CVE-2021-40940 in Monstra
Summary
by MITRE • 06/15/2022
Monstra 3.0.4 does not filter the case of php, which leads to an unrestricted file upload vulnerability.
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Analysis
by VulDB Data Team • 06/16/2022
The vulnerability identified as CVE-2021-40940 affects Monstra CMS version 3.0.4 and represents a critical security flaw in the file upload validation mechanism. This issue stems from the application's failure to properly sanitize file extensions during the upload process, specifically allowing uppercase php extensions to bypass security checks. The vulnerability creates a dangerous condition where attackers can upload malicious files with php extensions in uppercase letters such as .PHP or .Php, which the application incorrectly accepts as valid uploads. This weakness directly violates fundamental security principles for file upload controls and represents a clear failure in input validation and sanitization processes. The flaw exists in the application's core file handling logic where case-sensitive validation is insufficient to prevent malicious file uploads. According to CWE-434, this vulnerability falls under unrestricted file upload, a category that has been consistently identified as one of the most dangerous web application security flaws due to its potential for remote code execution. The vulnerability aligns with ATT&CK technique T1505.003 which covers the use of web shell deployment, as successful exploitation could enable attackers to establish persistent access through uploaded malicious scripts.
The technical exploitation of this vulnerability requires minimal effort from an attacker who can simply rename a malicious php shell to uppercase extensions and upload it through the application's file upload interface. The root cause lies in the application's inadequate validation logic that does not normalize file extensions to lowercase before checking against allowed extensions. This creates a scenario where the system accepts files with uppercase php extensions while rejecting those with lowercase extensions, effectively bypassing the intended security controls. The vulnerability demonstrates poor security implementation practices and highlights the importance of comprehensive input validation. The flaw can be exploited by an authenticated user with appropriate privileges to upload files, though in some cases it may be exploitable by unauthenticated users depending on the application's configuration. The impact is severe as it allows for arbitrary code execution on the target server, potentially leading to complete system compromise and data exfiltration.
The operational impact of CVE-2021-40940 extends beyond immediate code execution capabilities to encompass broader system compromise and data security risks. Successful exploitation enables attackers to deploy web shells, backdoors, and other malicious payloads that can persist across system reboots and maintain unauthorized access. The vulnerability creates a persistent threat vector that can be leveraged for lateral movement within networks, privilege escalation, and data theft. Organizations running affected versions of Monstra CMS face significant risk of unauthorized access and potential data breaches. The vulnerability also impacts the application's integrity and availability, as attackers can modify or delete system files through uploaded malicious content. Security monitoring becomes challenging as the uploaded files may not be immediately detectable by traditional security controls. The flaw represents a critical gap in the application's security architecture and demonstrates the importance of proper file extension validation, including case normalization, in preventing such attacks. Organizations should immediately implement patch management procedures to address this vulnerability and conduct comprehensive security assessments of their web applications.
Mitigation strategies for CVE-2021-40940 require both immediate and long-term security measures to prevent exploitation. The primary remediation involves applying the official security patch provided by the Monstra development team, which should include proper case normalization and comprehensive file extension validation. Organizations should implement multiple layers of defense including strict file type validation, content inspection, and proper file upload directory permissions. The application should normalize all file extensions to lowercase before validation and maintain a strict whitelist of allowed file types rather than relying on blacklists. Network-level protections such as web application firewalls should be configured to monitor and block suspicious upload attempts. Regular security audits and penetration testing should be conducted to identify similar validation flaws in other applications. The vulnerability underscores the necessity of following secure coding practices and implementing comprehensive input validation as outlined in OWASP top ten security principles. Additionally, organizations should establish proper security monitoring procedures to detect unauthorized file uploads and maintain regular security updates for all web applications to prevent similar vulnerabilities from being exploited.