diff --git a/NEWS b/NEWS
index a63a367d1f5..15202e9ad73 100644
--- a/NEWS
+++ b/NEWS
@@ -22,6 +22,9 @@ PHP                                                                        NEWS
 - MySQLnd:
   . Fixed bug GH-11440 (authentication to a sha256_password account fails over
     SSL). (nielsdos)
+  . Fixed bug GH-11438 (mysqlnd fails to authenticate with sha256_password
+    accounts using passwords longer than 19 characters).
+    (nielsdos, Kamil Tekiela)
 
 - Opcache:
   . Fixed bug GH-11715 (opcache.interned_strings_buffer either has no effect or
diff --git a/ext/mysqli/tests/gh11438.phpt b/ext/mysqli/tests/gh11438.phpt
new file mode 100644
index 00000000000..49c715e72c0
--- /dev/null
+++ b/ext/mysqli/tests/gh11438.phpt
@@ -0,0 +1,84 @@
+--TEST--
+GH-11438 (mysqlnd fails to authenticate with sha256_password accounts using passwords longer than 19 characters)
+--EXTENSIONS--
+mysqli
+--SKIPIF--
+<?php
+require_once 'skipifconnectfailure.inc';
+
+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");
+
+if (!$link = my_mysqli_connect($host, $user, $passwd, $db, $port, $socket))
+    die(printf("skip: [%d] %s\n", mysqli_connect_errno(), mysqli_connect_error()));
+
+if (mysqli_get_server_version($link) < 50606)
+    die("skip: SHA-256 requires MySQL 5.6.6+");
+
+if (!($res = $link->query("SHOW PLUGINS"))) {
+    die(sprintf("skip [%d] %s\n", $link->errno, $link->error));
+}
+
+$found = false;
+while ($row = $res->fetch_assoc()) {
+    if (($row['Name'] == 'sha256_password') && ($row['Status'] == 'ACTIVE')) {
+        $found = true;
+        break;
+    }
+}
+if (!$found)
+    die("skip SHA-256 server plugin unavailable");
+
+// Ignore errors because this variable exists only in MySQL 5.6 and 5.7
+$link->query("SET @@session.old_passwords=2");
+
+$link->query('DROP USER shatest');
+$link->query("DROP USER shatest@localhost");
+
+if (!$link->query('CREATE USER shatest@"%" IDENTIFIED WITH sha256_password') ||
+    !$link->query('CREATE USER shatest@"localhost" IDENTIFIED WITH sha256_password')) {
+    die(sprintf("skip CREATE USER failed [%d] %s", $link->errno, $link->error));
+}
+
+// Password of length 52, more than twice the length of the scramble data to ensure scramble is repeated correctly
+if (!$link->query('SET PASSWORD FOR shatest@"%" = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"') ||
+    !$link->query('SET PASSWORD FOR shatest@"localhost" = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"')) {
+    die(sprintf("skip SET PASSWORD failed [%d] %s", $link->errno, $link->error));
+}
+
+echo "nocache";
+?>
+--FILE--
+<?php
+require_once 'connect.inc';
+
+$link = new mysqli($host, 'shatest', 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ', null, $port, $socket);
+if ($link->connect_errno) {
+    printf("[001] [%d] %s\n", $link->connect_errno, $link->connect_error);
+} else {
+    if (!$res = $link->query("SELECT USER()"))
+        printf("[002] [%d] %s\n", $link->errno, $link->error);
+
+    if (!$row = mysqli_fetch_assoc($res)) {
+        printf("[003] [%d] %s\n", $link->errno, $link->error);
+    }
+
+    if (!is_string($row['USER()']) || !str_starts_with($row['USER()'], 'shatest')) {
+        printf("[004] Expecting 1 got %s/'%s'", gettype($row['USER()']), $row['USER()']);
+    }
+}
+
+print "done!";
+?>
+--CLEAN--
+<?php
+require_once 'connect.inc';
+$link->query('DROP USER shatest');
+$link->query('DROP USER shatest@localhost');
+?>
+--EXPECTF--
+done!
diff --git a/ext/mysqlnd/mysqlnd_auth.c b/ext/mysqlnd/mysqlnd_auth.c
index 6956440abdc..ea9755c982e 100644
--- a/ext/mysqlnd/mysqlnd_auth.c
+++ b/ext/mysqlnd/mysqlnd_auth.c
@@ -927,7 +927,10 @@ mysqlnd_sha256_auth_get_auth_data(struct st_mysqlnd_authentication_plugin * self
 			char *xor_str = do_alloca(passwd_len + 1, use_heap);
 			memcpy(xor_str, passwd, passwd_len);
 			xor_str[passwd_len] = '\0';
-			mysqlnd_xor_string(xor_str, passwd_len, (char *) auth_plugin_data, auth_plugin_data_len);
+			/* https://dev.mysql.com/doc/dev/mysql-server/latest/page_caching_sha2_authentication_exchanges.html
+			 * This tells us that the nonce is 20 (==SCRAMBLE_LENGTH) bytes long.
+			 * In a 5.5+ server we might get additional scramble data in php_mysqlnd_greet_read, not used by this authentication method. */
+			mysqlnd_xor_string(xor_str, passwd_len, (char *) auth_plugin_data, SCRAMBLE_LENGTH);
 			ret = mysqlnd_sha256_public_encrypt(conn, server_public_key, passwd_len, auth_data_len, xor_str);
 			free_alloca(xor_str, use_heap);
 		}