CVE-2020-21054 in FusionPBX
Summary
by MITRE • 05/21/2021
Cross Site Scripting (XSS) vulnerability in FusionPBX 4.5.7 allows remote malicious users to inject arbitrary web script or HTML via an unsanitized "f" variable in app\vars\vars_textarea.php.
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Analysis
by VulDB Data Team • 05/23/2021
This cross site scripting vulnerability exists within FusionPBX version 4.5.7, specifically in the apparsars_textarea.php component where user input is not properly sanitized before being rendered in web pages. The flaw occurs when the "f" parameter is passed through HTTP requests without adequate validation or encoding, creating an avenue for attackers to inject malicious scripts that execute in the context of other users' browsers. This represents a classic reflected cross site scripting vulnerability where malicious payloads are reflected back to users through the vulnerable application interface. The vulnerability falls under CWE-79 which categorizes improper neutralization of input during web page generation, specifically targeting the failure to sanitize user-controllable data before incorporating it into dynamic web content. From an operational security perspective, this vulnerability presents a significant risk as it allows remote attackers to execute arbitrary code within the browser context of authenticated users, potentially enabling session hijacking, credential theft, or redirection to malicious sites. The attack vector requires minimal prerequisites as it only requires sending a specially crafted URL to a victim, making it particularly dangerous in environments where users may click on links in emails or messaging platforms. The vulnerability impacts the confidentiality and integrity of user sessions, as malicious scripts can access session cookies, form data, and other sensitive information that users might enter into the application. According to ATT&CK framework, this vulnerability maps to T1059.007 for scripting and T1566.001 for spearphishing via web application, as it enables attackers to deliver malicious payloads through web interfaces. The technical exploitation involves crafting a URL containing malicious script within the "f" parameter that gets executed when the page loads, potentially allowing attackers to steal session tokens or perform unauthorized actions on behalf of users. The impact extends beyond simple script execution as it can lead to complete compromise of user accounts and potentially escalate to system-level access depending on user privileges within the FusionPBX environment.
The vulnerability stems from inadequate input validation and output encoding practices within the application's user interface components. FusionPBX fails to implement proper sanitization mechanisms for user-supplied parameters before rendering them in HTML contexts, allowing malicious payloads to persist and execute when the affected page is accessed. This failure to enforce secure coding practices violates fundamental web application security principles and demonstrates a lack of proper input filtering at the application layer. The specific file apparsars_textarea.php appears to directly incorporate user input from the "f" variable into HTML output without appropriate HTML entity encoding or script validation, creating the XSS condition. This issue is particularly concerning because FusionPBX serves as a unified communications platform that often handles sensitive business communications, making it an attractive target for attackers seeking to exploit user trust. The vulnerability's persistence across different user contexts means that any authenticated user who accesses the vulnerable page could be compromised, regardless of their privilege level. Security controls that should have been in place include input validation libraries, output encoding mechanisms, and proper parameter sanitization routines that would prevent the injection of executable code. Organizations using this version of FusionPBX should consider immediate remediation through patching or implementing compensating controls such as web application firewalls that can detect and block malicious script patterns. The vulnerability's classification as a remote code execution risk through browser-based attacks aligns with ATT&CK's T1203 which covers legitimate credentials for remote access, as successful exploitation could lead to unauthorized access to the system through stolen session information. Additionally, the vulnerability could facilitate further attacks through credential harvesting and session manipulation techniques that are commonly used in advanced persistent threat campaigns.
Mitigation strategies for this XSS vulnerability should include immediate patching of FusionPBX to version 4.5.8 or later where the input sanitization issue has been addressed. Organizations should also implement comprehensive input validation at multiple layers including web application firewalls, server-side validation routines, and client-side sanitization mechanisms to prevent similar issues in other components. The implementation of content security policies can provide additional protection against script injection attempts by restricting the sources from which scripts can be loaded. Security teams should conduct regular vulnerability assessments and penetration testing to identify similar input handling issues in other web applications and components within their environment. The remediation process should also include reviewing and updating security coding practices to ensure that all user-controllable parameters are properly validated and encoded before being used in dynamic content generation. Organizations should establish secure development lifecycle practices that include automated code scanning tools and security testing as part of their development process to prevent similar vulnerabilities from being introduced in future releases. The vulnerability serves as a reminder of the critical importance of input validation and output encoding in preventing web application attacks, particularly in systems handling sensitive communications data. Regular security awareness training for developers and system administrators can help prevent similar issues by emphasizing secure coding practices and the importance of proper input sanitization in web applications.