Directory Traversal: When Hackers Walk Through Your Filesystem By CyberDudeBivash – Your Daily Dose of Ruthless, Engineering-Grade Threat Intel

1. Introduction

Directory Traversal — also known as Path Traversal — is a classic yet devastating vulnerability that allows attackers to access files and directories outside the intended scope of a web application. By manipulating file path inputs, an attacker can jump out of the application’s working directory and read, modify, or exfiltrate sensitive files.

Why it’s still critical in 2025:

Legacy systems and insecure coding patterns still exist in production.
APIs, IoT devices, and microservices often have inadequate path sanitization.
Directory traversal remains a top entry point for data breaches, configuration leaks, and credential theft.

2. How Directory Traversal Works

When an application constructs file paths using user-supplied input without proper validation or sanitization, attackers can insert special path sequences to navigate up the directory tree.

Example payload:

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

Attack Flow:

Vulnerable Endpoint – File download/view feature uses direct file path concatenation.
Input Manipulation – Attacker injects ../ sequences to move to parent directories.
Target Access – Sensitive system files or application source code become accessible.

3. Common Targets in Directory Traversal Attacks

Unix/Linux: /etc/passwd, /etc/shadow
Windows: C:\Windows\System32\config\SAM
Application Configs: .env, config.php, database.yml
Source Code: .git/, /WEB-INF/web.xml
Cloud Metadata (via local services): /var/lib/cloud/instance/

4. Types of Directory Traversal

4.1 Relative Path Traversal

Using ../ sequences to navigate up the directory tree.

4.2 Absolute Path Traversal

Using a full path (e.g., /etc/passwd) if the application allows absolute references.

4.3 Encoded Path Traversal

Encoding traversal sequences to bypass filters:

%2e%2e%2f → ../
%252e%252e%252f (double encoding)

4.4 Null Byte Injection

On older systems, %00 can terminate a string early, bypassing file extension checks.

5. Real-World Incidents

Fortinet VPN (2023) – Directory traversal allowed reading arbitrary system files, aiding RCE.
GoAhead Web Server Exploit – Popular in IoT devices, traversal flaws exposed sensitive configs.
GitLab 2022 – Path traversal in file upload feature exposed private repository data.

6. MITRE ATT&CK Mapping

T1005 – Data from Local System
T1083 – File and Directory Discovery
T1552.001 – Credentials in Files
T1213 – Data from Information Repositories

7. Advanced Exploitation Techniques in 2025

Technique	Description	Example
Filter Bypass via Encoding	Obfuscating traversal strings to evade WAFs.	`%252e%252e%252f`
Parameter Pollution	Adding traversal sequences in unexpected parameters.	`lang=../../../../etc/passwd`
File Upload + Traversal	Combining malicious uploads with traversal to overwrite sensitive files.	Uploading shell to `/var/www/html/../../tmp/`
Symlink Abuse	Pointing a symlink to sensitive files and accessing them via the app.	`/uploads/symlink_to_shadow`

8. Detection & Prevention Strategies

A. Input Validation & Sanitization

Reject ../, ..\, or encoded equivalents in file paths.
Use whitelisting for allowed file names.

B. Path Normalization

Resolve and canonicalize paths before accessing files.

C. Least Privilege

The application process should not have read/write access to sensitive OS files.

D. Virtual Chroot/Jailing

Use chroot environments or containerized file access to isolate application directories.

E. Logging & Monitoring

Detect traversal patterns in server logs.
Use WAF rules to block common traversal payloads.

9. Threat Hunting Tips

Look for ../ patterns in HTTP request parameters and URLs.
Review access logs for file requests outside the application directory.
Monitor for repeated 403/404 errors targeting unusual file paths.

10. CyberDudeBivash Recommendations

Red Team: Actively test file handling features with traversal payloads during pentests.
Blue Team: Deploy anomaly detection for suspicious file access patterns.
DevSecOps: Integrate path traversal checks into CI/CD pipelines.

Conclusion

Directory Traversal is deceptively simple but devastatingly effective. A single insecure file-handling function can expose entire systems. In modern DevSecOps workflows, preventing it is about rigorous input validation, isolation, and proactive testing.

Bottom line: Don’t let attackers take a stroll through your filesystem.

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