SSRF Validator

Server-Side Request Forgery (SSRF) is a critical web vulnerability that allows attackers to induce the server-side application to make HTTP requests to an arbitrary destination of the attacker's choosing. Ranked in the OWASP Top 10, SSRF has been responsible for some of the most significant cloud security breaches in recent years, including the 2019 Capital One breach that exposed over 100 million customer records. This SSRF tutorial covers the fundamentals of this increasingly important attack class.

How SSRF Attacks Work

SSRF vulnerabilities arise when a web application fetches a remote resource based on a user-supplied URL without proper validation. Common features that are vulnerable include URL preview generators, webhook handlers, file importers, and PDF renderers. When an attacker can control the destination URL, they can redirect the server's request to internal resources that are not accessible from the internet - such as cloud metadata services, internal APIs, databases, and administration panels. The server acts as a proxy, bypassing network-level access controls.

SSRF Attack Techniques and Bypass Methods

Basic SSRF involves accessing internal addresses like http://localhost or http://169.254.169.254 (the cloud metadata endpoint). This SSRF tutorial also covers advanced bypass techniques needed when applications implement URL validation. These include using alternative IP representations (decimal, hexadecimal, octal), DNS rebinding attacks, URL parsing inconsistencies, protocol switching (using gopher:// or file://), and redirect-based bypasses where the attacker's server redirects to an internal address. Each technique exploits different aspects of how applications parse and validate URLs.

Cloud Environment Impact

SSRF is particularly devastating in cloud environments. AWS, GCP, and Azure all provide instance metadata services accessible via link-local addresses. An SSRF vulnerability in a cloud-hosted application can allow attackers to steal IAM credentials, access configuration data, and pivot to other cloud resources. The combination of SSRF and cloud metadata services has become one of the most critical attack patterns in modern cybersecurity, making SSRF testing an essential skill for penetration testers and security engineers.