docs(security): add IP address header configuration for Better Auth

docs/content/docs/reference/security.mdx

@@ -46,6 +46,26 @@ Plugins can also set custom cookie options to align with specific security needs
 
 Better Auth includes built-in rate limiting to safeguard against brute-force attacks. Rate limits are applied across all routes by default, with specific routes subject to stricter limits based on potential risk.
 
+## IP Address Headers
+
+Better Auth uses client IP addresses for rate limiting and security monitoring. By default, it reads the IP address from the standard `X-Forwarded-For` header. However, you can configure a specific trusted header to ensure accurate IP address detection and prevent IP spoofing attacks.
+
+You can configure the IP address header in your Better Auth configuration:
+
+```typescript
+{
+  security: {
+    ipAddress: {
+      ipAddressHeaders: ['cf-connecting-ip'] // or any other custom header
+    }
+  }
+}
+```
+
+This ensures that Better Auth only accepts IP addresses from your trusted proxy's header, making it more difficult for attackers to bypass rate limiting or other IP-based security measures by spoofing headers.
+
+> **Important**: When setting a custom IP address header, ensure that your proxy or load balancer is properly configured to set this header, and that it cannot be set by end users directly.
+
 ## Trusted Origins
 
 Trusted origins prevent CSRF attacks and block open redirects. You can set a list of trusted origins in the `trustedOrigins` configuration option. Requests from origins not on this list are automatically blocked.

packages/better-auth/src/utils/get-request-ip.ts

@@ -12,25 +12,33 @@ export function getIp(
 	if (isTest) {
 		return testIP;
 	}
-	const ipHeaders = options.advanced?.ipAddress?.ipAddressHeaders;
-	const keys = ipHeaders || [
-		"x-client-ip",
-		"x-forwarded-for",
-		"cf-connecting-ip",
-		"fastly-client-ip",
-		"x-real-ip",
-		"x-cluster-client-ip",
-		"x-forwarded",
-		"forwarded-for",
-		"forwarded",
-	];
+
 	const headers = "headers" in req ? req.headers : req;
-	for (const key of keys) {
+
+	const defaultHeaders = ["x-forwarded-for"];
+
+	const ipHeaders =
+		options.advanced?.ipAddress?.ipAddressHeaders || defaultHeaders;
+
+	for (const key of ipHeaders) {
 		const value = "get" in headers ? headers.get(key) : headers[key];
 		if (typeof value === "string") {
 			const ip = value.split(",")[0].trim();
-			if (ip) return ip;
+			if (isValidIP(ip)) {
+				return ip;
+			}
 		}
 	}
 	return null;
 }
+
+function isValidIP(ip: string): boolean {
+	const ipv4Regex = /^(\d{1,3}\.){3}\d{1,3}$/;
+	if (ipv4Regex.test(ip)) {
+		const parts = ip.split(".").map(Number);
+		return parts.every((part) => part >= 0 && part <= 255);
+	}
+
+	const ipv6Regex = /^([0-9a-fA-F]{1,4}:){7}[0-9a-fA-F]{1,4}$/;
+	return ipv6Regex.test(ip);
+}