diff --git a/src/Legacy/IntercomClient.php b/src/Legacy/IntercomClient.php
index b303b79d..e944a981 100644
--- a/src/Legacy/IntercomClient.php
+++ b/src/Legacy/IntercomClient.php
@@ -276,10 +276,20 @@ public function get($endpoint, $queryParams = [])
      *
      * @param  stdClass $pages
      * @return stdClass
+     * @throws \InvalidArgumentException if the pagination URL is not a valid https://  *.intercom.io address
      */
     public function nextPage($pages)
     {
-        $response = $this->sendRequest('GET', $pages->next);
+        $url = (string) $pages->next;
+        $parsed = parse_url($url);
+        $host = $parsed['host'] ?? '';
+        $validHost = str_ends_with($host, '.intercom.io');
+        if (($parsed['scheme'] ?? '') !== 'https' || !$validHost) {
+            throw new \InvalidArgumentException(
+                'nextPage URL must be an https:// address on the intercom.io domain.'
+            );
+        }
+        $response = $this->sendRequest('GET', $url);
         return $this->handleResponse($response);
     }
 
diff --git a/tests/Legacy/IntercomClientNextPageTest.php b/tests/Legacy/IntercomClientNextPageTest.php
new file mode 100644
index 00000000..77d3e161
--- /dev/null
+++ b/tests/Legacy/IntercomClientNextPageTest.php
@@ -0,0 +1,122 @@
+<?php
+
+namespace Intercom\Tests\Legacy;
+
+use GuzzleHttp\Client;
+use GuzzleHttp\Handler\MockHandler;
+use GuzzleHttp\HandlerStack;
+use GuzzleHttp\Psr7\Response;
+use Intercom\Legacy\IntercomClient;
+use InvalidArgumentException;
+use PHPUnit\Framework\TestCase;
+use stdClass;
+
+class IntercomClientNextPageTest extends TestCase
+{
+    private IntercomClient $client;
+    private MockHandler $mockHandler;
+
+    protected function setUp(): void
+    {
+        $this->mockHandler = new MockHandler();
+        $handlerStack = HandlerStack::create($this->mockHandler);
+        $httpClient = new Client(['handler' => $handlerStack]);
+
+        $this->client = new IntercomClient('test_token');
+        $this->client->setHttpClient($httpClient);
+    }
+
+    // Happy path — legitimate https://api.intercom.io URL must be followed with credentials attached.
+    public function testNextPageFollowsLegitimateUrl(): void
+    {
+        $this->mockHandler->append(new Response(200, [], json_encode(['data' => []])));
+
+        $pages = new stdClass();
+        $pages->next = 'https://api.intercom.io/contacts?page=2&per_page=50';
+
+        $result = $this->client->nextPage($pages);
+
+        $this->assertIsObject($result);
+        $lastRequest = $this->mockHandler->getLastRequest();
+        $this->assertNotNull($lastRequest);
+        $this->assertEquals(
+            'https://api.intercom.io/contacts?page=2&per_page=50',
+            (string) $lastRequest->getUri()
+        );
+        $this->assertStringStartsWith('Bearer ', $lastRequest->getHeaderLine('Authorization'));
+    }
+
+    // Vulnerability closed — attacker-controlled host must be rejected before any HTTP call.
+    // If the guard were absent MockHandler would throw OutOfBoundsException (no queued response),
+    // so receiving InvalidArgumentException proves the check fires first.
+    public function testNextPageRejectsAttackerControlledHost(): void
+    {
+        $pages = new stdClass();
+        $pages->next = 'https://attacker.com/steal';
+
+        $this->expectException(InvalidArgumentException::class);
+        $this->client->nextPage($pages);
+    }
+
+    // SSRF vector — AWS instance metadata endpoint must be rejected.
+    public function testNextPageRejectsAwsMetadataServiceUrl(): void
+    {
+        $pages = new stdClass();
+        $pages->next = 'http://169.254.169.254/latest/meta-data/iam/security-credentials/';
+
+        $this->expectException(InvalidArgumentException::class);
+        $this->client->nextPage($pages);
+    }
+
+    // Scheme enforcement — http:// to the correct host must still be rejected (no downgrade).
+    public function testNextPageRejectsPlainHttpEvenForApiIntercomIo(): void
+    {
+        $pages = new stdClass();
+        $pages->next = 'http://api.intercom.io/contacts?page=2';
+
+        $this->expectException(InvalidArgumentException::class);
+        $this->client->nextPage($pages);
+    }
+
+    // EU region — api.eu.intercom.io must be allowed without any configuration.
+    public function testNextPageAllowsEuRegionalUrl(): void
+    {
+        $this->mockHandler->append(new Response(200, [], json_encode(['data' => []])));
+
+        $pages = new stdClass();
+        $pages->next = 'https://api.eu.intercom.io/contacts?page=2&per_page=50';
+
+        $result = $this->client->nextPage($pages);
+
+        $this->assertIsObject($result);
+        $lastRequest = $this->mockHandler->getLastRequest();
+        $this->assertNotNull($lastRequest);
+        $this->assertEquals(
+            'https://api.eu.intercom.io/contacts?page=2&per_page=50',
+            (string) $lastRequest->getUri()
+        );
+    }
+
+    // AU region — api.au.intercom.io must be allowed without any configuration.
+    public function testNextPageAllowsAuRegionalUrl(): void
+    {
+        $this->mockHandler->append(new Response(200, [], json_encode(['data' => []])));
+
+        $pages = new stdClass();
+        $pages->next = 'https://api.au.intercom.io/contacts?page=2&per_page=50';
+
+        $result = $this->client->nextPage($pages);
+
+        $this->assertIsObject($result);
+    }
+
+    // Domain suffix bypass — evilintercom.io must not match (requires the dot).
+    public function testNextPageRejectsDomainThatMerelySuffixesIntercomIo(): void
+    {
+        $pages = new stdClass();
+        $pages->next = 'https://evilintercom.io/steal';
+
+        $this->expectException(InvalidArgumentException::class);
+        $this->client->nextPage($pages);
+    }
+}