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What is Server-Side Request Forgery?

Discover what Server-Side Request Forgery is and how to detect and prevent SSRF attacks effectively.

Explain Like I'm 5

Imagine your friend is at a party (the server) and you want to see inside some rooms (internal resources) without being invited. If you can trick your friend into going into those rooms and bringing something back for you, that's like a Server-Side Request Forgery. You make your friend think they're just doing a simple task, but really, they're helping you access things you're not supposed to see.

Sometimes, your friend can bring back exactly what you asked for (non-blind SSRF), like finding a piece of candy in a room and showing it to you. Other times, they come back with only hints about what's inside (blind SSRF), so you have to guess based on their clues.

This is important because if someone can fool a server into exploring its own place and revealing secrets, they might find keys to the server's treasures. That's why we teach servers not to trust just anyone who asks them to do things.

Technical Definition

Definition

Server-Side Request Forgery (SSRF) is a web security vulnerability that allows an attacker to induce the server-side application to make HTTP requests to an arbitrary domain of the attacker's choosing. This can lead to unauthorized access to internal resources, often bypassing firewall protections.

How It Works

  1. 1The attacker identifies a vulnerable server that processes URL input from the user.
  2. 2The attacker crafts a malicious request, tricking the server into fetching a URL chosen by the attacker.
  3. 3The server processes the request and returns data to the attacker, potentially exposing sensitive information.
  4. 4Blind SSRF occurs when the attacker cannot see the response but can infer information from side effects.

Key Characteristics

  • Exploits server trust and access to internal networks.
  • Can result in data exfiltration or unauthorized internal access.
  • Often targets cloud metadata services, like AWS 169.254.169.254.

Comparison

TermDescription
SSRFManipulates server requests to access internal resources.
CSRFTricks users into executing unwanted actions on websites.
XSSInjects scripts into web pages viewed by users.

Real-World Example

The Capital One breach in 2019 (CVE-2019-9193) involved an SSRF attack that exploited AWS metadata access, leading to unauthorized data access of over 100 million customer records.

Detection & Prevention

  • Detection: Use tools like Burp Suite or nuclei with SSRF templates to simulate and detect vulnerabilities.
  • Prevention:
- Implement network segmentation to limit server access to internal resources. - Use allowlists to specify allowable external domains. - Validate and sanitize all user inputs to prevent malicious URL requests.

Common Misconceptions

  • Myth: SSRF is only a concern for cloud services.
Reality: It can affect any server with internal network access.
  • Myth: Firewalls alone can prevent SSRF.
Reality: SSRF can bypass firewalls by exploiting trusted server access.

Keywords

what is Server-Side Request ForgeryServer-Side Request Forgery explainedSSRF detectionSSRF preventionServer-Side Request Forgery examplecloud metadata SSRFSSRF vulnerability

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