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What is server-side request forgery?
In SSRF, the server—not the requester’s browser—makes a network request on the requester’s behalf. OWASP describes SSRF as an attack that “abuses an application to interact with the internal/external network or the machine itself.” (OWASP SSRF Prevention Cheat Sheet)
A vulnerable application might fetch a user-submitted image, call a webhook URL, or import content from a supplied address. If the application accepts a destination without adequate controls, it can act as a proxy from its own network position. That may give the requester a path to systems unavailable from the public internet.
SSRF is different from cross-site request forgery (CSRF). SSRF causes an application server to make a request; CSRF tricks a user’s authenticated browser into making one.
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What does “pre-authentication SSRF” mean?
Pre-authentication describes when the vulnerable functionality can be reached without signing in. If a public page or endpoint accepts a URL and triggers a server-side fetch, an unauthenticated visitor may be able to invoke that request path.
Unauthenticated access alone does not establish an SSRF vulnerability. Exploitability also depends on whether the requester can control the destination, how the application parses and validates it, whether redirects are followed, which services the server can reach, and whether useful response data is exposed. OWASP’s guidance emphasizes validating user-supplied URLs and controlling where server-side requests can go. (OWASP SSRF Prevention Cheat Sheet; OWASP SSRF overview)
What internal services could be exposed?
The server’s reachable network and the application’s response behavior determine what an attacker might access or learn. Potential targets include:
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- Cloud instance metadata: Depending on the cloud configuration and workload permissions, metadata services may expose information or credentials.
- Internal APIs and HTTP services: These may be accessible from the application’s network even when they are not internet-facing.
- Databases and other internal systems: SSRF can sometimes help identify or interact with services reachable from the server, though access depends on the protocols and controls involved.
- Local resources: A request may target resources on the server itself, subject to how the application handles URLs and responses.
Possible consequences include information disclosure, internal service enumeration, bypass of network controls, or follow-on attacks against internal services. These are potential outcomes, not guaranteed results; an application may not expose the fetched response, and network or service protections may prevent access. OWASP discusses SSRF in its API and web application risk classifications. (OWASP API Security Top 10:2023, API7; OWASP Top 10:2021, A10)
How to reduce SSRF risk
Use controls at both the application and network layers. Application controls restrict requests to intended destinations; network egress controls limit what the server can reach if application validation fails.
Allow only required destinations
If the feature needs to contact a known set of services, use a positive allowlist of expected destinations. Validate the URL’s scheme, host, and port with a well-tested URL parser rather than relying on regular expressions alone. OWASP cautions that deny-lists are bypass-prone and recommends allowlisting where feasible. (OWASP SSRF Prevention Cheat Sheet)
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Control redirects and name resolution
Disable redirects where practical. If redirects are necessary, validate every redirect destination—not just the initial URL. Ensure that DNS resolution cannot turn an apparently permitted hostname into an internal address. For features that genuinely require arbitrary external destinations, apply explicit restrictions to prohibited address ranges and metadata endpoints, and account for both DNS resolution and redirects.
Limit what the requester can learn
Avoid returning raw upstream responses to users. Return only the information the feature needs, and handle errors without exposing internal response content. This limits disclosure even when a request reaches a service that should not have been accessible.
Restrict network egress
Configure the fetcher’s network access so it can reach only the services it needs. This complements URL allowlisting: application validation narrows the intended destinations, while egress rules contain the impact of a validation failure or bypass.
Harden cloud metadata access
In cloud deployments, review the instance metadata configuration and the credentials available to the workload. AWS IMDSv2 provides an additional defense-in-depth measure against some SSRF attempts, but it does not replace application validation or network controls. (AWS Prescriptive Guidance: IMDSv2 defense in depth)
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