CVE-2016-5303 in Horde Groupware
Summary
by MITRE
Cross-site scripting (XSS) vulnerability in the Horde Text Filter API in Horde Groupware and Horde Groupware Webmail Edition before 5.2.16 allows remote attackers to inject arbitrary web script or HTML via crafted data:text/html content in a form (1) action or (2) xlink attribute.
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Analysis
by VulDB Data Team • 08/27/2025
The CVE-2016-5303 vulnerability represents a critical cross-site scripting flaw within the Horde Text Filter API component of Horde Groupware and Horde Groupware Webmail Edition software versions prior to 5.2.16. This vulnerability resides in the text processing functionality that handles user input and form data, creating an avenue for remote attackers to execute malicious scripts within the context of affected applications. The flaw specifically manifests when the system processes crafted data:text/html content within form attributes, particularly targeting the action and xlink attributes that are commonly used in web forms and xml content. The vulnerability demonstrates a classic XSS weakness where untrusted input is not properly sanitized or validated before being rendered in web pages, allowing attackers to inject malicious code that executes in users' browsers.
The technical implementation of this vulnerability leverages the inherent trust placed in form processing within web applications. When the Horde Text Filter API encounters form elements containing data:text/html content in either the action attribute or xlink attribute, it fails to adequately sanitize these inputs, permitting the execution of arbitrary script code. This flaw operates at the application layer where user-supplied data flows through the text filtering system without sufficient validation controls. The vulnerability is particularly dangerous because it can be exploited through multiple vectors within the same form processing mechanism, making it more difficult to defend against completely. The use of data:text/html content allows attackers to bypass traditional input validation measures since this format is legitimate web content that may pass security checks designed for standard text input.
The operational impact of CVE-2016-5303 extends beyond simple script injection, potentially enabling attackers to perform session hijacking, steal user credentials, redirect users to malicious sites, or execute other harmful actions within the context of the vulnerable application. Attackers could craft malicious forms that, when processed by the vulnerable system, would execute scripts in users' browsers, potentially compromising user sessions and accessing sensitive data. The vulnerability affects both Horde Groupware and Horde Groupware Webmail Edition, which are widely deployed enterprise applications, amplifying the potential impact. Organizations using these platforms could face significant security breaches where user data is compromised, and the integrity of the web applications is undermined. The flaw particularly threatens environments where users trust the applications and may interact with forms containing malicious content, as the exploitation occurs silently in the background without user awareness.
Mitigation strategies for CVE-2016-5303 primarily focus on upgrading to the patched version 5.2.16 or later, which implements proper input sanitization for text/html content within form attributes. Organizations should also implement comprehensive input validation and output encoding mechanisms that prevent the execution of script code within form attributes. The implementation of Content Security Policy headers can provide additional protection against script execution, while regular security testing and code reviews should be conducted to identify similar vulnerabilities. This vulnerability aligns with CWE-79 which describes cross-site scripting flaws, and maps to ATT&CK technique T1059.008 for script injection, highlighting the importance of proper input validation and output encoding as defensive measures. Security teams should also consider implementing web application firewalls that can detect and block malicious form content, while maintaining regular patch management processes to ensure all components remain up-to-date with security fixes.