CVE-2021-3824 in Access Server
Summary
by MITRE • 09/24/2021
OpenVPN Access Server 2.9.0 through 2.9.4 allow remote attackers to inject arbitrary web script or HTML via the web login page URL.
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Analysis
by VulDB Data Team • 10/02/2021
The vulnerability identified as CVE-2021-3824 affects OpenVPN Access Server versions 2.9.0 through 2.9.4, representing a critical cross-site scripting vulnerability that enables remote attackers to execute malicious scripts within the context of the victim's browser. This flaw exists in the web login page URL handling mechanism, where insufficient input validation allows attackers to inject arbitrary web scripts or HTML content that gets executed when the page is rendered. The vulnerability stems from improper sanitization of user-supplied input parameters that are directly incorporated into the web interface without adequate security filtering.
The technical implementation of this vulnerability involves the web server's failure to properly escape or validate URL parameters before they are processed and displayed in the login interface. When a user navigates to the login page with maliciously crafted URL parameters, the server incorporates these parameters directly into the HTML response without proper sanitization. This creates an environment where an attacker can embed JavaScript code, HTML tags, or other malicious content that executes in the victim's browser session. The vulnerability is particularly dangerous because it occurs during the authentication process, when users are most likely to interact with the interface, and can potentially lead to session hijacking or credential theft.
From an operational perspective, this vulnerability presents significant risks to organizations relying on OpenVPN Access Server for remote access management. Attackers can exploit this flaw to perform session fixation attacks, steal user credentials, redirect users to malicious websites, or execute arbitrary commands within the user's browser context. The impact extends beyond simple script injection as it can facilitate more sophisticated attacks such as phishing campaigns where users are tricked into believing they are interacting with legitimate authentication pages. The vulnerability affects both authenticated and unauthenticated users since the flaw exists in the login page itself, making it accessible to anyone with network access to the VPN server.
The vulnerability maps directly to CWE-79 - Cross-Site Scripting and aligns with several ATT&CK techniques including T1566 - Phishing and T1071.004 - Application Layer Protocol: DNS, as attackers can leverage this vulnerability to redirect users or harvest credentials. Organizations should implement immediate mitigations including upgrading to OpenVPN Access Server version 2.9.5 or later, which contains the necessary patches to address the input validation issues. Additionally, network administrators should consider implementing web application firewalls to detect and block malicious URL parameters, and conduct thorough security assessments of the VPN infrastructure. Regular monitoring of login page access logs for suspicious URL patterns and user behavior anomalies should also be implemented to detect potential exploitation attempts. The vulnerability highlights the critical importance of input validation in web applications and demonstrates how seemingly minor security flaws can have significant operational impacts on enterprise security posture.