Understanding SSRF (Server-Side Request Forgery) Through CTF Events

Server-Side Request Forgery (SSRF) is one of the most impactful web vulnerabilities in modern applications. It allows an attacker to make a vulnerable server send requests to unintended destinations, often giving access to sensitive internal systems.

At Parrot CTFs, we frequently include SSRF scenarios in our CTF event hosting challenges to teach both beginners and professionals how to identify, exploit, and remediate this class of vulnerability.

---

What is SSRF?

SSRF occurs when an application takes user-supplied input (like a URL or resource location) and uses it to make a server-side request without proper validation.

Example Scenario:

A web app fetches an image from a user-provided URL:

GET /fetch?url=http://example.com/image.png

If input isn’t sanitized, an attacker could supply:

http://localhost/admin

The server, which can access internal resources, fetches this instead—exposing internal admin panels or metadata services (e.g., AWS IMDS at http://169.254.169.254).

---

Why SSRF Matters

Impact Area	Description
Internal Systems	Access databases, cloud metadata, or services not exposed externally.
Cloud Exploitation	Retrieve AWS/GCP/Azure credentials from metadata APIs.
Bypass Firewalls	The server can make requests attackers can’t directly send.
Data Exfiltration	Sensitive files or tokens can be leaked through crafted responses.

This makes SSRF one of the most dangerous vulnerabilities, especially in cloud-heavy environments.

---

How CTF Events Teach SSRF

At Parrot CTFs-hosted events, SSRF challenges are used to show:

1. Basic SSRF Exploitation
   • Redirecting server requests to internal services.
2. Advanced Payloads
   • Using URL encodings or DNS rebinding to bypass filters.
3. Cloud-Specific Attacks
   • Exploiting SSRF to access AWS EC2 metadata (http://169.254.169.254/latest/meta-data/).
4. Defense-in-Depth
   • How input validation, allow-lists, and proxy controls can mitigate SSRF.

---

Example SSRF CTF Challenge

Imagine a challenge where players are given an “image fetcher” app.

• Goal: Retrieve the flag hidden in the cloud provider’s metadata service.
• Steps:
  1. Identify the vulnerable url parameter.
  2. Craft a payload pointing to the metadata service.
  3. Parse the response to extract secrets.
  4. Submit the captured flag to the scoreboard.

By simulating these attacks in a controlled CTF environment, participants get hands-on experience with a vulnerability that has real-world consequences.

---

Hosting an SSRF Challenge with Parrot CTFs

Whether for university students, corporate staff training, or conference events, Parrot CTFs can host SSRF-focused scenarios.

Event Type	SSRF Training Example
University Class	Students learn SSRF exploitation as part of a secure coding course.
Corporate Training	Developers discover how SSRF can lead to cloud breaches and how to prevent it.
Conference Event	Attendees compete in advanced SSRF labs designed around modern cloud infrastructure.