Simple Directory Traversal

Directory traversal (also known as path traversal) is a fundamental web security vulnerability that allows attackers to access files and directories outside the intended scope of the application. By manipulating file path inputs with special character sequences, attackers can read sensitive system files, application source code, and configuration files containing credentials. This vulnerability type has been consistently present in web security assessments for decades.

How Directory Traversal Works

Web applications frequently serve files based on user-supplied input - whether through URL parameters, form fields, or API requests. When the application constructs file paths by directly concatenating user input without proper validation, attackers can use ../ (dot-dot-slash) sequences to traverse up the directory tree and access files outside the web root. For example, requesting ../../etc/passwd would attempt to read the Linux password file by climbing two directories above the application's file serving directory.

Common Attack Patterns and Bypass Techniques

Basic directory traversal uses simple ../ sequences, but real-world exploitation often requires bypass techniques. URL encoding (%2e%2e%2f), double URL encoding (%252e%252e%252f), and using backslashes on Windows systems (..\) can evade basic input filters. Some applications strip ../ sequences once, which can be bypassed with ....// (the remaining characters form ../ after stripping). Null byte injection (%00) has historically been used to truncate file extensions. Understanding these techniques is essential for thorough security testing.

Real-World Impact and Defense

Directory traversal vulnerabilities have been discovered in web servers, content management systems, file sharing applications, and cloud platforms. Successful exploitation can reveal database credentials, API keys, SSL certificates, and other sensitive configuration data. In severe cases, attackers combine directory traversal with file upload vulnerabilities to achieve remote code execution. Defense requires proper input validation, canonicalization of file paths before use, chroot jails or containerization, and the principle of least privilege for application file system access.