ext/mysqlnd: Guard against short server scramble in sha256_password auth

ext/mysqli/tests/fake_server.inc

@@ -1428,6 +1428,20 @@ function my_mysqli_test_stmt_response_field_len_premature(my_mysqli_fake_server_
 	$conn->read(65536);
 }
 
+function my_mysqli_test_sha256_password_short_scramble(my_mysqli_fake_server_conn $conn): void
+{
+    $conn->send_server_greetings();
+    $conn->read();
+    // AuthSwitchRequest (0xfe) to sha256_password carrying a scramble shorter
+    // than SCRAMBLE_LENGTH (20). mnd_emalloc sizes the heap copy to this
+    // length, so the sha256 plugin's 20-byte XOR over-reads it without the
+    // length guard.
+    $payload = "\xfe" . "sha256_password\x00" . str_repeat("\x41", 8);
+    $header = substr(pack('V', strlen($payload)), 0, 3) . "\x02";
+    $conn->send($header . $payload, "Auth Switch Request [short sha256 scramble]");
+    $conn->read();
+}
+
 function run_fake_server(string $test_function, int|string $port = 0): void
 {
     $host = '127.0.0.1';

ext/mysqli/tests/mysqli_sha256_password_short_scramble.phpt

@@ -0,0 +1,52 @@
+--TEST--
+mysqlnd sha256_password: reject server scramble shorter than 20 bytes
+--EXTENSIONS--
+mysqli
+--SKIPIF--
+<?php
+ob_start();
+phpinfo(INFO_MODULES);
+$tmp = ob_get_contents();
+ob_end_clean();
+if (!stristr($tmp, "auth_plugin_sha256_password"))
+    die("skip SHA256 auth plugin not built-in to mysqlnd");
+?>
+--FILE--
+<?php
+require_once 'fake_server.inc';
+
+mysqli_report(MYSQLI_REPORT_OFF);
+
+$process = run_fake_server_in_background('sha256_password_short_scramble');
+$process->wait();
+
+$key_file = __DIR__ . '/mysqli_sha256_password_short_scramble.pem';
+file_put_contents($key_file, <<<PEM
+-----BEGIN PUBLIC KEY-----
+MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAvz0SP+XekztaRD5Ghdx8
+ZLjXB8TKlF3tup4n6fJqhf6LyCX//AX/ppTZUmoiptxMucJrSaXYuI2YOeh2tfuR
+HFMLqlAnK96UxpQMfWSSXVTKaeA4ld0DwQjQeWUIN+2AAN6HNGp7ilnS+7DlSHhd
+7KLLZDJqrhQWtDynxGuIx55bAgPMp7adVn8CXg1vVMh62KYZDYmGU69HVVEuHq1n
+odJdC1S2HYSJ4LgVIKg9eUBPKO1Bvwrwxse7UDGUNCSOyF+r8/jTKdrPdXp1qRNQ
+9dU4/gMzMWd2NJ3+unXaLEEmpWEj/8x56X46PmpfbVAKm0tyn7YZH3tqamPo1piz
+rQIDAQAB
+-----END PUBLIC KEY-----
+PEM);
+
+$link = mysqli_init();
+$link->options(MYSQLI_SERVER_PUBLIC_KEY, $key_file);
+@$link->real_connect("127.0.0.1", "root", "password longer than the scramble", "", $process->getPort());
+printf("[%d] %s\n", mysqli_connect_errno(), mysqli_connect_error());
+
+$process->terminate();
+print "done!\n";
+?>
+--CLEAN--
+<?php
+@unlink(__DIR__ . '/mysqli_sha256_password_short_scramble.pem');
+?>
+--EXPECTF--
+[*] Server started on 127.0.0.1:%d
+%A
+[2027] The server sent wrong length for scramble
+done!

ext/mysqlnd/mysqlnd_auth.c

@@ -908,6 +908,11 @@ mysqlnd_sha256_auth_get_auth_data(struct st_mysqlnd_authentication_plugin * self
 	DBG_ENTER("mysqlnd_sha256_auth_get_auth_data");
 	DBG_INF_FMT("salt(%zu)=[%.*s]", auth_plugin_data_len, (int) auth_plugin_data_len, auth_plugin_data);
 
+	if (auth_plugin_data_len < SCRAMBLE_LENGTH) {
+		SET_CLIENT_ERROR(conn->error_info, CR_MALFORMED_PACKET, UNKNOWN_SQLSTATE, "The server sent wrong length for scramble");
+		DBG_ERR_FMT("The server sent wrong length for scramble %zu. Expected %u", auth_plugin_data_len, SCRAMBLE_LENGTH);
+		DBG_RETURN(NULL);
+	}
 
 	if (conn->vio->data->ssl) {
 		DBG_INF("simple clear text under SSL");