What is Path traversal ?

Path traversal, also known as directory traversal, is a web security vulnerability that allows an attacker to access files and directories on a server that are outside the intended access scope of the application. This occurs when an application does not properly sanitize user input and allows manipulation of file paths.

How Path Traversal Works

  1. User Input: Many web applications accept user input to construct file paths (e.g., to fetch or save a file).
  2. Exploitation: An attacker provides input containing directory traversal sequences like ../ or ..\ to move up the directory structure and access files outside the designated folder.
  3. Result: If not properly validated, the application accesses unintended files, potentially exposing sensitive data.

Example

Consider a web application that loads a file based on user input:

# Pseudocode
file_path = "/var/www/app/files/" + user_input
open(file_path, "r")

If the attacker provides input like ../../etc/passwd, the resulting file_path becomes:

/var/www/app/files/../../etc/passwd

This could allow the attacker to read the system’s password file (/etc/passwd), which is outside the intended directory.

Risks of Path Traversal

  1. Unauthorized Access: Attackers can access sensitive files, such as configuration files, credentials, or system logs.
  2. Data Leakage: Exposure of sensitive data to unauthorized parties.
  3. Further Exploitation: Reading application source code or server configurations might reveal additional vulnerabilities.

Mitigation Strategies

  1. Input Validation:
    • Reject suspicious input such as ../, ..\, or absolute paths.
    • Use allowlists to specify valid file names or paths.
  2. Path Normalization:
    • Resolve the user input to an absolute path and verify it stays within the intended directory.
  3. Access Control:
    • Use proper file permissions to ensure the application has minimal access to sensitive files.
  4. Environment Isolation:
    • Run the application in a sandboxed environment where access to sensitive files is restricted.
  5. Library/Framework Features:
    • Use secure functions or libraries that handle file paths properly, avoiding manual concatenation.

Understanding and addressing path traversal vulnerabilities is critical to securing web applications and preventing unauthorized access to sensitive server data.