fix(voice-call): require Twilio signature in ngrok loopback mode

extensions/voice-call/src/config.ts

@@ -207,8 +207,10 @@ export const VoiceCallTunnelConfigSchema = z
     ngrokDomain: z.string().min(1).optional(),
     /**
      * Allow ngrok free tier compatibility mode.
-     * When true, signature verification failures on ngrok-free.app URLs
-     * will be allowed only for loopback requests (ngrok local agent).
+     * When true, forwarded headers may be trusted for loopback requests
+     * to reconstruct the public ngrok URL used for signing.
+     *
+     * IMPORTANT: This does NOT bypass signature verification.
      */
     allowNgrokFreeTierLoopbackBypass: z.boolean().default(false),
   })

extensions/voice-call/src/webhook-security.test.ts

@@ -222,7 +222,39 @@ describe("verifyTwilioWebhook", () => {
     expect(result.reason).toMatch(/Invalid signature/);
   });
 
-  it("allows invalid signatures for ngrok free tier only on loopback", () => {
+  it("accepts valid signatures for ngrok free tier on loopback when compatibility mode is enabled", () => {
+    const authToken = "test-auth-token";
+    const postBody = "CallSid=CS123&CallStatus=completed&From=%2B15550000000";
+    const webhookUrl = "https://local.ngrok-free.app/voice/webhook";
+
+    const signature = twilioSignature({
+      authToken,
+      url: webhookUrl,
+      postBody,
+    });
+
+    const result = verifyTwilioWebhook(
+      {
+        headers: {
+          host: "127.0.0.1:3334",
+          "x-forwarded-proto": "https",
+          "x-forwarded-host": "local.ngrok-free.app",
+          "x-twilio-signature": signature,
+        },
+        rawBody: postBody,
+        url: "http://127.0.0.1:3334/voice/webhook",
+        method: "POST",
+        remoteAddress: "127.0.0.1",
+      },
+      authToken,
+      { allowNgrokFreeTierLoopbackBypass: true },
+    );
+
+    expect(result.ok).toBe(true);
+    expect(result.verificationUrl).toBe(webhookUrl);
+  });
+
+  it("does not allow invalid signatures for ngrok free tier on loopback", () => {
     const authToken = "test-auth-token";
     const postBody = "CallSid=CS123&CallStatus=completed&From=%2B15550000000";
 
@@ -243,9 +275,9 @@ describe("verifyTwilioWebhook", () => {
       { allowNgrokFreeTierLoopbackBypass: true },
     );
 
-    expect(result.ok).toBe(true);
+    expect(result.ok).toBe(false);
+    expect(result.reason).toMatch(/Invalid signature/);
     expect(result.isNgrokFreeTier).toBe(true);
-    expect(result.reason).toMatch(/compatibility mode/);
   });
 
   it("ignores attacker X-Forwarded-Host without allowedHosts or trustForwardingHeaders", () => {

extensions/voice-call/src/webhook-security.ts

@@ -339,7 +339,13 @@ export function verifyTwilioWebhook(
   options?: {
     /** Override the public URL (e.g., from config) */
     publicUrl?: string;
-    /** Allow ngrok free tier compatibility mode (loopback only, less secure) */
+    /**
+     * Allow ngrok free tier compatibility mode (loopback only).
+     *
+     * IMPORTANT: This does NOT bypass signature verification.
+     * It only enables trusting forwarded headers on loopback so we can
+     * reconstruct the public ngrok URL that Twilio used for signing.
+     */
     allowNgrokFreeTierLoopbackBypass?: boolean;
     /** Skip verification entirely (only for development) */
     skipVerification?: boolean;
@@ -401,18 +407,6 @@ export function verifyTwilioWebhook(
   const isNgrokFreeTier =
     verificationUrl.includes(".ngrok-free.app") || verificationUrl.includes(".ngrok.io");
 
-  if (isNgrokFreeTier && options?.allowNgrokFreeTierLoopbackBypass && isLoopback) {
-    console.warn(
-      "[voice-call] Twilio signature validation failed (ngrok free tier compatibility, loopback only)",
-    );
-    return {
-      ok: true,
-      reason: "ngrok free tier compatibility mode (loopback only)",
-      verificationUrl,
-      isNgrokFreeTier: true,
-    };
-  }
-
   return {
     ok: false,
     reason: `Invalid signature for URL: ${verificationUrl}`,