🔒 Fix IP Address Spoofing via X-Forwarded-For

src/app/api/paste/route.ts

@@ -14,8 +14,8 @@ import {
     calculateExpiration,
     calculateTTL,
     validatePasteSize,
-    getClientIp,
 } from '@/lib/validation';
+import { getClientIp } from '@/lib/ip';
 
 // Rate limiting: 10 pastes per hour per IP
 const RATE_LIMIT_MAX = 10;

src/lib/ip.test.ts

@@ -0,0 +1,46 @@
+import { test } from 'node:test';
+import assert from 'node:assert';
+import { getClientIp } from './ip.ts';
+
+test('getClientIp prioritizes x-real-ip', () => {
+    const headers = new Headers();
+    headers.set('x-real-ip', '1.2.3.4');
+    headers.set('x-forwarded-for', '5.6.7.8');
+    const ip = getClientIp(headers);
+    assert.strictEqual(ip, '1.2.3.4');
+});
+
+test('getClientIp takes the LAST IP in x-forwarded-for to prevent spoofing', () => {
+    const headers = new Headers();
+    // 1.1.1.1 is spoofed by client, 2.2.2.2 is real client IP added by proxy
+    headers.set('x-forwarded-for', '1.1.1.1, 2.2.2.2');
+    const ip = getClientIp(headers);
+    assert.strictEqual(ip, '2.2.2.2');
+});
+
+test('getClientIp handles multiple IPs in x-forwarded-for', () => {
+    const headers = new Headers();
+    headers.set('x-forwarded-for', '1.1.1.1, 2.2.2.2, 3.3.3.3');
+    const ip = getClientIp(headers);
+    assert.strictEqual(ip, '3.3.3.3');
+});
+
+test('getClientIp handles x-forwarded-for with whitespace', () => {
+    const headers = new Headers();
+    headers.set('x-forwarded-for', ' 1.1.1.1 ,  2.2.2.2 ');
+    const ip = getClientIp(headers);
+    assert.strictEqual(ip, '2.2.2.2');
+});
+
+test('getClientIp returns unknown if no headers are present', () => {
+    const headers = new Headers();
+    const ip = getClientIp(headers);
+    assert.strictEqual(ip, 'unknown');
+});
+
+test('getClientIp handles empty x-forwarded-for', () => {
+    const headers = new Headers();
+    headers.set('x-forwarded-for', '');
+    const ip = getClientIp(headers);
+    assert.strictEqual(ip, 'unknown');
+});

src/lib/ip.ts

@@ -0,0 +1,28 @@
+/**
+ * Utility to securely retrieve client IP from request headers
+ */
+
+/**
+ * Get client IP from request headers
+ * Handles proxy headers securely by prioritizing x-real-ip and
+ * taking the last entry in x-forwarded-for to prevent spoofing.
+ */
+export function getClientIp(headers: Headers): string {
+    // 1. Check x-real-ip (often set by Vercel, Nginx, etc.)
+    const xRealIp = headers.get('x-real-ip');
+    if (xRealIp) return xRealIp;
+
+    // 2. Check x-forwarded-for
+    const xForwardedFor = headers.get('x-forwarded-for');
+    if (xForwardedFor) {
+        // We take the LAST IP in the list.
+        // If the client spoofs XFF, their spoofed IP will be at the beginning.
+        // The real client IP (seen by the proxy) will be appended to the list.
+        const ips = xForwardedFor.split(',').map(ip => ip.trim()).filter(Boolean);
+        if (ips.length > 0) {
+            return ips[ips.length - 1];
+        }
+    }
+
+    return 'unknown';
+}

src/lib/validation.ts

@@ -144,13 +144,3 @@ export function validatePasteSize(ciphertext: string): boolean {
     return estimatedSize <= MAX_PASTE_SIZE;
 }
 
-/**
- * Get client IP from request headers
- */
-export function getClientIp(headers: Headers): string {
-    return (
-        headers.get('x-forwarded-for')?.split(',')[0] ||
-        headers.get('x-real-ip') ||
-        'unknown'
-    );
-}