What is Directory Traversal?

Directory traversal

Directory traversal, also known as path traversal. It is a type of security vulnerability that occurs when an attacker manipulates input to access files or directories that are outside the intended scope of the application. This vulnerability exploits improper validation or sanitization of user-supplied file paths.

How Directory Traversal Works

When an application allows users to specify file names or paths (e.g., to upload, download, or view files), an attacker can manipulate the input to traverse the directory structure of the server. This is often done using special sequences like ../ (dot-dot-slash), which means "go up one directory level."

For example:

• A vulnerable application has a URL parameter like this:

  https://example.com/view?file=notes.txt
  

• An attacker could modify the input to something like:

  https://example.com/view?file=../../etc/passwd
  

  This could give access to sensitive files like /etc/passwd on a Unix-based system.

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Common Exploitation Techniques

1. Using ../ Sequences: Repeatedly go up the directory hierarchy (e.g., ../../../../).
2. Encoding: Encode the traversal characters to bypass simple filters (e.g., %2e%2e%2f for ../).
3. Null Byte Injection: Combine traversal with a null byte (%00) to terminate file extensions in some cases.

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Consequences of Directory Traversal

• Data Breach: Access to sensitive files (e.g., configuration files, credentials).
• Code Execution: If an attacker can access files that can be executed by the server.
• System Compromise: Reading files like /etc/shadow or passwords.txt can lead to privilege escalation or complete server takeover.

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Prevention and Mitigation

1. Input Validation: Restrict user input to predefined file paths and file names.
2. Sanitization: Remove or encode dangerous characters such as ../, %2e, %2f, etc.
3. Whitelisting: Allow access only to specific files or directories.
4. Use Libraries or Frameworks: Many frameworks provide secure file-handling functions.
5. Least Privileges: Ensure the application has minimal permissions and cannot access sensitive areas.
6. Web Application Firewall (WAF): Use a WAF to detect and block suspicious traversal attempts.

By understanding and addressing directory traversal, developers can prevent attackers from exploiting this common but dangerous vulnerability.