CVE-2020-4043 in phpMussel
Summary
by MITRE
phpMussel from versions 1.0.0 and less than 1.6.0 has an unserialization vulnerability in PHP's phar wrapper. Uploading a specially crafted file to an affected version allows arbitrary code execution (discovered, tested, and confirmed by myself), so the risk factor should be regarded as very high. Newer phpMussel versions don't use PHP's phar wrapper, and are therefore unaffected. This has been fixed in version 1.6.0.
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Analysis
by VulDB Data Team • 10/24/2020
The vulnerability identified as CVE-2020-4043 represents a critical unserialization flaw within phpMussel version 1.0.0 through 1.5.9, exploiting PHP's phar wrapper functionality to enable remote code execution. This vulnerability resides in the file handling mechanism of the phpMussel security suite, which is designed to scan and detect malicious files on web servers. The flaw specifically manifests when the application processes specially crafted phar files that contain serialized objects, creating a pathway for attackers to execute arbitrary code on the target system. The vulnerability has been classified as having a very high risk factor due to its ability to provide full remote code execution capabilities without requiring authentication or specific user interaction.
The technical exploitation of this vulnerability leverages PHP's phar wrapper functionality, which allows PHP to treat files as archives and automatically deserialize objects when the phar file is accessed. When phpMussel processes a malicious phar file through its file scanning mechanism, the phar wrapper triggers automatic deserialization of the serialized object contained within the archive. This deserialization process occurs without proper input validation, allowing attackers to inject malicious payloads that execute with the privileges of the web server process. The vulnerability directly maps to CWE-502, which describes "Deserialization of Untrusted Data" as a critical weakness that can lead to arbitrary code execution, privilege escalation, and denial of service attacks. The attack vector requires only successful file upload to the target system, making it particularly dangerous in environments where users can upload files to directories accessible by the web server.
The operational impact of this vulnerability extends beyond simple code execution, as it can enable attackers to gain full control over affected systems and potentially escalate privileges to access sensitive data or compromise the entire server infrastructure. Since phpMussel is typically deployed as a web application security tool, the vulnerability creates a dangerous scenario where the security tool itself becomes a potential attack surface. The risk is compounded by the fact that many organizations rely on phpMussel for malware detection and prevention, making the compromise of such a tool particularly damaging. Attackers can leverage this vulnerability to establish persistent access, deploy additional malware, or use the compromised system as a launch point for further attacks within the network. The vulnerability also aligns with ATT&CK technique T1059.007, which covers "Command and Scripting Interpreter: PowerShell," as the executed code can include PowerShell commands or other scripting languages that can be used to further compromise the system.
The fix implemented in phpMussel version 1.6.0 addresses this vulnerability by removing the dependency on PHP's phar wrapper functionality, thereby eliminating the automatic deserialization process that enabled the attack. This remediation approach follows security best practices by avoiding the use of potentially dangerous features that can lead to deserialization attacks. Organizations affected by this vulnerability should immediately upgrade to phpMussel version 1.6.0 or later, as earlier versions remain vulnerable to exploitation. The patch demonstrates a proper security response that addresses the root cause rather than implementing superficial workarounds, which aligns with the principle of defense in depth. Additionally, system administrators should implement proper file upload restrictions, validate all uploaded files through multiple security layers, and monitor for suspicious file uploads to prevent exploitation attempts. The vulnerability serves as a reminder of the importance of secure coding practices and the dangers of automatic deserialization in web applications, particularly when dealing with user-provided input that can be processed through archive formats like phar files.