CVE-2018-20129 in DeDeCMS
Summary
by MITRE
An issue was discovered in DedeCMS V5.7 SP2. uploads/include/dialog/select_images_post.php allows remote attackers to upload and execute arbitrary PHP code via a double extension and a modified ".php" substring, in conjunction with the image/jpeg content type, as demonstrated by the filename=1.jpg.p*hp value.
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Analysis
by VulDB Data Team • 04/20/2020
This vulnerability exists within the DedeCMS content management system version 5.7 SP2 where the file upload functionality in uploads/include/dialog/select_images_post.php fails to properly validate file extensions and content types. The flaw allows attackers to bypass security measures by using a double extension technique where a file named 1.jpg.php gets processed with the .jpg extension but retains executable PHP code within the .php portion. The vulnerability specifically leverages the image/jpeg content type header to mask malicious uploads, making the attack more stealthy and effective. This represents a classic file upload vulnerability that can be classified under CWE-434 as an insecure file upload without proper validation of file types and extensions.
The technical implementation of this vulnerability exploits the way the system processes file uploads by not properly sanitizing filenames or validating the actual file content against the declared content type. When an attacker submits a file with a double extension, the system accepts the first extension as legitimate while ignoring the second extension that contains the malicious payload. The modified ".php" substring in the filename allows the system to treat the file as an image while simultaneously embedding PHP code that can be executed on the server. This attack vector demonstrates a failure in input validation and sanitization, which are fundamental security controls that should prevent such bypasses.
The operational impact of this vulnerability is significant as it provides remote attackers with arbitrary code execution capabilities on the target server. Once successfully exploited, attackers can upload backdoors, web shells, or other malicious payloads that allow persistent access to the system. The vulnerability enables complete compromise of the web application and potentially the underlying server infrastructure. This represents a critical security risk that can lead to data breaches, system infiltration, and further lateral movement within network environments. The attack requires minimal privileges and can be automated, making it particularly dangerous in production environments.
Mitigation strategies for this vulnerability involve implementing strict file extension validation and content type verification mechanisms. Organizations should enforce whitelist-based validation that only allows specific, safe file extensions such as .jpg, .png, and .gif while rejecting any files with double extensions or suspicious patterns. The system must validate that the actual file content matches the declared content type and implement proper file name sanitization to prevent manipulation of extensions. Additionally, the web server configuration should be adjusted to prevent execution of PHP files in upload directories, and regular security audits should be conducted to identify similar vulnerabilities. This vulnerability aligns with ATT&CK technique T1106 for execution through file upload and T1059 for command and scripting interpreter usage, highlighting the need for comprehensive defensive measures across multiple security domains.