CVE-2017-16221 in yzt
Summary
by MITRE
yzt is a simple file server. yzt is vulnerable to a directory traversal issue, giving an attacker access to the filesystem by placing "../" in the url.
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Analysis
by VulDB Data Team • 02/16/2020
The vulnerability identified as CVE-2017-16221 affects yzt, a simple file server implementation that suffers from a critical directory traversal flaw. This issue stems from insufficient input validation and sanitization within the application's URL handling mechanism, allowing remote attackers to manipulate file paths through crafted requests containing directory traversal sequences. The vulnerability manifests when the server fails to properly sanitize user-supplied URL parameters, enabling attackers to navigate outside the intended directory structure and access arbitrary files on the underlying filesystem. This represents a fundamental security weakness in the application's path resolution logic, where the server processes user input without adequate restrictions on path components.
The technical exploitation of this vulnerability follows the classic directory traversal attack pattern, where attackers append sequences such as "../" to manipulate the file system path resolution. When the yzt server processes these malicious URLs, it fails to validate or sanitize the path components, allowing the traversal to occur. The vulnerability directly maps to CWE-22, which describes improper limitation of a pathname to a restricted directory, commonly known as path traversal or directory traversal. This weakness enables attackers to access files outside the intended directory scope, potentially exposing sensitive system files, configuration data, or user information. The attack vector is particularly dangerous as it requires no authentication and can be executed remotely through standard HTTP requests.
The operational impact of CVE-2017-16221 is severe and multifaceted, potentially exposing the entire file system to unauthorized access. An attacker could retrieve sensitive files such as configuration files, database credentials, application source code, or system logs that might contain confidential information. The vulnerability could also enable further attacks by allowing access to system binaries or libraries that might be exploited for privilege escalation. Depending on the server's configuration and file permissions, attackers might gain access to critical system files, user documents, or even administrative interfaces. This vulnerability essentially provides a backdoor into the server's file system, potentially compromising the confidentiality and integrity of all data stored on the affected system.
Mitigation strategies for this vulnerability should focus on implementing robust input validation and sanitization mechanisms. The primary fix involves ensuring that all URL parameters containing file paths are properly validated to prevent directory traversal sequences from being processed. This can be achieved through path normalization techniques that strip or encode dangerous characters such as "../", "..\", or similar traversal patterns. Implementing a whitelist approach for allowed file paths or using secure file access libraries that automatically handle path validation can effectively prevent this class of vulnerabilities. Organizations should also consider implementing proper access controls and file system permissions to limit what files can be accessed even if the traversal vulnerability is exploited. The solution aligns with ATT&CK technique T1083, which describes the use of file and directory discovery methods, and emphasizes the importance of proper input validation as a defensive measure. Additionally, regular security audits and penetration testing should be conducted to identify and remediate similar vulnerabilities in other applications and systems.