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A hostname allowlist alone does not reliably stop server-side request forgery (SSRF). To protect an API that fetches user-influenced URLs, validate the parsed destination, resolve its IPv4 and IPv6 addresses, reject prohibited addresses, and ensure the HTTP connection uses an address that passed those checks. Reapply the policy to every redirect, and restrict outbound traffic as a separate layer of defense.
Why can an allowlisted URL still reach localhost or cloud metadata?
An allowlist can approve the text of a URL or its hostname without checking the network destination the server ultimately contacts. A hostname is a name; DNS supplies one or more IP addresses for the client to connect to. If an attacker can control the name or its DNS answers, a URL that looks permitted can lead to an unintended destination.
For example, an API might accept a URL because its hostname matches an allowed domain. That hostname could resolve to a loopback or private address, a link-local address, an IPv6 unique-local address, or a cloud metadata endpoint. An attacker may also change DNS answers over time. OWASP puts the core weakness plainly: “Domain allowlisting alone does not prevent DNS rebinding.”
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The practical question is therefore not just “Is this hostname allowed?” It is also “Which address will receive the connection, and can the application guarantee that the connection goes to that address?”
What should an API validate: the URL, hostname, or resolved IP?
Validate the URL’s structure and policy, but make the resolved IP address part of the decision. A hostname check is useful for identifying the intended service; it is not a substitute for checking and enforcing the actual network destination.
A defensible decision has several parts:
- URL parsing: Interpret input with a standards-compliant parser and reject malformed or ambiguous input.
- Destination policy: If the feature only needs to reach known services, match a normalized hostname against an explicit allowlist. Prefer accepting a host or resource identifier and building the URL yourself over accepting an unrestricted complete URL.
- Address policy: Resolve the name immediately before connecting and evaluate every returned address against the prohibited-range policy.
- Connection enforcement: Route the socket to an address that passed the check rather than letting the HTTP client perform a fresh, independent lookup.
OWASP recommends using an explicit hostname allowlist and constructing a request with a fixed scheme, port, and controlled path where that fits the feature. This reduces the number of choices an untrusted caller can influence. It does not eliminate the need to check the resolved destination: DNS can still return an unsafe address for a hostname that appears on the list.
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DNS rebinding exploits a gap between validating a name and using it. The application checks a hostname at one point in time, but the HTTP client resolves it again later. If the answers differ, the connection may go somewhere the validation never approved.
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GitLab describes this as a time-of-check/time-of-use race. A hostname can resolve to a permitted public address during validation and a prohibited address before the request is made. The reverse design flaw is also possible: an application resolves and checks a safe address but discards it, then asks a client library to connect by hostname. That second lookup means the earlier check does not bind the decision to the connection.
The safer pattern is to use the IP address that passed validation for the network connection, while preserving the original hostname for the HTTP Host header, TLS SNI, and certificate verification. These are distinct roles: the IP determines where the socket is routed; the original hostname identifies the intended HTTPS service and is used to verify its certificate. If a library or proxy cannot preserve that separation, its behavior needs careful review before it is trusted for this use.
How should an API safely validate and fetch a URL?
Use a pipeline that narrows the caller’s control and carries the validated address through to the connection. Exact APIs differ by language and HTTP client, so verify that your chosen implementation can pin the connection to the checked IP without breaking hostname-based TLS verification.
- Parse and normalize the input. Use a standards-compliant URL parser. Reject malformed URLs, ambiguous interpretations, embedded credentials, unexpected backslashes, and any scheme the feature does not require. Do not rely on substring checks or ad hoc string splitting to identify a host.
- Reduce caller-controlled choices. When practical, accept a hostname or resource identifier rather than a full URL. Match the normalized host against an explicit allowlist, then construct the URL with a fixed scheme, port, and controlled path. If full URLs are necessary, apply equivalent checks to every component the caller can set.
- Resolve just before the request. Obtain the hostname’s A and AAAA answers immediately before connecting. Evaluate all returned addresses, not just the first or a preferred address family. Treat an empty result, a resolution error, or any prohibited answer according to a deny-by-default policy.
- Classify the complete address set. Reject loopback, RFC1918 private, link-local, IPv6 unique-local, multicast, broadcast, cloud metadata, and other ranges prohibited by the feature’s policy. Account for alternate textual IP forms and IPv4-mapped IPv6 addresses so a different representation cannot evade the check.
- Pin the connection to an approved address. Configure the client or its connection layer to connect to the exact address that passed validation. Preserve the original hostname for the HTTP
Hostheader, TLS SNI, and certificate verification. Do not resolve again through an unrelated client lookup, proxy, retry path, or fallback address-family path. - Control the request and response. Permit only required schemes—normally HTTPS for a web fetcher—and fixed ports. Set strict timeouts and response-size limits. Normalize the response before exposing it, and do not return raw upstream headers, raw bodies, or detailed connection errors to an untrusted caller.
- Revalidate each redirect. Prefer disabling automatic redirects. If redirects are a required feature, treat every target as a new untrusted URL: parse it, apply the hostname and scheme policy, resolve and classify its addresses, and pin the next connection. A safe first destination does not make the redirect destination safe.
These stages are interdependent. Parsing does not establish that an address is safe; DNS classification does not help if the client performs another lookup; and connection pinning does not make an unsupported scheme or an unsafe redirect acceptable.
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Which addresses and URL forms need to be blocked?
Block destinations that the fetching feature has no legitimate reason to reach. At a minimum, the policy described by OWASP includes loopback, private, link-local, IPv6 unique-local, multicast, broadcast, and cloud metadata addresses where those destinations are not explicitly needed.
Apply classification to the resolved address values, not only to how an address was written in the input. A parser may accept multiple textual representations of an IP, and an IPv4 address can also appear in IPv4-mapped IPv6 form. Check every A and AAAA answer. If one answer is prohibited, a deny-by-default fetcher should reject the hostname rather than hope the client selects a different one.
Scheme restrictions matter as well. A server-side fetcher intended for web resources generally has no reason to accept file:, gopher:, ftp:, dict:, or data:. Reject unused schemes explicitly instead of assuming that a hostname allowlist applies meaningfully to every protocol. The permitted scheme, port, and path should match the feature’s actual needs.
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Every mechanism that can change the destination must remain inside the same policy boundary. Automatic redirects are especially easy to miss: a request can begin at an approved host and then instruct the client to fetch another host or address.
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The simplest policy is to disable automatic redirects and reject redirect responses. If the product needs redirects, process them one hop at a time and repeat parsing, destination checks, DNS resolution, address classification, and connection pinning for each new target. Limit behavior to what the product requires; do not assume that a redirect to the same-looking hostname or scheme is safe without evaluating its actual destination.
Review retries, connection pools, proxies, and fallback between IPv4 and IPv6 too. A retry may trigger a new lookup; a pool may reuse a connection established under a different policy context; and a proxy may perform DNS resolution on the application’s behalf. Ensure these paths cannot bypass the validation decision. Where a proxy performs the connection, establish how its destination resolution and address restrictions are enforced rather than treating the application’s local check as proof of the proxy’s destination.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How do application checks compare with network egress controls?
Application validation and network restrictions solve different parts of the problem. The application knows what destinations a particular feature is intended to use; network controls can limit the damage if that application logic fails. The strongest design combines both.
| Control | What it checks or limits | What it does not prove by itself |
|---|---|---|
| URL-string or hostname allowlist | Whether the submitted URL or normalized hostname matches an application rule. | That DNS resolves to a permitted address or that the connection uses an address that was checked. |
| Resolved-IP validation | Whether the returned IPv4 and IPv6 addresses are allowed by the destination policy. | That the later socket uses those same addresses unless the connection is pinned. |
| Connection-time enforcement | Whether the socket is routed to an address that passed validation while the original hostname remains available for HTTP and TLS identity checks. | That redirects, retries, proxies, or other request paths follow the same policy unless they are included. |
| Redirect-by-redirect validation | Whether each new target is checked before the next connection. | That unrelated outbound traffic from the service is constrained. |
| Network egress restrictions | Which destinations the service can reach at the network layer, reducing impact when application validation fails. | That a particular URL is a legitimate or intended destination for the application feature. |
Use deny-by-default egress rules, network segmentation, least-privilege identities, metadata-service hardening, and logging and monitoring as defense in depth. These measures reduce the consequences of a validation failure; they do not replace destination checks in the fetcher.
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What should a security review check in a server-side fetcher?
Review the complete path from user input to socket creation, including behavior hidden in libraries and infrastructure. A hostname check in one function is not enough if the eventual connection is made elsewhere.
- Is policy evaluated on resolved addresses as well as the original hostname?
- Are all A and AAAA records checked, including IPv4-mapped IPv6 and alternate textual IP representations?
- Does the actual connection use an address that passed validation?
- Is the original hostname used for
Host, SNI, and certificate verification without triggering an unchecked lookup? - Are redirects rejected or revalidated one hop at a time?
- Are unused schemes such as
file:,gopher:,ftp:,dict:, anddata:rejected? - Do retries, connection pools, proxies, and IPv4/IPv6 fallback paths enforce the same policy?
- Are loopback, private, link-local, unique-local, multicast, broadcast, and relevant cloud metadata ranges blocked when the feature does not need them?
- Are outbound connections restricted and monitored at the network layer?
- Are response bodies, headers, sizes, timeouts, and errors constrained before anything is returned to the caller?
If any answer is unclear, trace how the HTTP client resolves the hostname and opens the socket. The critical verification is whether an address validated by the application is the address used for the connection, including on redirects and retries.
Frequently Asked Questions
Can a public DNS record safely point to a private IP for an API fetcher?
Not by default. If the feature has a documented need to reach that private destination, scope an exception to the specific service and network path, and retain connection-time enforcement and egress controls; a generally usable fetcher should reject it.
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For a deny-by-default fetcher, yes: reject the hostname if any returned address violates policy. Otherwise a client’s address selection, fallback, or retry behavior could choose the prohibited result.
Does HTTPS prevent SSRF or DNS rebinding?
No. HTTPS protects the connection and authenticates a hostname under certificate rules, but it does not establish that the destination IP is permitted for the application. The fetcher still needs address validation and connection pinning.
Is blocking only 169.254.169.254 enough to protect cloud metadata?
No. The policy must cover the metadata destinations and access paths relevant to the deployment, as well as other prohibited internal ranges. Metadata-service hardening and network controls are useful additional safeguards, not replacements for validating each outbound destination.
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