Server-Side Request Forgery, or SSRF, is a web security vulnerability where attackers can force a server to make requests to arbitrary domains of their choosing. This allows attackers to access internal systems, scan internal networks, retrieve sensitive data, and bypass security controls. In a typical scenario, a legitimate user makes normal requests to a web server, which processes them and returns appropriate responses. However, an attacker can exploit vulnerable functionality by sending specially crafted requests that cause the server to make additional requests to internal resources that should not be accessible from the outside.
Let's look at how SSRF attacks work in practice. Attackers typically exploit vulnerable URL input fields, webhooks, document processors, or API integrations. A common example is a web application that fetches external resources based on user input. When the application doesn't properly validate the URL, attackers can provide internal addresses like localhost or private IP ranges. This vulnerable Python code shows how a server might accept a URL parameter without validation and make a request to it, potentially exposing internal resources. The attack flow starts with the attacker sending a malicious URL from the internet, and then the server makes an internal request to sensitive resources like databases.
Attackers using SSRF vulnerabilities typically target several key resources. Cloud metadata endpoints, like the AWS metadata service at 169.254.169.254, can reveal sensitive credentials and configuration details. Internal admin interfaces that aren't exposed to the internet but accessible locally can be compromised. Internal APIs may contain sensitive business logic or data. Services running on localhost that assume they're only accessible from the local machine are vulnerable. And attackers can use SSRF for network scanning to map out internal networks. The diagram shows these common attack paths, where a compromised cloud server can be used to access the metadata service, admin interfaces, internal APIs, and databases.
Preventing SSRF attacks requires implementing several defense layers. First, always validate and sanitize all URLs provided by users, ensuring they're well-formed and point to legitimate resources. Use allowlists for domains rather than blocklists, as allowlists specify exactly which domains are permitted. Block requests to internal IP ranges and localhost to prevent internal service access. Implement proper access controls on all internal services as a defense-in-depth measure. And use dedicated service accounts with minimal privileges for server-to-server communications. The secure code example shows how to implement URL validation by checking against an allowlist and blocking private IP addresses. The diagram illustrates the defense-in-depth approach with multiple security layers protecting internal resources.
Let's look at how SSRF attacks work in practice. Attackers typically exploit vulnerable URL input fields, webhooks, document processors, or API integrations. A common example is a web application that fetches external resources based on user input. When the application doesn't properly validate the URL, attackers can provide internal addresses like localhost or private IP ranges. This vulnerable Python code shows how a server might accept a URL parameter without validation and make a request to it, potentially exposing internal resources. The attack flow starts with the attacker sending a malicious URL from the internet, and then the server makes an internal request to sensitive resources like databases.
Attackers using SSRF vulnerabilities typically target several key resources. Cloud metadata endpoints, like the AWS metadata service at 169.254.169.254, can reveal sensitive credentials and configuration details. Internal admin interfaces that aren't exposed to the internet but accessible locally can be compromised. Internal APIs may contain sensitive business logic or data. Services running on localhost that assume they're only accessible from the local machine are vulnerable. And attackers can use SSRF for network scanning to map out internal networks. The diagram shows these common attack paths, where a compromised cloud server can be used to access the metadata service, admin interfaces, internal APIs, and databases.
Preventing SSRF attacks requires implementing several defense layers. First, always validate and sanitize all URLs provided by users, ensuring they're well-formed and point to legitimate resources. Use allowlists for domains rather than blocklists, as allowlists specify exactly which domains are permitted. Block requests to internal IP ranges and localhost to prevent internal service access. Implement proper access controls on all internal services as a defense-in-depth measure. And use dedicated service accounts with minimal privileges for server-to-server communications. The secure code example shows how to implement URL validation by checking against an allowlist and blocking private IP addresses. The diagram illustrates the defense-in-depth approach with multiple security layers protecting internal resources.
To summarize what we've learned about Server-Side Request Forgery: SSRF is a critical web vulnerability that allows attackers to make server-side applications send requests to arbitrary destinations of their choosing. Common targets include internal networks, cloud metadata services, and administrative interfaces that should not be accessible from the outside. Vulnerable code typically accepts user-provided URLs without proper validation or sanitization, creating a pathway for attackers. Prevention requires implementing multiple layers of defense, including strict URL validation, domain allowlists, internal IP filtering, and proper access controls on all internal services. Remember that a defense-in-depth approach is essential for comprehensive protection against SSRF attacks.