CVE-2006-5503 in Simple Machines
Summary
by MITRE
Cross-site scripting (XSS) vulnerability in index.php in Simple Machines Forum (SMF) 1.1 RC2 allows remote attackers to inject arbitrary web script or HTML via the action parameter.
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Analysis
by VulDB Data Team • 04/25/2026
The vulnerability identified as CVE-2006-5503 represents a critical cross-site scripting flaw within Simple Machines Forum version 1.1 RC2, specifically affecting the index.php script. This vulnerability resides in the handling of the action parameter, which fails to properly sanitize user input before processing. The flaw enables remote attackers to execute malicious scripts in the context of other users' browsers, potentially leading to session hijacking, credential theft, or unauthorized actions performed on behalf of victims. The vulnerability is categorized under CWE-79 as Improper Neutralization of Input During Web Page Generation, which is a fundamental weakness in web application security. The issue demonstrates the classic characteristics of reflected XSS attacks where malicious input is immediately reflected back to the user without proper validation or encoding mechanisms.
The technical implementation of this vulnerability stems from insufficient input validation within the SMF application's parameter handling system. When the action parameter is passed to index.php without adequate sanitization, the application directly incorporates this input into dynamically generated web content. This creates an environment where attackers can inject malicious HTML or JavaScript code that executes in the victim's browser context. The vulnerability is particularly dangerous because it affects a core forum component that handles user interactions and navigation. Attackers can craft malicious URLs containing script payloads in the action parameter, which when clicked by unsuspecting users, execute the injected code in their browser sessions. This behavior aligns with ATT&CK technique T1566.001 for Initial Access through Spearphishing Attachment, where users are tricked into executing malicious code through crafted web requests.
The operational impact of this vulnerability extends beyond simple script execution to encompass significant security risks for forum administrators and users. Successful exploitation could allow attackers to steal session cookies, redirect users to malicious sites, deface forum content, or perform administrative actions if the victim has elevated privileges. The vulnerability affects the integrity and availability of the forum's user data and communication channels. Organizations relying on SMF 1.1 RC2 for community forums, discussion boards, or user interaction platforms face potential data breaches and reputational damage. The vulnerability also impacts the trust relationship between forum operators and their user base, as users may lose confidence in the platform's security measures. This type of vulnerability commonly leads to cascading security issues where compromised user sessions can be leveraged for further attacks within the network.
Mitigation strategies for CVE-2006-5503 require immediate implementation of input validation and output encoding measures. The most effective approach involves implementing proper parameter sanitization within the index.php script to ensure that all user-supplied input is validated and escaped before being processed or displayed. Organizations should implement Content Security Policy headers to limit script execution contexts and prevent unauthorized code injection. The recommended solution aligns with OWASP Top 10 2021 category A03: Injection, specifically addressing the need for proper input validation and output encoding. Additionally, upgrading to a patched version of SMF 1.1 RC2 or migrating to a more recent stable release is essential. Security teams should conduct regular vulnerability assessments and implement web application firewalls to detect and block malicious payloads targeting similar XSS vulnerabilities. The remediation process must include comprehensive testing to ensure that all input parameters are properly sanitized while maintaining the forum's intended functionality and user experience.