OWASP Top 10 Series – Server-Side Request Forgery

In these series, I will try to explain each one of the security risks in the OWASP Top 10 (2021) list in the simplest way possible. Hopefully this can help myself and other’s to recall these concepts quickly.

Understanding OWASP Server-Side Request Forgery

OWASP Server-Side Request Forgery refers to vulnerabilities that allow attackers to manipulate an application into making arbitrary requests on behalf of the server. This can lead to unauthorized access to internal resources, data leaks, and potentially, remote code execution. Addressing this vulnerability is pivotal as SSRF attacks can undermine an application’s security architecture and compromise sensitive information.

Examples and Risks of Server-Side Request Forgery

  1. Access to Internal Resources: Attackers can exploit SSRF to access internal resources that should not be exposed to the public internet, including databases, backend systems, and internal APIs.
  2. Remote Code Execution: In certain cases, SSRF vulnerabilities can be escalated to allow remote code execution on the server, potentially leading to full system compromise.
  3. Data Leakage: Attackers can use SSRF to exfiltrate data from other websites, APIs, or systems, potentially exposing sensitive information.

Real-world Example: Exploiting Server-Side Request Forgery

Consider a web application that allows users to input a URL to fetch metadata from external websites. The application doesn’t validate the URL and directly sends requests to the provided URL. An attacker exploits this vulnerability by inputting a malicious URL that points to an internal server. This manipulation causes the server to make requests to the internal server, potentially leading to unauthorized data exposure or even remote code execution.

Prevention Strategies and Best Practices

  1. Input Validation: Implement strict input validation and whitelisting of allowed URLs to prevent attackers from injecting malicious URLs.
  2. Restrict Network Access: Configure firewalls and network settings to restrict outbound requests from the server to trusted destinations.
  3. Use Whitelists: Only allow connections to known and trusted external resources. Use whitelists to define the permissible targets.
  4. Use DNS Resolution Safeguards: Employ DNS rebinding protections to ensure that the resolved IP addresses match the intended targets.
  5. Request Sanitization: Filter and sanitize user-provided URLs to remove potentially malicious components.