CVE-2021-35955 in Contao
Summary
by MITRE • 08/12/2021
Contao >=4.0.0 allows backend XSS via HTML attributes to an HTML field. Fixed in 4.4.56, 4.9.18, 4.11.7.
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Analysis
by VulDB Data Team • 08/18/2021
The vulnerability CVE-2021-35955 represents a cross-site scripting vulnerability affecting the Contao content management system version 4.0.0 and later. This security flaw exists within the backend interface where HTML attributes can be injected into HTML fields, creating a persistent XSS vector that allows attackers to execute malicious scripts in the context of authenticated admin sessions. The vulnerability specifically impacts the HTML field handling functionality within Contao's backend administration panel, where user input containing HTML attributes is not properly sanitized or validated before being rendered back to users. This flaw enables attackers to inject malicious JavaScript code through HTML attributes such as onclick, onmouseover, or other event handlers that get executed when the malicious content is displayed in the backend interface.
The technical implementation of this vulnerability stems from insufficient input validation and output encoding mechanisms within Contao's HTML field processing. When administrators view or edit content containing malicious HTML attributes, the system fails to properly escape or filter these attributes before rendering them in the browser context. This creates a classic XSS scenario where an attacker can craft malicious HTML content that gets stored in the database and subsequently executed when other administrators view the content. The vulnerability affects multiple versions of Contao including the 4.4.x, 4.9.x, and 4.11.x release lines, indicating it was present across a significant portion of the Contao 4.x lineage. The issue is particularly concerning because it targets the backend interface where administrators have elevated privileges, making successful exploitation potentially devastating for system security.
The operational impact of this vulnerability is severe as it allows attackers to gain administrative privileges or execute arbitrary commands within the context of the Contao backend. An attacker who successfully exploits this vulnerability could modify or delete content, create new administrator accounts, access sensitive data, or even escalate their privileges to full system compromise. The vulnerability is especially dangerous because it operates within the administrative interface where users have the highest level of access to the system. According to CWE classification, this vulnerability maps to CWE-79 which represents Cross-site Scripting, specifically focusing on improper neutralization of input during web page generation. The attack vector falls under the ATT&CK framework category of T1059.007 for Command and Scripting Interpreter with JavaScript, and potentially T1566 for Phishing with Social Engineering techniques to gain initial access to the backend.
The recommended mitigation strategy involves upgrading to the fixed versions of Contao 4.4.56, 4.9.18, or 4.11.7, which contain proper input validation and output encoding fixes. Organizations should also implement additional security measures including regular security audits of backend interfaces, monitoring for suspicious administrative activities, and implementing proper input sanitization at multiple layers of the application. Network segmentation and privileged access controls should be enforced to limit the potential damage from any successful exploitation. Security teams should also consider implementing Content Security Policy headers and regular security scanning of backend interfaces to detect similar vulnerabilities. The fix addresses the core issue by ensuring that HTML attributes are properly escaped and validated before being processed and rendered within the backend interface, preventing malicious scripts from executing in the context of authenticated administrator sessions.