CVE-2017-6902 in b2evolution
Summary
by MITRE
Unrestricted file upload vulnerability in 'file upload' modules in b2evolution 6.8.8 allows authenticated users to upload malicious code (shell) by visiting the admin.php?ctrl=files page, even though the system has restricted the .php extension.
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Analysis
by VulDB Data Team • 08/19/2020
The CVE-2017-6902 vulnerability represents a critical unrestricted file upload flaw in the b2evolution content management system version 6.8.8. This vulnerability specifically targets the file upload modules within the administrative interface, creating a dangerous pathway for authenticated attackers to bypass security controls and execute malicious code on the affected system. The vulnerability exists in the admin.php?ctrl=files page where users can upload files, despite the system having implemented restrictions on .php file extensions. This represents a fundamental flaw in the application's security architecture, as the protection mechanisms are insufficient to prevent code execution through alternative means.
The technical implementation of this vulnerability stems from inadequate input validation and sanitization within the file upload functionality. When authenticated users attempt to upload files through the administrative interface, the system fails to properly validate file types beyond simple extension checks. Attackers can exploit this by uploading malicious files with extensions that are not properly blocked, such as .php3, .php4, .php5, or other variants that may be allowed by the restrictive filtering mechanism. The vulnerability operates at the application layer and can be exploited by authenticated users who have administrative privileges or can gain such access through other means, making it particularly dangerous as it requires minimal additional attack vectors to achieve code execution.
The operational impact of this vulnerability extends beyond simple code execution capabilities, as it provides attackers with persistent access to the compromised system. Once an attacker successfully uploads malicious code, they can establish backdoors, escalate privileges, or use the compromised system as a staging ground for further attacks against the network. The vulnerability allows for remote code execution, which can lead to complete system compromise, data exfiltration, and potential lateral movement within the network. This represents a severe threat to system integrity and confidentiality, as the attacker can manipulate the application's behavior and potentially gain access to sensitive user data or administrative functions.
Security professionals should address this vulnerability through immediate patching of the b2evolution platform to version 6.9.0 or later, which includes proper file upload validation and sanitization mechanisms. Organizations should also implement additional security controls such as implementing proper file type validation, using Content Security Policies, and restricting file upload functionality to trusted users only. The vulnerability aligns with CWE-434, which describes unrestricted upload of code, and can be mapped to ATT&CK technique T1190 for exploit public-facing application, as it allows for remote code execution through web application vulnerabilities. Regular security audits and proper input validation practices should be enforced to prevent similar issues in other applications, as this vulnerability demonstrates the critical importance of robust file handling mechanisms in web applications.