CVE-2021-37626 in Contao
Summary
by MITRE • 08/12/2021
Contao is an open source CMS that allows you to create websites and scalable web applications. In affected versions it is possible to load PHP files by entering insert tags in the Contao back end. Installations are only affected if they have untrusted back end users who have the rights to modify fields that are shown in the front end. Update to Contao 4.4.56, 4.9.18 or 4.11.7 to resolve. If you cannot update then disable the login for untrusted back end users.
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Analysis
by VulDB Data Team • 08/16/2021
CVE-2021-37626 represents a critical server-side request forgery vulnerability within the Contao content management system that stems from improper input validation and privilege escalation mechanisms. This vulnerability specifically affects versions prior to 4.4.56, 4.9.18, and 4.11.7, creating a dangerous attack surface where untrusted back end users can potentially execute arbitrary PHP code through insert tags. The flaw operates by allowing malicious actors with limited back end permissions to manipulate insert tag functionality, which then enables them to load and execute PHP files on the server. This vulnerability falls under CWE-94, representing improper control of generation of code, and aligns with ATT&CK technique T1505.003 for server-side include attacks. The security implications extend beyond simple code execution as this vulnerability enables attackers to perform privilege escalation and potentially gain full administrative control over the affected systems.
The technical exploitation of this vulnerability requires an attacker to possess credentials for a back end user account with permissions to modify fields that are rendered on the front end. This creates a specific attack vector where the attacker can leverage the insert tag functionality to load malicious PHP files, effectively bypassing traditional access controls and authorization mechanisms. The vulnerability demonstrates a fundamental flaw in Contao's input sanitization processes, where user-supplied insert tag content is not properly validated or escaped before being processed. When legitimate insert tags are manipulated to include malicious file paths or code snippets, the system processes these inputs without adequate security checks, leading to arbitrary code execution. The impact is particularly severe in environments where multiple users have back end access, as it allows for lateral movement and privilege escalation within the CMS infrastructure.
Organizations running affected Contao installations face significant operational risks including data breaches, system compromise, and potential regulatory violations. The vulnerability enables attackers to access sensitive data, modify content, install malware, and potentially use the compromised system as a pivot point for attacking other systems within the network. The attack requires minimal technical expertise to exploit, making it particularly dangerous for organizations that cannot immediately implement security updates. The vulnerability also impacts compliance with various security standards including iso 27001, pci dss, and gdpr, as it creates potential data exposure and unauthorized access scenarios. Organizations may experience service disruption, reputational damage, and financial losses due to the exploitation of this vulnerability.
The recommended mitigation strategy involves immediate patching to versions 4.4.56, 4.9.18, or 4.11.7, which contain the necessary security fixes and input validation improvements. Organizations unable to update immediately should implement strict access controls by disabling login capabilities for untrusted back end users, thereby preventing unauthorized access to the vulnerable functionality. Additional mitigations include implementing web application firewalls, monitoring for suspicious insert tag usage patterns, and conducting regular security audits of back end user permissions. Security teams should also consider implementing principle of least privilege models, ensuring that only trusted personnel have access to back end modification capabilities. The vulnerability highlights the importance of proper input validation and privilege separation in web applications, serving as a reminder of the critical need for comprehensive security testing and regular vulnerability assessments in content management systems.