within the action parameter of the forum's main index.php file. When SMF processes these parameters without adequate sanitization, it allows malicious base64 encoded strings to be interpreted as executable code. This creates a situation where an attacker can inject arbitrary HTML or JavaScript content that gets rendered in the browser of unsuspecting forum users. The exploitation process typically requires the attacker to craft a URL containing the malicious base64 encoded payload within the action parameter, which when visited by a victim, executes the injected code in the victim's browser context.
The operational impact of this vulnerability extends beyond simple script execution, as it provides attackers with the capability to perform various malicious activities through the compromised user sessions. An attacker could potentially steal session cookies, redirect users to malicious sites, modify forum content, or even escalate privileges within the forum environment. The vulnerability affects the integrity and confidentiality of user data, as well as the overall security posture of the forum. Users who visit the malicious link could have their personal information compromised, and the forum administrators might lose control over their platform's content and user interactions. This type of vulnerability also enables further attack vectors such as credential theft, account takeover, and potential use in phishing campaigns.
Mitigation strategies for CVE-2006-5504 should focus on input validation and output encoding practices that align with industry standards and security frameworks. The most effective approach involves implementing proper sanitization of all user-supplied input, particularly parameters that undergo base64 decoding. Security measures should include validating the action parameter against a whitelist of allowed values, implementing proper HTML escaping for all dynamic content, and ensuring that base64 decoded data undergoes strict validation before processing. Organizations should also consider implementing Content Security Policy headers, which can provide additional protection against XSS attacks by restricting the sources from which scripts can be executed. The vulnerability demonstrates the importance of following secure coding practices and the ATT&CK framework's emphasis on preventing code injection vulnerabilities. Regular security updates and patches are essential, as this vulnerability was addressed in subsequent versions of SMF. Additionally, implementing web application firewalls and conducting regular security assessments can help detect and prevent exploitation attempts. The incident highlights the critical need for thorough input validation and output encoding in web applications, as specified in OWASP Top 10 and other security standards that emphasize the prevention of XSS vulnerabilities through proper data sanitization and context-appropriate output encoding.