CVE-2019-7216 in FileChucker
Summary
by MITRE
An issue was discovered in FileChucker 4.99e-free-e02. filechucker.cgi has a filter bypass that allows a malicious user to upload any type of file by using % characters within the extension, e.g., file.%ph%p becomes file.php.
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Analysis
by VulDB Data Team • 05/07/2020
The vulnerability identified as CVE-2019-7216 resides within FileChucker version 4.99e-free-e02, specifically in the filechucker.cgi component that governs file upload operations. This represents a critical security flaw that undermines the intended file type validation mechanisms. The issue manifests through a sophisticated filter bypass technique that exploits how the application processes file extensions containing percent encoding characters. The vulnerability stems from insufficient input sanitization and validation logic that fails to properly interpret encoded file extensions, allowing attackers to circumvent security controls designed to restrict file uploads to specific types.
The technical implementation of this vulnerability demonstrates a classic case of improper input validation and sanitization, which aligns with CWE-20 - Improper Input Validation. The flaw occurs when the application processes file extensions that contain percent characters, such as file.%ph%p, which should be rejected as invalid but are instead interpreted as legitimate file extensions. This bypass mechanism operates by exploiting the way the application parses and validates file extensions, where the percent encoding is not properly decoded or validated before the file upload decision is made. The vulnerability specifically targets the file type filtering logic that should prevent execution of potentially malicious files such as php scripts, perl scripts, or other web shells.
The operational impact of this vulnerability is severe and far-reaching for any system utilizing the affected FileChucker software. An attacker can leverage this flaw to upload malicious web shells or scripts that can then be executed on the target server, potentially leading to complete system compromise. The ability to bypass file type restrictions enables unauthorized code execution, data exfiltration, and persistent access to the compromised system. This vulnerability particularly affects web applications that rely on FileChucker for file management operations, making it a prime target for attackers seeking to establish backdoors or gain unauthorized access to web servers. The implications extend beyond simple file upload restrictions as this vulnerability can lead to full system compromise and persistent unauthorized access.
Mitigation strategies for this vulnerability should focus on implementing robust input validation and sanitization techniques that properly handle encoded characters in file extensions. Organizations should immediately upgrade to a patched version of FileChucker if available, or implement proper input sanitization that decodes and validates all file extensions before processing. The solution must ensure that percent-encoded characters are properly interpreted and validated against a strict whitelist of allowed file extensions. Security measures should include implementing multiple layers of validation including server-side checks, proper file type detection, and content verification mechanisms. Additionally, implementing proper access controls and monitoring for unusual file upload activities can help detect exploitation attempts. This vulnerability highlights the importance of comprehensive input validation and the need for security controls that address not just direct input but also encoded or obfuscated inputs, as outlined in the ATT&CK framework under T1105 - Ingress Tool Transfer and T1059 - Command and Scripting Interpreter.