What is Server Side Request Forgery SSRF?

Understanding Server Side Request Forgery SSRF

Server-Side Request Forgery (SSRF) occurs when an application accepts remote resource URLs from user input and fetches them server-side without proper validation. Attackers can abuse SSRF to make the server connect to internal services (metadata endpoints in cloud environments, admin panels, or databases) bypassing firewall rules, or to exfiltrate data. Common SSRF examples include file import/export features, webhooks, or image fetchers referencing user-provided URLs. Defenses include strict input validation (whitelisting permissible hostnames or using DNS resolvers that block private IP ranges), restricting network access for the application server (no outbound connectivity or egress filtering), and disabling unused URL schemes. Cloud providers often highlight SSRF’s risk, as reading `http://169.254.169.254/` can yield instance credentials. Attackers also attempt encoded or redirected URLs to circumvent naive checks. Sophisticated solutions integrate a “safe” proxy that only permits certain domains or IPs, preventing accidental internal calls. SSRF escalations might lead to code injection if the internal service lacks authentication. Overall, SSRF highlights the need for zero-trust principles on internal endpoints. Combined with network segmentation, application-level defenses drastically reduce SSRF exploit success.