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Prevent server-side request forgery (SSRF) by ensuring gateway features cannot make outbound requests to destinations an attacker can control. Prefer a short allowlist of necessary destinations, bind validation to the address the client actually connects to, control redirects, and restrict outbound network access. These safeguards matter wherever a gateway or adjacent service fetches a URL, delivers a webhook, or makes another server-side request based on user input.
What SSRF means for a remote access gateway
SSRF occurs when an attacker can influence a server-side feature into making an unintended request. The request originates from the gateway or another service, so it may reach internal services or sensitive cloud metadata endpoints that the user could not contact directly.
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Remote access products can have outbound-request paths in URL previews, webhook delivery, callback handling, custom single sign-on (SSO) integrations, and image, document, or other URL-based imports. OWASP identifies these kinds of API features as common SSRF exposure points. The security question is not only whether a user may access the gateway, but also which destinations its server-side functions are allowed to reach.
How to reduce SSRF risk
1. Inventory every outbound-request feature
Review the gateway and adjacent services for every feature that fetches content or contacts a destination influenced by a user, administrator, integration, or callback. Treat those destinations as untrusted until a documented business requirement establishes which ones are necessary. Include background jobs and retries, not just the initial web request.
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2. Prefer named destinations over arbitrary URLs
If the required destinations are known, accept a short destination identifier and map it to a server-controlled address. If users must supply a host, restrict it to an explicit positive allowlist. Enforce the required scheme, port, and destination together; do not accept URL components the feature does not need.
Avoid accepting a complete arbitrary URL when a constrained destination model will work. URL parsers can interpret ambiguous input differently, so raw string prefix or suffix checks—and regular expressions alone—are not a reliable destination policy. If arbitrary external destinations are a genuine requirement, use a maintained URL-parsing library, explicitly define permitted schemes, and reject malformed or ambiguous inputs and embedded credentials.
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3. Validate the address used by the connection
For an allowed hostname, resolve it and inspect every returned IPv4 and IPv6 address against the destination policy. Reject any address outside that policy. Most importantly, make the HTTP client connect only to an address that has been checked. Validating one DNS lookup and then allowing the client to perform a fresh, unchecked lookup leaves a time-of-check/time-of-use gap.
When connecting to the checked address, preserve the original hostname for the HTTP Host header, TLS SNI, and certificate verification. Apply the same checks to new resolutions, retries, and fallback connections; none should silently use a different destination.
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4. Prevent redirects from changing the destination
Disable automatic redirect following where practical. If the feature needs redirects, validate each redirect target and its resolved addresses under the same policy before following it. A request that begins at an allowed host must not inherit permission to reach whatever destination that host names in a later response.
Also review the request client’s proxy configuration, retry behavior, supported protocols, and timeouts. These settings should not expand the set of destinations or behaviors the feature is permitted to use.
5. Restrict outbound traffic at the network layer
Run remote-fetch functionality in a separately restricted network zone where practical. Use deny-by-default firewall or network access-control rules, allowing only the routes the feature needs. Log both permitted and blocked flows, assign ownership to each rule, and review rules when application dependencies change. Network restrictions reduce the impact of an application-layer validation defect or bypass.
6. Treat cloud metadata as a protected destination
Block unintended access to cloud metadata services in both application destination policy and network controls. For AWS, OWASP recommends migrating to Instance Metadata Service Version 2 (IMDSv2) and disabling IMDSv1 as an additional defense-in-depth measure. Metadata protections supplement a general destination policy; they do not replace it.
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Which destination model should you choose?
Use a fixed allowlist when the business-required destinations can be enumerated. Allow arbitrary external fetching only when the product genuinely needs it and the system can safely enforce destination checks, including at connection time.
| Consideration | Fixed destination allowlist | Arbitrary external fetching |
|---|---|---|
| Business flexibility | Limited to configured destinations | Supports destinations beyond a predefined set |
| Destination inventory | Best when required destinations can be enumerated | Requires a policy that safely handles destinations that cannot all be listed in advance |
| DNS and connection handling | Still validate resolved addresses and bind checks to the actual connection | Must validate resolved addresses and ensure the client connects only to a checked address |
| Redirects and retries | Every hop and new connection must remain within the allowlist | Every hop and new connection must satisfy the external-destination policy |
| Network egress | Restrict routes to those required by the configured destinations | Use network isolation and deny-by-default rules to constrain permitted routes |
| Operational overhead | Review and update allowlist and firewall changes as dependencies change | Maintain the broader validation policy and review network rules as dependencies change |
What to verify during implementation
- Every user-influenced outbound request path has an identified owner and a documented business purpose.
- Destination policy constrains scheme, port, and host or address; it does not rely on a raw string match alone.
- All resolved IPv4 and IPv6 addresses are checked, and the client connects only to an approved, checked address.
- Redirects are disabled or each target is revalidated; retries and fallback connections receive the same checks.
- Outbound network rules deny by default, and permitted and blocked flows are logged.
- Cloud metadata endpoints are blocked unless access is explicitly required; AWS workloads use IMDSv2 with IMDSv1 disabled as OWASP recommends.
These controls follow the OWASP Server-Side Request Forgery Prevention Cheat Sheet, OWASP Top 10:2021 A10, OWASP Foundation’s SSRF guidance, OWASP’s Open Redirect guidance, and OWASP API Security Project’s API7:2023 guidance.
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