What is File Path Traversal and how do hackers exploit it? The Detailed Guide

Table of Contents

• What is File Path Traversal?
• How File Path Traversal Works
• Real-World Example of File Path Traversal
• Common Attack Payloads
• Why File Path Traversal is Dangerous
• How to Prevent File Path Traversal
• File Path Traversal in Different Languages
• Tools to Detect Path Traversal
• Comparison of Path Traversal Impacts
• Conclusion
• Frequently Asked Questions (FAQs)

File Path Traversal is a common and dangerous web security vulnerability that allows attackers to access files and directories that are stored outside the intended folder. By exploiting weak or improperly validated user input, a threat actor can manipulate file paths and gain unauthorized access to sensitive system files—potentially leading to system compromise, data leakage, or complete server takeover.

In this blog, we’ll explore what file path traversal is, how it works, real-world examples, and how to prevent it effectively.

What is File Path Traversal?

File Path Traversal (also known as Directory Traversal) is a vulnerability that allows users to access directories and files that they shouldn't be allowed to access by manipulating file path variables.

This typically happens when the application allows file input via HTTP parameters (such as GET or POST), and doesn’t properly sanitize input paths. An attacker can use characters like ../ (dot-dot-slash) to move up the directory structure to access restricted files like:

/etc/passwd
/var/www/html/config.php
C:\Windows\System32\drivers\etc\hosts

How File Path Traversal Works

Most web applications serve dynamic files. Suppose you have a URL like:

http://example.com/page?file=about.html

The backend may be loading files like this:

file = request.GET['file']
open("pages/" + file)

If the input is not sanitized, an attacker can replace the file value with:

../../../../etc/passwd

Which changes the path to:

pages/../../../../etc/passwd

This can lead the server to expose sensitive OS files.

Real-World Example of File Path Traversal

In 2022, a vulnerability in a WordPress plugin allowed unauthenticated users to exploit a file path traversal flaw to download arbitrary files from the web server. Attackers used the vulnerability to gain access to critical configuration files, such as wp-config.php, which contained database credentials and secret keys.

Common Attack Payloads

Here are some typical payloads used in path traversal attacks:

• ../../../../etc/passwd – Linux file

• ../../../boot.ini – Windows file

• %2e%2e%2f – URL encoded for ../

• ..%5c..%5c..%5cwindows\win.ini – Windows backward slash version

Why File Path Traversal is Dangerous

Path traversal can lead to:

• Sensitive file disclosure: Accessing password files or app configuration files.

• Remote Code Execution: Uploading web shells or modifying executable scripts.

• Authentication bypass: Reading session tokens or cookie storage files.

• System compromise: If attacker accesses SSH keys, password hashes, etc.

How to Prevent File Path Traversal

1. Input Validation and Sanitization
Reject input with suspicious patterns like ../, ..%2F, or backward slashes.

2. Use Whitelisting
Allow access only to pre-approved filenames or paths stored in a safe list.

3. Normalize Paths
Use secure libraries to canonicalize and sanitize paths before use.

4. Implement File Access Restrictions
Use proper permission settings and restrict server access to only what is necessary.

5. Avoid User-Supplied Paths
Never allow users to directly specify file paths. Use IDs or mapped filenames instead.

6. Use Secure APIs
Some languages/frameworks have built-in protection for file operations. Use them where available.

File Path Traversal in Different Languages

• PHP: Use realpath() to resolve paths.

• Python: Use os.path.realpath() and validate base paths.

• Java: Use java.nio.file.Paths and restrict paths using startsWith().

Tools to Detect Path Traversal

• Burp Suite – Use Intruder or Scanner with payload lists.

• OWASP ZAP – Passive and active scanning for traversal patterns.

• Nikto – Includes checks for common path traversal issues.

• DirBuster – Can identify exposed directory listings.

Comparison of Path Traversal Impacts

Conclusion

File Path Traversal is a serious security vulnerability that continues to affect web applications, especially when input is not properly sanitized. Organizations must adopt secure coding practices, enforce strict access control, and perform regular vulnerability scans to detect and fix such issues early.

By understanding the mechanics behind path traversal and applying layered defenses, developers and security professionals can protect their applications from these potentially devastating attacks.

FAQs

What is a File Path Traversal vulnerability?

A File Path Traversal vulnerability allows attackers to access files and directories outside the web root by manipulating file path inputs.

How does a path traversal attack work?

An attacker injects sequences like ../ into a file path input to move up the directory tree and access restricted files.

Can path traversal be used to execute code?

Yes, if combined with Local File Inclusion (LFI) or Remote Code Execution (RCE) vulnerabilities.

What files do attackers usually target?

Common targets include /etc/passwd, configuration files, source code, logs, and credentials.

What are common indicators of a traversal attack?

Unusual file path requests in logs, access to unauthorized files, and encoded traversal patterns.

What tools are used to detect path traversal?

Burp Suite, OWASP ZAP, Nikto, and manual input fuzzing are popular tools.

How do you prevent File Path Traversal?

Use strict input validation, whitelist files, avoid using raw file names from input, and restrict file access.

What is double encoding in path traversal?

It’s when characters are encoded more than once (e.g., %252e%252e%252f) to bypass filters.

Is File Path Traversal the same as LFI?

They are related—path traversal helps access files, while LFI allows inclusion or execution of them.

Why is path traversal dangerous?

It can expose confidential files and lead to full system compromise if combined with other flaws.

Does HTTPS stop file path traversal?

No, HTTPS secures the transport, not the logic of file handling on the server.

Is path traversal only possible in PHP apps?

No, it can affect any language or framework that improperly handles file paths.

Can input sanitization fully prevent it?

Not always. Whitelisting and secure file handling are more effective.

Are there automated scanners for this?

Yes, tools like Burp Suite and OWASP ZAP can scan for it.

What does ../ mean in path traversal?

It means “go up one directory level” in Unix-style file systems.

What is URL encoding in traversal?

It’s the practice of hiding ../ as %2e%2e%2f to bypass input filters.

Can Windows systems be affected?

Yes, Windows path traversal uses ..\ instead of ../.

How does whitelisting help?

It restricts access to only known and safe files, blocking unauthorized ones.

What’s a sandbox directory?

A confined directory where users can only access specific files.

Can logs help detect path traversal?

Yes, by identifying unusual or unauthorized file access attempts.

What programming languages are most affected?

All—PHP, Java, Python, Node.js, etc.—if not properly secured.

How can developers avoid this?

By using secure file access methods and avoiding user-supplied file paths.

Is file ID safer than file name?

Yes, file IDs reduce the risk of path manipulation.

What is Local File Inclusion (LFI)?

LFI allows inclusion of local files, and path traversal can enable it.

Can firewalls block traversal?

Some WAFs (Web Application Firewalls) can detect and block known patterns.

Is this listed in OWASP Top 10?

Yes, it's part of the "A05:2021 – Security Misconfiguration" and related categories.

Should I log user file requests?

Yes, to detect abnormal behavior and trace attacks.

Can antivirus detect this?

No, antivirus software doesn't typically detect web application logic flaws.

Can this be tested manually?

Yes, by inputting ../ patterns and checking the server’s response.

Is encoding alone safe?

Encoding helps, but should be used with other security measures.