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How to fix SSRF (Server Side Request Forgery)
in Dart Frog

Executive Summary

SSRF in Dart Frog manifests when the backend fetches a remote resource based on user-controllable input without strict validation. This allows an attacker to coerce the server into making requests to internal infrastructure, cloud metadata services (169.254.169.254), or loopback interfaces, effectively bypassing network perimeters.

The Vulnerable Pattern

VULNERABLE CODE
import 'package:dart_frog/dart_frog.dart';
import 'package:http/http.dart' as http;

// VULNERABLE: Blindly trusts the ‘url’ query parameter
Future onRequest(RequestContext context) async {
final url = context.request.uri.queryParameters[‘url’];
if (url == null) return Response(statusCode: 400);


try {
final response = await http.get(Uri.parse(url));
return Response(body: response.body);
} catch (e) {
return Response(statusCode: 500);
}
}

The Secure Implementation

The fix employs a defense-in-depth strategy. First, we use Uri.tryParse to ensure the input is a valid URI structure. Second, we enforce the 'https' scheme to prevent protocol smuggling (e.g., file:// or gopher://). Third, we implement a strict allowlist for the 'host' component, which prevents the server from hitting internal IP ranges (10.0.0.0/8, 127.0.0.1) or cloud provider metadata endpoints. For high-security environments, it is also recommended to resolve the IP address and verify it is not a private/loopback address to mitigate DNS Rebinding attacks.

SECURE CODE
import 'package:dart_frog/dart_frog.dart';
import 'package:http/http.dart' as http;

const ALLOWED_HOSTS = {‘api.trusted-partner.com’, ‘cdn.myapp.com’};


Future onRequest(RequestContext context) async {
final urlParam = context.request.uri.queryParameters[‘url’];
if (urlParam == null) return Response(statusCode: 400);


final uri = Uri.tryParse(urlParam);
if (uri == null) return Response(statusCode: 400);


// 1. Enforce HTTPS only
if (uri.scheme != ‘https’) {
return Response(statusCode: 403, body: ‘Insecure protocol prohibited’);
}


// 2. Strict Domain Allowlisting
if (!ALLOWED_HOSTS.contains(uri.host)) {
return Response(statusCode: 403, body: ‘Target host not permitted’);
}


try {
// 3. Use a timeout and limit response size to prevent DoS
final client = http.Client();
final response = await client.get(uri).timeout(Duration(seconds: 5));
return Response(body: response.body);
} catch (e) {
return Response(statusCode: 502);
}
}

System Alert • ID: 4734
Target: Dart Frog API
Potential Vulnerability

Your Dart Frog API might be exposed to SSRF (Server Side Request Forgery)

74% of Dart Frog apps fail this check. Hackers use automated scanners to find this specific flaw. Check your codebase before they do.

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