feat: enhance request handling with method validation and target URL checks

src/index.ts

@@ -3,11 +3,23 @@ import {
 	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") || "/";
 
@@ -22,6 +34,17 @@ export default async function handler(req: Request): Promise<Response> {
 		);
 	}
 
+	// 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);
 

src/relay-utils.ts

@@ -1,7 +1,80 @@
+// 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,
@@ -10,12 +83,40 @@ export function normalizeTargetUrl(
 	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 {
-	const stripped = new Headers(headers);
-	stripped.delete("x-relay-target");
-	stripped.delete("x-relay-path");
-	stripped.delete("host");
-	return stripped;
+	// Use the secure filter instead of manual deletion
+	return filterHeaders(headers);
 }
 
 export function shouldSendBody(method: string): boolean {