CVE-2021-40104 in Concrete
Summary
by MITRE • 09/27/2021
An issue was discovered in Concrete CMS through 8.5.5. There is an SVG sanitizer bypass.
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Analysis
by VulDB Data Team • 10/02/2021
The vulnerability identified as CVE-2021-40104 represents a critical security flaw in Concrete CMS versions up to 8.5.5, specifically concerning the SVG (Scalable Vector Graphics) sanitizer implementation. This issue enables attackers to bypass the content validation mechanisms that are designed to prevent malicious code execution within SVG files uploaded to the system. The vulnerability stems from insufficient input validation and sanitization processes that fail to properly filter out potentially dangerous SVG elements and attributes that could be exploited for cross-site scripting attacks or other malicious activities.
The technical flaw manifests in the SVG sanitization logic where certain SVG elements and attributes that should be restricted or removed are not properly handled by the security filters. This bypass allows attackers to inject malicious code within SVG files that are then processed by the CMS, potentially leading to unauthorized access, data exfiltration, or system compromise. The vulnerability is particularly concerning because SVG files are commonly used for images and graphics within web applications, making them a frequent vector for exploitation. According to CWE guidelines, this vulnerability maps to CWE-79 which describes Cross-Site Scripting (XSS) vulnerabilities, specifically those occurring in input validation contexts where security filters are bypassed.
The operational impact of this vulnerability is significant for organizations using Concrete CMS versions 8.5.5 or earlier, as it provides attackers with a method to execute arbitrary code on affected systems through SVG file uploads. This could lead to complete system compromise, unauthorized data access, and potential lateral movement within network environments where the CMS is deployed. Attackers can leverage this vulnerability to inject malicious JavaScript code within SVG files that are then rendered by web browsers, creating persistent XSS attack vectors that can be used to steal user sessions, redirect traffic, or perform other malicious activities. The attack surface is broad since SVG files are commonly accepted for user uploads and content management purposes.
Mitigation strategies for CVE-2021-40104 should prioritize immediate patching of Concrete CMS to versions that address the SVG sanitization bypass. Organizations should also implement additional security controls including strict file type validation, enhanced content filtering, and regular security audits of uploaded files. The implementation of Web Application Firewalls (WAF) with specific rules for SVG content validation can provide additional protection layers. Security teams should conduct thorough assessments of existing SVG files within the CMS to identify and remove any potentially compromised content. According to ATT&CK framework, this vulnerability aligns with T1566 which covers Phishing with Malicious Attachments, and T1059 which encompasses Command and Scripting Interpreter techniques. Organizations should also consider implementing principle of least privilege access controls and monitoring for unusual file upload activities to detect potential exploitation attempts. Regular security training for administrators and users regarding the risks of SVG file handling can further reduce the attack surface and improve overall security posture.