CVE-2006-3761 in MyBB
Summary
by MITRE
Cross-site scripting (XSS) vulnerability in inc/functions_post.php in MyBB (aka MyBulletinBoard) 1.0 RC2 through 1.1.4 allows remote attackers to inject arbitrary web script or HTML via a javascript URI with an SGML numeric character reference in the url BBCode tag, as demonstrated using "javascript".
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Analysis
by VulDB Data Team • 04/30/2019
The vulnerability described in CVE-2006-3761 represents a critical cross-site scripting flaw within the MyBulletinBoard forum software ecosystem. This vulnerability exists specifically within the inc/functions_post.php file of MyBB versions ranging from 1.0 RC2 through 1.1.4, making it a significant security concern for users of these older software versions. The flaw manifests when the application processes BBCode tags, particularly the url tag, without adequate input sanitization or validation mechanisms. Attackers can exploit this weakness by crafting malicious posts containing javascript URIs that utilize SGML numeric character references to bypass standard security filters. The demonstration example shows the exploitation technique using "javascript" where the numeric character reference s represents the lowercase letter s, effectively creating a javascript URI that evades simple pattern matching defenses.
The technical implementation of this vulnerability stems from insufficient sanitization of user input within the BBCode processing pipeline. When MyBB encounters a url BBCode tag containing a javascript URI, the application fails to properly validate or escape the content before rendering it in web pages. This allows attackers to inject malicious scripts that execute in the context of other users' browsers when they view the compromised content. The use of SGML numeric character references represents a sophisticated evasion technique that leverages the fact that numeric character references are often processed before security filters, allowing attackers to bypass simple string matching protections. The vulnerability specifically targets the url BBCode tag processing functionality, which is commonly used for linking to external resources within forum posts, making it a high-value target for malicious actors seeking to compromise user sessions or steal sensitive information.
The operational impact of this vulnerability extends beyond simple script injection to encompass potential session hijacking, credential theft, and data exfiltration capabilities. When users browse forum pages containing maliciously crafted posts, their browsers execute the injected javascript code, potentially leading to unauthorized access to their accounts, modification of forum content, or redirection to malicious websites. The vulnerability affects all users of affected MyBB versions, creating a widespread security risk for forum administrators and their communities. Given that forum platforms often contain sensitive user information, including personal details, private messages, and potentially confidential discussions, the exploitation of this vulnerability could result in significant data breaches and privacy violations. The long-term consequences include potential compromise of user trust, reputational damage to forum administrators, and increased risk of further exploitation through secondary attacks that leverage the compromised user sessions.
Security mitigations for this vulnerability should focus on immediate patching of affected MyBB installations to versions that properly sanitize BBCode inputs and validate URL schemes. Administrators should implement comprehensive input validation mechanisms that properly escape or filter numeric character references and javascript URIs before rendering user-generated content. The implementation should follow established security practices such as those outlined in the CWE-79 category for cross-site scripting vulnerabilities, which emphasizes the importance of proper output encoding and input validation. Additionally, organizations should consider implementing web application firewalls that can detect and block suspicious numeric character reference patterns, as well as regular security audits of forum content to identify and remove malicious posts. The ATT&CK framework categorizes this type of vulnerability under T1059.007 for "Command and Scripting Interpreter: JavaScript' and T1566.001 for 'Phishing: Spearphishing Attachment', highlighting the need for both defensive measures against script injection and user education regarding potential security risks in forum environments.