Defending Against SSRF

Preventing SSRF requires defense in depth, starting with strict input validation. Maintain allowlists of permitted domains and protocols rather than trying to blocklist dangerous ones. It's easier to specify "only fetch from these image hosting sites" than to block every possible internal address. Validate URLs at multiple levels: check the protocol (typically only HTTPS should be allowed), verify the domain against your allowlist, and resolve the domain to ensure it doesn't point to internal IP ranges.

Implement network segregation to limit SSRF impact. Run services that fetch external resources in isolated network segments without access to sensitive internal resources. Use cloud platform features like AWS VPC endpoints and security groups to restrict metadata service access. Never trust user input to construct URLs directly—parse and validate every component. For cloud environments, use IMDSv2 (Instance Metadata Service version 2) which requires special headers, making SSRF attacks more difficult.

Consider implementing a proxy service for all external requests. This centralizes URL validation and logging while providing a single point for security controls. Monitor for suspicious patterns like requests to internal IP ranges, metadata endpoints, or unusual protocols. When possible, avoid features that require fetching arbitrary external resources—the security risk often outweighs the functionality benefit. Remember, SSRF is particularly dangerous because it abuses legitimate application functionality, making prevention through careful design crucial.

This comprehensive guide to the OWASP Top 10 provides the foundation for building more secure web applications. Security isn't a destination but a journey—threats evolve, new vulnerabilities emerge, and defensive techniques improve. Use this guide as a starting point, but continue learning and adapting your security practices. Remember, perfect security doesn't exist, but by understanding and addressing these top risks, you make your applications harder targets, encouraging attackers to look elsewhere. Start with the basics, implement systematically, and build security into your development culture. Your users' data depends on it.