CVE-2015-8976 in MyBB
Summary
by MITRE
Cross-site scripting (XSS) vulnerability in MyBB (aka MyBulletinBoard) before 1.6.18 and 1.8.x before 1.8.6 and MyBB Merge System before 1.8.6 might allow remote attackers to inject arbitrary web script or HTML via vectors related to "old upgrade files."
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Analysis
by VulDB Data Team • 08/08/2020
The CVE-2015-8976 vulnerability represents a critical cross-site scripting flaw affecting MyBB forum software versions prior to 1.6.18 and 1.8.x versions before 1.8.6, along with the MyBB Merge System before 1.8.6. This vulnerability stems from insufficient input validation and sanitization mechanisms within the software's upgrade and migration processes, specifically when handling legacy upgrade files. The flaw allows remote attackers to inject malicious web scripts or HTML content into the vulnerable system, creating a persistent threat vector that can be exploited across multiple user sessions.
The technical implementation of this vulnerability occurs during the upgrade process where MyBB fails to properly sanitize user-supplied data from old upgrade files. These files often contain configuration settings, database schema information, or migration scripts that are processed without adequate validation. When the system encounters untrusted input from these legacy files, it fails to strip or encode potentially dangerous characters that could be interpreted as executable script code by web browsers. This processing gap creates an injection point where attackers can place malicious payloads that execute in the context of other users' browsers, fundamentally undermining the security boundaries of the forum platform.
The operational impact of this vulnerability extends beyond simple script injection, creating a significant risk for forum administrators and users alike. Attackers can leverage this vulnerability to execute malicious scripts that may steal session cookies, redirect users to phishing sites, deface forum content, or establish persistent backdoors within the platform. The vulnerability affects not just the current user base but also creates a potential attack surface for social engineering campaigns where malicious actors can craft convincing phishing content that appears legitimate within the forum environment. The persistence of this flaw in upgrade processes means that even systems that appear to be properly updated could remain vulnerable if they contain legacy upgrade files from previous installations.
This vulnerability maps directly to CWE-79, which describes Cross-Site Scripting flaws in software applications. The attack pattern aligns with ATT&CK technique T1566.001, specifically targeting credential access through phishing with malicious links. Organizations should implement immediate mitigations including thorough removal of legacy upgrade files, implementation of proper input sanitization for all upgrade processes, and deployment of web application firewalls with XSS detection capabilities. The recommended remediation involves upgrading to MyBB versions 1.6.18 or 1.8.6 and newer, followed by comprehensive audit of upgrade directories to eliminate any remaining legacy files that could contain malicious payloads. Additionally, implementing Content Security Policy headers and regular security scanning of upgrade processes can provide additional defense layers against exploitation attempts.