Identification of Server-Side Request Forgery (SSRF)

Server-Side Request Forgery (SSRF) is a security vulnerability where an attacker induces the server to make a request to an unintended location, potentially accessing sensitive data or interacting with internal systems. Here's how a penetration tester can identify SSRF vulnerabilities:

Steps for Identification

  1. Identify User-Controlled Input: Look for any functionality where a user can influence URLs or other data used in server-side requests. This often includes features like importing data from a URL, image loading, webhooks, or integration with external services.
  2. Craft Malicious Requests: Attempt to craft requests that change the destination of the server-side request to an internal service or an unintended external service. For example, modifying a request to fetch an image from to http://localhost/admin.
  3. Test Internal and External Schemes: Try various schemes in the URLs to access internal resources (file://, ftp://, dict://, etc.) or cloud metadata services, which could reveal sensitive information.
  4. Observe Responses: Analyze the responses from the server to determine if the payload was successful in accessing internal services or data. Even error messages or timing differences can provide clues.
  5. Automated Scanning Tools: Utilize tools designed to identify SSRF vulnerabilities. These tools can automate the process of sending various payloads to see if the server makes unexpected external or internal requests.

Examples of SSRF Attacks

  1. Accessing Internal Services: The attacker might access internal services like databases, admin interfaces, or APIs that are not exposed to the external network.
  2. Interacting with Cloud Services: On cloud platforms, SSRF can be used to interact with metadata services, potentially allowing an attacker to access credentials and other sensitive data.
  3. Bypassing IP Whitelisting: SSRF can be used to bypass IP-based whitelisting, allowing attackers to interact with services that only accept internal traffic.

Implications of SSRF

  • Data Exposure: Accessing internal services can lead to unauthorized data exposure or data leakage.
  • Network Reconnaissance: Attackers can use SSRF to map out internal network architecture, identifying other vulnerable systems or services.
  • Elevated Attacks: SSRF can be a stepping stone to more severe attacks, like Remote Code Execution (RCE) or lateral movement within the network.

Mitigation Strategies

  • Whitelist Allowable Resources: Only allow the server to make requests to a predefined list of safe resources.
  • Validate and Sanitize Input: Ensure all user-controlled input used in server-side requests is strictly validated and sanitized.
  • Use Allowlists for URL Schemes: Restrict the URL schemes that the server can use in requests (e.g., only allow http or https).
  • Implement Network Segregation: Ensure that the server cannot access critical internal resources directly, using network segmentation and firewalls.
  • Monitor and Log Requests: Keep detailed logs of server-side requests to detect and investigate potential SSRF attempts.

By systematically testing and identifying SSRF vulnerabilities, penetration testers can uncover critical weaknesses that could lead to serious security breaches, thus playing a vital role in securing applications against such attack vectors.