fix(base-account): parse the SIWE nonce correctly and bind verification to the app domain

docs/base-account/guides/authenticate-users.mdx

@@ -147,12 +147,22 @@ try {
 ```ts Backend (Viem)
 import { createPublicClient, http } from 'viem';
 import { base } from 'viem/chains';
+import { verifySiweMessage } from 'viem/siwe';
+
+// The domain your app is served from. Anything signed for another
+// domain must not be accepted here.
+const APP_DOMAIN = 'yourapp.com';
 
 const client = createPublicClient({ chain: base, transport: http() });
 
 export async function verifySig(req, res) {
   const { address, message, signature } = req.body;
-  const valid = await client.verifyMessage({ address, message, signature });
+  const valid = await verifySiweMessage(client, {
+    address,
+    message,
+    signature,
+    domain: APP_DOMAIN,
+  });
   if (!valid) return res.status(401).json({ error: 'Invalid signature' });
   // create session / JWT
   res.json({ ok: true });
@@ -178,20 +188,33 @@ export async function verifySig(req, res) {
   regardless of where it originated.
 </Note>
 
+<Warning>
+  Always check the `domain` field of the signed message against your own domain.
+  A signature is valid for whichever domain it was signed for, so a signature a
+  user produced on another site is cryptographically valid on yours too.
+  `verifySiweMessage` performs this check when you pass `domain`; a bare
+  `verifyMessage` call does not, and accepts the signature.
+</Warning>
+
 ### Example Express Server
 
 ```ts title="server/auth.ts" expandable
 import crypto from "crypto";
 import express from "express";
 import { createPublicClient, http } from "viem";
 import { base } from "viem/chains";
+import { parseSiweMessage, verifySiweMessage } from "viem/siwe";
 
 const app = express();
 app.use(express.json());
 
 // Simple in-memory nonce store (swap for Redis or DB in production)
 const nonces = new Set<string>();
 
+// The domain your app is served from. Anything signed for another
+// domain must not be accepted here.
+const APP_DOMAIN = "yourapp.com";
+
 app.get("/auth/nonce", (_, res) => {
   const nonce = crypto.randomBytes(16).toString("hex");
   nonces.add(nonce);
@@ -203,14 +226,19 @@ const client = createPublicClient({ chain: base, transport: http() });
 app.post("/auth/verify", async (req, res) => {
   const { address, message, signature } = req.body;
 
-  // 1. Check nonce hasn\'t been reused
-  const nonce = message.match(/at (\w{32})$/)?.[1];
+  // 1. Check this server issued the nonce and hasn't seen it before
+  const { nonce } = parseSiweMessage(message);
   if (!nonce || !nonces.delete(nonce)) {
     return res.status(400).json({ error: "Invalid or reused nonce" });
   }
 
-  // 2. Verify signature
-  const valid = await client.verifyMessage({ address, message, signature });
+  // 2. Verify the signature and bind it to your domain
+  const valid = await verifySiweMessage(client, {
+    address,
+    message,
+    signature,
+    domain: APP_DOMAIN,
+  });
   if (!valid) return res.status(401).json({ error: "Invalid signature" });
 
   // 3. Create session / JWT here