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What is SSRF?
In an SSRF attack, the application—not the attacker’s browser—makes a network request to a destination influenced by the attacker. OWASP describes the core risk as an API fetching a remote resource from a user-supplied URL without validating that URL. The server’s network position and attached identity can give the request access the attacker does not have. OWASP API7:2023
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Features that make outbound requests deserve scrutiny, but are not automatically vulnerable. Examples include webhooks, URL-based file fetching, custom single sign-on flows, and URL previews. The issue arises when an attacker can steer the request and the application does not adequately constrain its destination or behavior.
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Why can SSRF compromise a gateway?
A gateway or reverse proxy is built to receive requests and communicate with other systems. If an attacker can influence where it connects, the gateway can become a request-making deputy: it may be able to reach internal APIs, management interfaces, or cloud metadata endpoints that are not publicly exposed. The gateway’s reachable networks and identity permissions determine the possible blast radius.
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If the feature returns fetched content, an attacker may directly see data from the destination. If it does not, a blind SSRF may still trigger outbound actions or requests to internal services. The consequences can include disclosure of internal data or cloud credentials and tokens, access to internal management services, proxying, or denial of service—but none is guaranteed by the presence of SSRF alone. Request methods and headers, network access, response handling, and the gateway’s privileges all matter. OWASP’s SSRF Prevention Cheat Sheet
Cloud metadata is a specific concern, not an automatic account takeover
Cloud metadata services may expose credentials or access tokens usable by the workload. OWASP identifies metadata services across AWS, Azure, and Google Cloud as potential targets. MITRE ATT&CK also describes adversaries exploiting SSRF in a public-facing web proxy to reach a cloud Instance Metadata API. Whether exposed credentials can be used—and what they can do—depends on the identity’s permissions and the services it can reach. MITRE ATT&CK: Cloud Instance Metadata API
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How do you prevent SSRF in a gateway?
Use several layers of control. Application validation should limit what the feature is intended to fetch; network egress rules should independently limit where the gateway can connect.
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- Validate the parsed destination and its resolved addresses. Use a well-defined URL parser, restrict acceptable schemes and hostnames, and check resolved IP addresses. Apply controls consistently across the request flow; parser differences and DNS changes can undermine checks made only at initial input.
- Control redirects and request features. Decide whether redirects are needed, and validate every redirect destination rather than trusting the initial URL. Limit methods and headers to what the feature requires. These choices matter because they affect where the request goes and what it can do.
- Restrict outbound network access. Keep the gateway or fetcher from reaching loopback, private, link-local, multicast, and metadata destinations unless the feature has a specific authorized need. Egress restrictions provide a separate safeguard if application checks fail.
- Reduce metadata exposure. AWS recommends Instance Metadata Service Version 2 (IMDSv2) as defense in depth. It is not a substitute for destination validation or egress controls; AWS notes limits to static-header protections when an SSRF can control arbitrary headers. AWS: Defense in depth for EC2 Instance Metadata Service
What should a gateway security review check?
Assess the actual request path, not just the URL field. OWASP’s SSRF testing guidance highlights the importance of local trust relationships; a useful review examines how input is parsed, resolved, redirected, sent, and returned. OWASP Testing Guide: Testing for Server-Side Request Forgery
- Are destinations fixed, allowlisted, or user-configurable?
- Are hostnames resolved and the resulting addresses checked, including after DNS changes or redirects?
- Which URL schemes, methods, and headers can the caller influence?
- Can the gateway reach internal, link-local, or metadata networks?
- Does the feature return response content, status details, or other information that could reveal data?
- What permissions does the gateway’s cloud identity have, and are they limited to the workload’s needs?
Testing should account for redirect behavior, DNS resolution, IPv4 and IPv6 address forms, methods, headers, and response visibility. The right controls depend on the feature’s intended destinations, but a design should not rely on a single URL check where network-layer restrictions can also reduce risk.
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