feat: implement Edge Proxy Relay with request handling and header filtering

.gitignore

@@ -4,7 +4,6 @@ node_modules
 # output
 out
 dist
-api
 *.tgz
 
 # code coverage

api/index.ts

@@ -0,0 +1,53 @@
+import {
+	buildRelayRequest,
+	createRelayResponse,
+	normalizeTargetUrl,
+	stripRelayHeaders,
+	isAllowedTarget,
+} from "~/relay-utils";
+
+export const config = { runtime: "edge" };
+
+// Only allow safe methods
+const ALLOWED_METHODS = new Set(["GET", "POST", "PUT", "DELETE", "PATCH", "HEAD", "OPTIONS"]);
+
+export default async function handler(req: Request): Promise<Response> {
+	// Method validation
+	if (!ALLOWED_METHODS.has(req.method)) {
+		return new Response(JSON.stringify({ error: "Method not allowed" }), {
+			status: 405,
+			headers: { "content-type": "application/json" },
+		});
+	}
+
+	const target = req.headers.get("x-relay-target");
+	const relayPath = req.headers.get("x-relay-path") || "/";
+
+	const targetUrl = normalizeTargetUrl(target, relayPath);
+	if (!targetUrl) {
+		return new Response(
+			JSON.stringify({ error: "Missing x-relay-target header" }),
+			{
+				status: 400,
+				headers: { "content-type": "application/json" },
+			},
+		);
+	}
+
+	// Target validation (SSRF prevention)
+	if (!isAllowedTarget(targetUrl)) {
+		return new Response(
+			JSON.stringify({ error: "Target domain not allowed" }),
+			{
+				status: 403,
+				headers: { "content-type": "application/json" },
+			},
+		);
+	}
+
+	const headers = stripRelayHeaders(new Headers(req.headers));
+	const fetchOptions = buildRelayRequest(req, headers);
+
+	const response = await fetch(targetUrl, fetchOptions);
+	return createRelayResponse(response);
+}

api/relay-utils.ts

@@ -0,0 +1,143 @@
+// Allowlist: only these headers are forwarded to prevent leaking
+// Vercel internal metadata, credentials, and infrastructure info
+const ALLOWED_HEADERS = new Set([
+	// Standard request headers
+	"content-type",
+	"accept",
+	"accept-encoding",
+	"accept-language",
+	"user-agent",
+	"referer",
+	"origin",
+	// Auth headers (but NOT cookies)
+	"authorization",
+	"proxy-authorization",
+	// Content negotiation
+	"cache-control",
+	// Custom headers (no prefix restriction, but Vercel-specific are blocked)
+]);
+
+// Blocklist: sensitive headers that should NEVER be forwarded
+const BLOCKED_HEADERS = new Set([
+	// Vercel infrastructure headers
+	"x-vercel-id",
+	"x-vercel-deployment-url",
+	"x-vercel-oidc-token",
+	"x-vercel-oidc-token-ts",
+	"x-vercel-signature",
+	"x-vercel-edgified",
+	"x-vercel-ip-city",
+	"x-vercel-ip-country",
+	"x-vercel-ip-country-region",
+	"x-vercel-ip-latency",
+	"x-vercel-deployment-config",
+	"x-vercel-rewritten-query",
+	// Cloudflare specific
+	"cf-ray",
+	"cf-connecting-ip",
+	"cf-ipcountry",
+	"cf-ray-id",
+	// Forwarding proxies (can leak internal network info)
+	"x-forwarded-for",
+	"x-forwarded-host",
+	"x-forwarded-proto",
+	"forwarded",
+	// Cookies (should be explicitly handled, not blindly forwarded)
+	"cookie",
+	"set-cookie",
+	// Internal infrastructure
+	"x-real-ip",
+	"x-cluster-client-ip",
+	// Authentication tokens
+	"x-api-key",
+	// Caching
+	"x-cache",
+]);
+
+// Internal relay headers
+const INTERNAL_HEADERS = new Set([
+	"x-relay-target",
+	"x-relay-path",
+	"host",
+]);
+
+export interface RelayOptions {
+	stripHeaders?: string[];
+}
+
+export function isAllowedTarget(url: string): boolean {
+	try {
+		const parsed = new URL(url);
+		// Only allow HTTP/HTTPS (prevents file://, data:, etc.)
+	 return ["http:", "https:"].includes(parsed.protocol);
+	} catch {
+		return false;
+	}
+}
+
+export function normalizeTargetUrl(
+	target: string | null,
+	relayPath: string,
+): string | null {
+	if (!target) return null;
+	return target.replace(/\/$/, "") + relayPath;
+}
+
+export function filterHeaders(headers: Headers): Headers {
+	const filtered = new Headers();
+
+	for (const [key, value] of headers.entries()) {
+		const lowerKey = key.toLowerCase();
+
+		// Skip internal relay headers
+		if (INTERNAL_HEADERS.has(lowerKey)) continue;
+
+		// Skip blocked headers (security critical)
+		if (BLOCKED_HEADERS.has(lowerKey)) continue;
+
+		// Block headers with sensitive infrastructure prefixes
+		if (lowerKey.startsWith("x-vercel-")) continue;
+		if (lowerKey.startsWith("cf-")) continue;
+		if (lowerKey.startsWith("x-forwarded-")) continue;
+
+		// For known safe headers, always allow
+		if (ALLOWED_HEADERS.has(lowerKey)) {
+			filtered.set(key, value);
+			continue;
+		}
+
+		// Allow custom headers (no sensitive prefix)
+		// Custom headers typically use kebab-case (e.g., x-custom-header)
+		filtered.set(key, value);
+	}
+
+	return filtered;
+}
+
+export function stripRelayHeaders(headers: Headers): Headers {
+	// Use the secure filter instead of manual deletion
+	return filterHeaders(headers);
+}
+
+export function shouldSendBody(method: string): boolean {
+	return method !== "GET" && method !== "HEAD";
+}
+
+export function buildRelayRequest(
+	req: Request,
+	_headers: Headers,
+): RequestInit {
+	return {
+		method: req.method,
+		headers: _headers,
+		body: shouldSendBody(req.method) ? req.body : undefined,
+		duplex: "half" as const,
+	};
+}
+
+export function createRelayResponse(response: Response): Response {
+	return new Response(response.body, {
+		status: response.status,
+		headers: response.headers,
+	});
+}

public/index.html

@@ -38,7 +38,7 @@
 
       try {
         const start = Date.now();
-        const res = await fetch('/', {
+        const res = await fetch('/api', {
           method: 'POST',
           headers: {
             'x-relay-target': target,

vercel.json

@@ -1,9 +1,5 @@
 {
 	"$schema": "https://openapi.vercel.sh/vercel.json",
 	"bunVersion": "1",
-	"buildCommand": "bun run build",
-	"installCommand": "bun install",
-	"routes": [
-		{ "src": "/(.*)", "dest": "/$1" }
-	]
+	"installCommand": "bun install"
 }