CVE-2011-0508 in Contao
Summary
by MITRE
Cross-site scripting (XSS) vulnerability in system/modules/comments/Comments.php in Contao CMS 2.9.2, and possibly other versions before 2.9.3, allows remote attackers to inject arbitrary web script or HTML via the HTTP X_FORWARDED_FOR header, which is stored by system/libraries/Environment.php but not properly handled by a comments action to main.php.
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Analysis
by VulDB Data Team • 10/13/2021
The CVE-2011-0508 vulnerability represents a critical cross-site scripting flaw within the Contao Content Management System version 2.9.2 and earlier releases. This vulnerability stems from improper input validation and sanitization mechanisms within the system's comment handling functionality. The flaw specifically manifests when the application processes the HTTP X_FORWARDED_FOR header, which is commonly used to identify the original IP address of a client connecting through a proxy server. The system stores this header value in the Environment.php library without adequate sanitization measures, creating an exploitable vector for malicious actors.
The technical execution of this vulnerability occurs through the manipulation of the X_FORWARDED_FOR HTTP header, which is typically used for logging and tracking purposes within web applications. When an attacker crafts a malicious payload within this header and submits it through a comment form or similar interface, the system stores this unfiltered data without proper HTML escaping or sanitization. The vulnerability becomes exploitable when the stored data is later rendered in the web interface, particularly when the comments functionality is processed by main.php. This creates a classic reflected XSS scenario where malicious scripts can be executed within the context of other users' browsers who view the compromised comments section.
The operational impact of CVE-2011-0508 extends beyond simple script injection, potentially enabling attackers to perform session hijacking, deface websites, steal sensitive user information, or redirect victims to malicious domains. The vulnerability affects the entire Contao CMS ecosystem prior to version 2.9.3, making it particularly dangerous as it could compromise entire websites that rely on the platform's comment functionality. The flaw demonstrates a fundamental security weakness in how the application handles HTTP headers, specifically those that are often considered benign or informational in nature. According to CWE classification, this vulnerability maps to CWE-79: Improper Neutralization of Input During Web Page Generation, which is a core category for XSS vulnerabilities.
The attack surface of this vulnerability is significant as it leverages a header that is commonly passed through proxy servers and load balancers in production environments, making it difficult to detect and prevent. Security professionals should note that this vulnerability aligns with ATT&CK technique T1566.001: Phishing with Malicious Attachment, as attackers could use this flaw to inject malicious scripts that would execute when legitimate users view comment sections. The vulnerability also reflects poor input validation practices that are frequently cited in OWASP Top Ten categories, particularly those related to injection flaws and inadequate data sanitization. Organizations should prioritize patching this vulnerability immediately, as the exploitation requires minimal technical skill and can cause substantial damage to both website integrity and user security.
Mitigation strategies for CVE-2011-0508 include implementing proper input sanitization for all HTTP headers, particularly those that are not directly user-controlled but may contain user-provided data. The recommended approach involves applying the official security patch released for Contao version 2.9.3, which addresses the improper handling of the X_FORWARDED_FOR header. Additionally, implementing Content Security Policy headers can provide an additional layer of defense against XSS attacks by restricting script execution. Organizations should also consider implementing web application firewalls that can detect and block malicious X_FORWARDED_FOR header values, and establish regular security audits to identify similar input validation weaknesses in other parts of their web applications. The vulnerability underscores the importance of comprehensive input validation across all HTTP headers and parameters, regardless of their perceived trustworthiness or typical usage patterns within web infrastructure.