CVE-2017-5542 in Symphony
Summary
by MITRE
Cross-site scripting (XSS) vulnerability in template/usererror.missing_extension.php in Symphony CMS before 2.6.10 allows remote attackers to inject arbitrary web script or HTML via the existing-folder parameter.
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Analysis
by VulDB Data Team • 05/14/2026
The vulnerability identified as CVE-2017-5542 represents a critical cross-site scripting flaw within Symphony CMS versions prior to 2.6.10, specifically affecting the template/usererror.missing_extension.php file. This vulnerability resides in the application's handling of user input within the existing-folder parameter, creating an exploitable condition that allows remote attackers to inject malicious web scripts or HTML code into the application's response. The flaw demonstrates characteristics consistent with CWE-79, which defines cross-site scripting vulnerabilities as weaknesses that occur when an application includes untrusted data in a new web page without proper validation or escaping, or when it fails to escape output appropriately. The attack vector leverages the CMS's error handling mechanism where missing extension errors are displayed to users, making this a particularly insidious vulnerability as it can be triggered through normal user interactions with the application's error reporting system.
The technical implementation of this vulnerability stems from inadequate input sanitization within the Symphony CMS error template processing system. When the application encounters a missing extension error, it renders the usererror.missing_extension.php template and incorporates the existing-folder parameter directly into the rendered HTML output without appropriate HTML escaping or sanitization. This allows attackers to inject malicious payloads that will execute in the context of other users' browsers who view the error page. The vulnerability is classified as a reflected XSS attack since the malicious input is reflected back to the user through the application's response, making it particularly dangerous in multi-user environments where administrators or other users might encounter these error pages. The ATT&CK framework categorizes this as a technique involving the exploitation of web application vulnerabilities to execute arbitrary code in user browsers, specifically under the T1059.007 sub-technique for script-based attacks.
The operational impact of this vulnerability extends beyond simple script injection, as it can enable attackers to perform various malicious activities including session hijacking, credential theft, data exfiltration, and privilege escalation within the CMS environment. An attacker who successfully exploits this vulnerability could potentially gain access to administrative accounts, modify content, or establish persistent access points within the web application. The vulnerability is particularly concerning because it affects the core error handling mechanism of the CMS, meaning that any user interaction that triggers an extension error could potentially lead to exploitation. This makes the attack surface quite broad and increases the likelihood of successful exploitation in real-world scenarios. Organizations using Symphony CMS versions prior to 2.6.10 face significant risk of compromise, especially in environments where the CMS is used for content management with multiple user roles and administrative privileges.
The recommended mitigation strategy for this vulnerability involves immediate upgrading to Symphony CMS version 2.6.10 or later, which includes the necessary patches to address the XSS vulnerability in the error handling template. Additionally, organizations should implement comprehensive input validation and output encoding mechanisms throughout their web applications, particularly in error handling and user-facing components. Security best practices dictate that all user-supplied input should be properly escaped or sanitized before being incorporated into dynamic web content, with HTML escaping being the preferred approach for preventing XSS attacks. The implementation of Content Security Policy (CSP) headers can provide additional defense-in-depth measures to mitigate the impact of successful XSS attempts, although this should not be considered a replacement for proper input validation. Organizations should also conduct regular security assessments and vulnerability scanning to identify similar issues in their web applications, as this vulnerability demonstrates the importance of secure coding practices in error handling and template rendering processes. The remediation process should include thorough testing to ensure that the patch does not introduce regressions in the CMS functionality while maintaining the security improvements.