CVE-2005-4621 in vBulletin
Summary
by MITRE
Cross-site scripting (XSS) vulnerability in the editavatar page in vBulletin 3.5.1 allows remote attackers to inject arbitrary web script or HTML via a URL in the remote avatar url field, in which the URL generates a parsing error, and possibly requiring a trailing extension such as .jpg.
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Analysis
by VulDB Data Team • 07/16/2018
The vulnerability identified as CVE-2005-4621 represents a classic cross-site scripting flaw within the vBulletin 3.5.1 forum software, specifically affecting the editavatar page functionality. This security weakness stems from inadequate input validation and output encoding mechanisms that fail to properly sanitize user-supplied data before it is rendered back to other users within the forum environment. The vulnerability manifests when users attempt to set their avatar by providing a remote URL, creating a pathway for malicious actors to inject harmful scripts that execute in the context of other users' browsers.
The technical exploitation of this vulnerability occurs through the manipulation of the remote avatar URL field, where attackers craft malicious URLs designed to trigger parsing errors in the application's handling of these inputs. The vulnerability requires that the injected payload be positioned within a URL structure that causes the application to parse the input in a way that allows script execution rather than proper validation. The specific requirement for a trailing extension such as .jpg suggests that the vulnerability is tied to the application's content type detection mechanisms, where the system may be treating certain URL patterns differently based on file extensions, creating a window for injection attacks that bypass standard validation checks.
This XSS vulnerability operates under CWE-79 which specifically addresses Cross-Site Scripting flaws in software applications, where improper validation of user input allows malicious code to be executed in the context of other users' browsers. The operational impact of this vulnerability extends beyond simple script injection, as it can enable attackers to steal session cookies, perform unauthorized actions on behalf of users, redirect victims to malicious sites, or even establish persistent backdoors within the forum environment. The attack vector is particularly dangerous because it leverages the trust relationship between users and the forum application, allowing malicious code to execute in the context of legitimate user sessions.
The implications of this vulnerability align with ATT&CK technique T1531 which covers "Run-time Process Injection" and T1059 which covers "Command and Scripting Interpreter" as attackers can leverage the XSS flaw to execute malicious scripts that can then perform further exploitation activities. The vulnerability's impact is amplified by the fact that vBulletin forums typically host numerous users who may not be aware of the security implications of avatar URLs, making the attack surface particularly broad. The presence of a trailing extension requirement suggests that the vulnerability may be related to how the application handles different content types or how it validates file extensions, which could provide additional attack vectors for sophisticated adversaries seeking to escalate their privileges or maintain persistent access to the forum infrastructure.
Organizations utilizing vBulletin 3.5.1 systems should implement immediate mitigations including input validation that strips or encodes potentially dangerous characters from URL inputs, output encoding that prevents script execution in HTML contexts, and the implementation of Content Security Policy headers to limit script execution. The recommended approach involves upgrading to patched versions of vBulletin, implementing proper input sanitization mechanisms, and establishing monitoring for suspicious avatar URL patterns. Additionally, administrators should consider disabling remote avatar functionality entirely if it is not essential to forum operations, as this would eliminate the attack vector while maintaining core forum functionality. The vulnerability demonstrates the critical importance of proper input validation and output encoding in web applications, particularly in community-driven platforms where user-generated content processing creates numerous potential attack surfaces that must be carefully managed to prevent exploitation.