diff --git a/src/auth/auth-metadata.ts b/src/auth/auth-metadata.ts
index d06b28d..bf0903f 100644
--- a/src/auth/auth-metadata.ts
+++ b/src/auth/auth-metadata.ts
@@ -20,17 +20,25 @@ const corsHeaders = {
  *                        These should match the "issuer" field in the authorization servers'
  *                        OAuth metadata (RFC 8414).
  * @param resourceUrl - Optional explicit resource URL override. When provided, this URL is
- *                      used instead of deriving it from the request. Use this when running
- *                      behind a proxy that doesn't set standard forwarding headers.
- *                      If not provided, the URL is automatically detected from proxy headers
- *                      (X-Forwarded-Host, X-Forwarded-Proto, Forwarded) or falls back to req.url.
+ *                      used instead of deriving it from the request. Strongly recommended
+ *                      in production to avoid relying on request-derived origins.
+ *                      If not provided, the URL is derived from `req.url` (and from proxy
+ *                      headers only when `trustProxy` is `true`).
+ * @param trustProxy - Whether to trust X-Forwarded-Host / X-Forwarded-Proto / Forwarded
+ *                     headers when deriving the resource URL. Defaults to `false`.
+ *                     Only set this to `true` when the request has demonstrably traversed
+ *                     a trusted reverse proxy that strips/overwrites these headers,
+ *                     otherwise clients can spoof the published `resource` identifier
+ *                     (CWE-918 / origin spoofing).
  */
 export function protectedResourceHandler({
     authServerUrls,
     resourceUrl: explicitResourceUrl,
+    trustProxy = false,
 }: {
     authServerUrls: string[];
     resourceUrl?: string;
+    trustProxy?: boolean;
 }) {
     return (req: Request) => {
         let resource: string;
@@ -39,8 +47,8 @@ export function protectedResourceHandler({
             // Use explicit override if provided
             resource = explicitResourceUrl;
         } else {
-            // Auto-detect from proxy headers or req.url
-            const publicUrl = getPublicUrl(req);
+            // Auto-detect from req.url (and proxy headers only if trustProxy is true)
+            const publicUrl = getPublicUrl(req, { trustProxy });
 
             publicUrl.pathname = publicUrl.pathname
               .replace(/^\/\.well-known\/[^\/]+/, "");
@@ -110,4 +118,3 @@ export function metadataCorsOptionsRequestHandler() {
         });
     };
 }
-
diff --git a/src/auth/auth-wrapper.ts b/src/auth/auth-wrapper.ts
index be7c096..9ecfa0d 100644
--- a/src/auth/auth-wrapper.ts
+++ b/src/auth/auth-wrapper.ts
@@ -24,24 +24,40 @@ export function withMcpAuth(
     resourceMetadataPath = "/.well-known/oauth-protected-resource",
     requiredScopes,
     resourceUrl,
+    trustProxy = false,
   }: {
     required?: boolean;
     resourceMetadataPath?: string;
     requiredScopes?: string[];
     /**
      * Explicit resource URL override. When provided, this URL is used as the
-     * origin for constructing the resource_metadata URL. Use this when running
-     * behind a proxy that doesn't set standard forwarding headers, or when you
-     * need to specify a specific public URL.
+     * origin for constructing the resource_metadata URL. Strongly recommended
+     * for production deployments — relying on request-derived origins lets
+     * clients influence the URL advertised in WWW-Authenticate responses
+     * unless the deployment is behind a proxy that sanitizes forwarding
+     * headers.
      *
-     * If not provided, the origin is automatically detected from proxy headers
-     * (X-Forwarded-Host, X-Forwarded-Proto, Forwarded) or falls back to req.url.
+     * If not provided, the origin is derived from `req.url` (and from proxy
+     * headers only when `trustProxy` is `true`).
      */
     resourceUrl?: string;
+    /**
+     * Whether to trust X-Forwarded-Host / X-Forwarded-Proto / Forwarded
+     * headers when deriving the origin used to build the
+     * `resource_metadata` URL in WWW-Authenticate responses. Defaults to
+     * `false`.
+     *
+     * Only set this to `true` when the request has demonstrably traversed
+     * a trusted reverse proxy that strips/overwrites these headers.
+     * Otherwise, an attacker can spoof the advertised metadata URL,
+     * potentially redirecting OAuth clients to attacker-controlled servers
+     * (CWE-918 / origin spoofing). Ignored when `resourceUrl` is set.
+     */
+    trustProxy?: boolean;
   } = {}
 ) {
   return async (req: Request) => {
-    const origin = resourceUrl ?? getPublicOrigin(req);
+    const origin = resourceUrl ?? getPublicOrigin(req, { trustProxy });
     const resourceMetadataUrl = `${origin}${resourceMetadataPath}`;
 
     const authHeader = req.headers.get("Authorization");
diff --git a/src/lib/url.ts b/src/lib/url.ts
index c4ca247..8f082cb 100644
--- a/src/lib/url.ts
+++ b/src/lib/url.ts
@@ -1,38 +1,72 @@
 /**
- * Get the public-facing origin from a request, respecting proxy headers.
+ * Options controlling how the public-facing origin is derived from a request.
+ */
+export interface PublicOriginOptions {
+  /**
+   * Whether to trust proxy-supplied forwarding headers
+   * (X-Forwarded-Host, X-Forwarded-Proto, Forwarded).
+   *
+   * Defaults to `false`. When false, these headers are ignored and the origin
+   * is derived from `req.url`. This is the safe default: clients can otherwise
+   * spoof the public origin by sending these headers directly when the
+   * deployment is not behind a reverse proxy that strips/overwrites them.
+   *
+   * Set to `true` only when the request has demonstrably traversed a trusted
+   * reverse proxy that sanitizes these headers (e.g., a properly configured
+   * load balancer, Vercel, Cloudflare).
+   *
+   * SECURITY: Trusting proxy headers in deployments without such a proxy
+   * allows attackers to control the `resource_metadata` URL in
+   * WWW-Authenticate responses and the `resource` field of the OAuth
+   * Protected Resource Metadata document, potentially redirecting OAuth
+   * clients to attacker-controlled servers (CWE-918 / origin spoofing).
+   */
+  trustProxy?: boolean;
+}
+
+/**
+ * Get the public-facing origin from a request.
  *
- * When running behind a reverse proxy (e.g., nginx, Vercel, Cloudflare),
- * the `req.url` typically reflects the internal URL (e.g., http://localhost:3000).
- * This function reconstructs the public-facing origin using standard proxy headers.
+ * By default (and unless `trustProxy: true` is explicitly passed), this
+ * function ignores X-Forwarded-* / Forwarded headers and derives the origin
+ * from `req.url`. This prevents clients from spoofing the public origin in
+ * deployments that are not behind a reverse proxy that sanitizes these
+ * headers. See {@link PublicOriginOptions.trustProxy}.
  *
- * Header precedence:
+ * When `trustProxy` is `true`, header precedence is:
  * 1. X-Forwarded-Host + X-Forwarded-Proto (most common)
  * 2. Forwarded header (RFC 7239)
  * 3. Falls back to req.url origin
  *
  * @param req - The incoming request
+ * @param options - Origin-derivation options
  * @returns The public-facing origin (e.g., "https://example.org")
  */
-export function getPublicOrigin(req: Request): string {
-  const forwardedHost = req.headers.get("x-forwarded-host");
-  const forwardedProto = req.headers.get("x-forwarded-proto");
+export function getPublicOrigin(
+  req: Request,
+  options: PublicOriginOptions = {}
+): string {
+  if (options.trustProxy) {
+    const forwardedHost = req.headers.get("x-forwarded-host");
+    const forwardedProto = req.headers.get("x-forwarded-proto");
 
-  // If we have X-Forwarded-Host, construct origin from forwarded headers
-  if (forwardedHost) {
-    // X-Forwarded-Host can contain multiple comma-separated values; use the first (leftmost)
-    const host = forwardedHost.split(",")[0].trim();
-    // X-Forwarded-Proto can also be comma-separated
-    const proto = forwardedProto?.split(",")[0].trim() || "https";
-    return `${proto}://${host}`;
-  }
+    // If we have X-Forwarded-Host, construct origin from forwarded headers
+    if (forwardedHost) {
+      // X-Forwarded-Host can contain multiple comma-separated values; use the first (leftmost)
+      const host = forwardedHost.split(",")[0].trim();
+      // X-Forwarded-Proto can also be comma-separated
+      const proto = forwardedProto?.split(",")[0].trim() || "https";
+      return `${proto}://${host}`;
+    }
 
-  // Check RFC 7239 Forwarded header (less common but standardized)
-  const forwarded = req.headers.get("forwarded");
-  if (forwarded) {
-    const parsed = parseForwardedHeader(forwarded);
-    if (parsed.host) {
-      const proto = parsed.proto || "https";
-      return `${proto}://${parsed.host}`;
+    // Check RFC 7239 Forwarded header (less common but standardized)
+    const forwarded = req.headers.get("forwarded");
+    if (forwarded) {
+      const parsed = parseForwardedHeader(forwarded);
+      if (parsed.host) {
+        const proto = parsed.proto || "https";
+        return `${proto}://${parsed.host}`;
+      }
     }
   }
 
@@ -41,14 +75,20 @@ export function getPublicOrigin(req: Request): string {
 }
 
 /**
- * Get the public-facing URL from a request, respecting proxy headers.
+ * Get the public-facing URL from a request.
+ *
+ * See {@link getPublicOrigin} for the security semantics of `trustProxy`.
  *
  * @param req - The incoming request
+ * @param options - Origin-derivation options
  * @returns The public-facing URL with the correct origin
  */
-export function getPublicUrl(req: Request): URL {
+export function getPublicUrl(
+  req: Request,
+  options: PublicOriginOptions = {}
+): URL {
   const url = new URL(req.url);
-  const publicOrigin = getPublicOrigin(req);
+  const publicOrigin = getPublicOrigin(req, options);
 
   // Construct a new URL with the public origin but preserve pathname, search, and hash
   const result = new URL(url.pathname + url.search + url.hash, publicOrigin);
diff --git a/tests/auth.test.ts b/tests/auth.test.ts
index 86bd43e..612a3b5 100644
--- a/tests/auth.test.ts
+++ b/tests/auth.test.ts
@@ -47,9 +47,10 @@ describe("auth", () => {
     });
   });
 
-  describe("proxy header support", () => {
+  describe("proxy header support (trustProxy: true)", () => {
     const handler = protectedResourceHandler({
       authServerUrls: ["https://auth-server.com"],
+      trustProxy: true,
     });
 
     it("uses X-Forwarded-Host and X-Forwarded-Proto headers", async () => {
@@ -118,6 +119,35 @@ describe("auth", () => {
     });
   });
 
+  describe("origin spoofing protection (default trustProxy=false)", () => {
+    const handler = protectedResourceHandler({
+      authServerUrls: ["https://auth-server.com"],
+    });
+
+    it("ignores attacker-supplied X-Forwarded-Host by default", async () => {
+      const req = new Request("https://real-server.com/.well-known/oauth-protected-resource", {
+        headers: {
+          "X-Forwarded-Host": "attacker.example",
+          "X-Forwarded-Proto": "https",
+        },
+      });
+      const res = handler(req);
+      const json = await res.json();
+      expect(json.resource).toBe("https://real-server.com");
+    });
+
+    it("ignores attacker-supplied Forwarded header by default", async () => {
+      const req = new Request("https://real-server.com/.well-known/oauth-protected-resource", {
+        headers: {
+          "Forwarded": "host=attacker.example;proto=https",
+        },
+      });
+      const res = handler(req);
+      const json = await res.json();
+      expect(json.resource).toBe("https://real-server.com");
+    });
+  });
+
   describe("explicit resourceUrl override", () => {
     it("uses explicit resourceUrl when provided", async () => {
       const handler = protectedResourceHandler({