diff --git a/app/Console/Commands/ValidateJsonFile.php b/app/Console/Commands/ValidateJsonFile.php
new file mode 100644
index 00000000..c7db73a2
--- /dev/null
+++ b/app/Console/Commands/ValidateJsonFile.php
@@ -0,0 +1,45 @@
+<?php
+
+namespace App\Console\Commands;
+
+use App\Console\VerifyJSON;
+use Illuminate\Console\Command;
+use Symfony\Component\Console\Command\Command as CommandAlias;
+
+class ValidateJsonFile extends Command
+{
+    use VerifyJSON;
+    /**
+     * The name and signature of the console command.
+     *
+     * @var string
+     */
+    protected $signature = 'import:validate-json {file : The JSON file to validate}';
+
+    /**
+     * The console command description.
+     *
+     * @var string
+     */
+    protected $description = 'Checks if a JSON file is valid according to the v3 import configuration file standard.';
+
+    /**
+     * Execute the console command.
+     */
+    public function handle(): int
+    {
+        $file = (string)$this->argument('file');
+        if (!is_file($file) || !is_readable($file)) {
+            $this->error(sprintf('File %s does not exist or is not readable.', $file));
+            return CommandAlias::FAILURE;
+        }
+        $result = $this->verifyJSON($file);
+        if(false === $result) {
+            $this->error('File is not valid JSON.');
+            return CommandAlias::FAILURE;
+        }
+
+        $this->info('File is valid JSON.');
+        return CommandAlias::SUCCESS;
+    }
+}
diff --git a/app/Console/Commands/ValidateJsonFiles.php b/app/Console/Commands/ValidateJsonFiles.php
new file mode 100644
index 00000000..82ac94c6
--- /dev/null
+++ b/app/Console/Commands/ValidateJsonFiles.php
@@ -0,0 +1,56 @@
+<?php
+
+namespace App\Console\Commands;
+
+use App\Console\VerifyJSON;
+use Illuminate\Console\Command;
+use Symfony\Component\Console\Command\Command as CommandAlias;
+
+class ValidateJsonFiles extends Command
+{
+    use VerifyJSON;
+    /**
+     * The name and signature of the console command.
+     *
+     * @var string
+     */
+    protected $signature = 'import:validate-json-directory {directory : The directory with JSON files to validate}';
+
+    /**
+     * The console command description.
+     *
+     * @var string
+     */
+    protected $description = 'Recursively validate all JSON files in a directory. Stops after 100 files.';
+
+    /**
+     * Execute the console command.
+     */
+    public function handle(): int
+    {
+        $directory = (string)$this->argument('directory');
+        if (!is_dir($directory) || !is_readable($directory)) {
+            $this->error(sprintf('Cannot read directory %s.', $directory));
+            return CommandAlias::FAILURE;
+        }
+
+        // check each file in the directory and see if it needs action.
+        // collect recursively:
+        $it        = new \RecursiveIteratorIterator(new \RecursiveDirectoryIterator($directory, \RecursiveDirectoryIterator::SKIP_DOTS));
+        $Regex     = new \RegexIterator($it, '/^.+\.json$/i', \RecursiveRegexIterator::GET_MATCH);
+        $fullPaths = [];
+        foreach ($Regex as $item) {
+            $path        = $item[0];
+            $fullPaths[] = $path;
+        }
+        foreach ($fullPaths as $file) {
+            $result = $this->verifyJSON($file);
+            if (false === $result) {
+                $this->error(sprintf('File "%s" is not valid JSON.', $file));
+                return CommandAlias::FAILURE;
+            }
+            $this->info(sprintf('File "%s" is valid JSON.', $file));
+        }
+        return CommandAlias::SUCCESS;
+    }
+}
diff --git a/app/Console/VerifyJSON.php b/app/Console/VerifyJSON.php
index 5712eeb6..3d4e7bd9 100644
--- a/app/Console/VerifyJSON.php
+++ b/app/Console/VerifyJSON.php
@@ -35,31 +35,40 @@ use Swaggest\JsonSchema\Schema;
  */
 trait VerifyJSON
 {
+    protected string $errorMessage = '';
+
     private function verifyJSON(string $file): bool
     {
         // basic check on the JSON.
-        $json       = (string)file_get_contents($file);
+        $json = (string)file_get_contents($file);
 
         try {
             $config = json_decode($json, null, 512, JSON_THROW_ON_ERROR);
         } catch (JsonException $e) {
-            Log::error(sprintf('The importer can\'t import: could not decode the JSON in the config file: %s', $e->getMessage()));
+            $message = sprintf('The importer can\'t import: could not decode the JSON in the config file: %s', $e->getMessage());
+            Log::error($message);
+            $this->errorMessage = $message;
 
             return false;
         }
         // validate JSON schema.
         $schemaFile = resource_path('schemas/v3.json');
         if (!file_exists($schemaFile)) {
-            Log::error(sprintf('The schema file "%s" does not exist.', $schemaFile));
+            $message = sprintf('The schema file "%s" does not exist.', $schemaFile);
+            Log::error($message);
+            $this->errorMessage = $message;
 
             return false;
         }
-        $schema     = json_decode(file_get_contents($schemaFile));
+        $schema = json_decode(file_get_contents($schemaFile));
 
         try {
             Schema::import($schema)->in($config);
         } catch (Exception|\Exception $e) {
-            Log::error(sprintf('Configuration file "%s" does not adhere to the v3 schema: %s', $file, $e->getMessage()));
+            $message = sprintf('Configuration file "%s" does not adhere to the v3 schema: %s', $file, $e->getMessage());
+
+            Log::error($message);
+            $this->errorMessage = $message;
 
             return false;
         }
diff --git a/app/Http/Controllers/Import/DownloadController.php b/app/Http/Controllers/Import/DownloadController.php
index 9c24a40f..58652695 100644
--- a/app/Http/Controllers/Import/DownloadController.php
+++ b/app/Http/Controllers/Import/DownloadController.php
@@ -53,6 +53,14 @@ class DownloadController extends Controller
         if(is_array($array['mapping']) && 0 === count($array['mapping'])) {
             $array['mapping'] = new \stdClass();
         }
+        // same for "accounts"
+        if(is_array($array['accounts']) && 0 === count($array['accounts'])) {
+            $array['accounts'] = new \stdClass();
+        }
+        // same for "accounts"
+        if(is_array($array['nordigen_requisitions']) && 0 === count($array['nordigen_requisitions'])) {
+            $array['nordigen_requisitions'] = new \stdClass();
+        }
 
 
         $result        = json_encode($array, JSON_PRETTY_PRINT | JSON_THROW_ON_ERROR);
diff --git a/app/Http/Controllers/Import/UploadController.php b/app/Http/Controllers/Import/UploadController.php
index 0b59edb5..edfc2a44 100644
--- a/app/Http/Controllers/Import/UploadController.php
+++ b/app/Http/Controllers/Import/UploadController.php
@@ -25,6 +25,7 @@ declare(strict_types=1);
 
 namespace App\Http\Controllers\Import;
 
+use App\Console\VerifyJSON;
 use App\Exceptions\ImporterErrorException;
 use App\Http\Controllers\Controller;
 use App\Http\Middleware\UploadControllerMiddleware;
@@ -53,6 +54,7 @@ use Storage;
 class UploadController extends Controller
 {
     use RestoresConfiguration;
+    use VerifyJSON;
 
     private string $configFileName;
     private string $contentType;
@@ -76,22 +78,22 @@ class UploadController extends Controller
     public function index(Request $request)
     {
         Log::debug(sprintf('Now at %s', __METHOD__));
-        $mainTitle          = 'Upload your file(s)';
-        $subTitle           = 'Start page and instructions';
-        $flow               = $request->cookie(Constants::FLOW_COOKIE);
+        $mainTitle = 'Upload your file(s)';
+        $subTitle  = 'Start page and instructions';
+        $flow      = $request->cookie(Constants::FLOW_COOKIE);
 
         // simplefin settings.
         $simpleFinToken     = config('simplefin.token');
         $simpleFinOriginUrl = config('simplefin.origin_url');
 
         // get existing configs.
-        $disk               = Storage::disk('configurations');
+        $disk = Storage::disk('configurations');
         Log::debug(sprintf('Going to check directory for config files: %s', config('filesystems.disks.configurations.root')));
-        $all                = $disk->files();
+        $all = $disk->files();
 
         // remove files from list
-        $list               = [];
-        $ignored            = config('importer.ignored_files');
+        $list    = [];
+        $ignored = config('importer.ignored_files');
         foreach ($all as $entry) {
             if (!in_array($entry, $ignored, true)) {
                 $list[] = $entry;
@@ -113,13 +115,13 @@ class UploadController extends Controller
     public function upload(Request $request)
     {
         Log::debug(sprintf('Now at %s', __METHOD__));
-        $importedFile  = $request->file('importable_file');
-        $configFile    = $request->file('config_file');
-        $flow          = $request->cookie(Constants::FLOW_COOKIE);
-        $errors        = new MessageBag();
+        $importedFile = $request->file('importable_file');
+        $configFile   = $request->file('config_file');
+        $flow         = $request->cookie(Constants::FLOW_COOKIE);
+        $errors       = new MessageBag();
 
         // process uploaded file (if present)
-        $errors        = $this->processUploadedFile($flow, $errors, $importedFile);
+        $errors = $this->processUploadedFile($flow, $errors, $importedFile);
 
         // process config file (if present)
         if (0 === count($errors) && null !== $configFile) {
@@ -127,7 +129,7 @@ class UploadController extends Controller
         }
 
         // process pre-selected file (if present):
-        $errors        = $this->processSelection($errors, (string)$request->get('existing_config'), $configFile);
+        $errors = $this->processSelection($errors, (string)$request->get('existing_config'), $configFile);
 
         if ($errors->count() > 0) {
             return redirect(route('003-upload.index'))->withErrors($errors)->withInput();
@@ -185,7 +187,7 @@ class UploadController extends Controller
                     // https://stackoverflow.com/questions/11066857/detect-eol-type-using-php
                     // because apparently there are banks that use "\r" as newline. Looking at the morons of KBC Bank, Belgium.
                     // This one is for you: 🤦‍♀️
-                    $eol     = $this->detectEOL($content);
+                    $eol = $this->detectEOL($content);
                     if ("\r" === $eol) {
                         Log::error('Your bank is dumb. Tell them to fix their CSV files.');
                         $content = str_replace("\r", "\n", $content);
@@ -195,7 +197,7 @@ class UploadController extends Controller
                 if ('camt' === $this->contentType) {
                     $content = (string)file_get_contents($file->getPathname());
                 }
-                $fileName          = StorageService::storeContent($content);
+                $fileName = StorageService::storeContent($content);
                 session()->put(Constants::UPLOAD_DATA_FILE, $fileName);
                 session()->put(Constants::HAS_UPLOAD, true);
             }
@@ -215,9 +217,9 @@ class UploadController extends Controller
     private function detectEOL(string $string): string
     {
         $eols     = ['\n\r' => "\n\r", // 0x0A - 0x0D - acorn BBC
-            '\r\n'          => "\r\n", // 0x0D - 0x0A - Windows, DOS OS/2
-            '\n'            => "\n", // 0x0A -      - Unix, OSX
-            '\r'            => "\r", // 0x0D -      - Apple ][, TRS80
+                     '\r\n' => "\r\n", // 0x0D - 0x0A - Windows, DOS OS/2
+                     '\n'   => "\n", // 0x0A -      - Unix, OSX
+                     '\r'   => "\r", // 0x0D -      - Apple ][, TRS80
         ];
         $curCount = 0;
         $curEol   = '';
@@ -247,19 +249,27 @@ class UploadController extends Controller
         // upload the file to a temp directory and use it from there.
         if (0 === $errorNumber) {
             Log::debug('Config file uploaded.');
-            $this->configFileName = StorageService::storeContent((string)file_get_contents($file->getPathname()));
+            $path       = $file->getPathname();
+            $validation = $this->verifyJSON($path);
+            if (false === $validation) {
+                $errors->add('config_file', $this->errorMessage);
+                return $errors;
+            }
+
+            $content              = (string)file_get_contents($path);
+            $this->configFileName = StorageService::storeContent($content);
 
             session()->put(Constants::UPLOAD_CONFIG_FILE, $this->configFileName);
 
             // process the config file
-            $success              = false;
-            $configuration        = null;
+            $success       = false;
+            $configuration = null;
 
             try {
                 $configuration = ConfigFileProcessor::convertConfigFile($this->configFileName);
                 $configuration->setContentType($this->contentType);
                 session()->put(Constants::CONFIGURATION, $configuration->toSessionArray());
-                $success       = true;
+                $success = true;
             } catch (ImporterErrorException $e) {
                 $errors->add('config_file', $e->getMessage());
             }
@@ -303,12 +313,12 @@ class UploadController extends Controller
      */
     private function handleSimpleFINFlow(Request $request, Configuration $configuration): RedirectResponse
     {
-        $errors           = new MessageBag();
+        $errors = new MessageBag();
         Log::debug('UploadController::handleSimpleFINFlow() INVOKED'); // Unique entry marker
 
-        $setupToken       = (string)$request->get('simplefin_token');
-        $isDemo           = $request->boolean('use_demo');
-        $accessToken      = $configuration->getAccessToken();
+        $setupToken  = (string)$request->get('simplefin_token');
+        $isDemo      = $request->boolean('use_demo');
+        $accessToken = $configuration->getAccessToken();
         Log::debug(sprintf('handleSimpleFINFlow("%s")', $setupToken));
 
         if ($isDemo) {
@@ -348,7 +358,7 @@ class UploadController extends Controller
         $configuration->setAccessToken($accessToken);
 
         try {
-            $accountsData   = $simpleFINService->fetchAccounts();
+            $accountsData = $simpleFINService->fetchAccounts();
 
             // save configuration in session and on disk: TODO needs a trait.
             Log::debug('Save config to disk after setting access token.');
diff --git a/composer.json b/composer.json
index 0185ab7d..a8e642f7 100644
--- a/composer.json
+++ b/composer.json
@@ -83,7 +83,8 @@
         "spatie/enum": "^3.10",
         "swaggest/json-schema": "^0.12.43",
         "symfony/http-client": "^7.3",
-        "symfony/mailgun-mailer": "^7.3"
+        "symfony/mailgun-mailer": "^7.3",
+        "thecodingmachine/safe": "^3.3"
     },
     "require-dev": {
         "barryvdh/laravel-debugbar": "^3.16",
diff --git a/composer.lock b/composer.lock
index 07b0ca87..778f2618 100644
--- a/composer.lock
+++ b/composer.lock
@@ -4,7 +4,7 @@
         "Read more about it at https://getcomposer.org/doc/01-basic-usage.md#installing-dependencies",
         "This file is @generated automatically"
     ],
-    "content-hash": "90550ddfcbcb4c8043368ef25abd0c9d",
+    "content-hash": "b7cf2032b370512926f502bdd4038b70",
     "packages": [
         {
             "name": "brick/math",
@@ -6316,6 +6316,145 @@
             ],
             "time": "2025-08-13T11:49:31+00:00"
         },
+        {
+            "name": "thecodingmachine/safe",
+            "version": "v3.3.0",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/thecodingmachine/safe.git",
+                "reference": "2cdd579eeaa2e78e51c7509b50cc9fb89a956236"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/thecodingmachine/safe/zipball/2cdd579eeaa2e78e51c7509b50cc9fb89a956236",
+                "reference": "2cdd579eeaa2e78e51c7509b50cc9fb89a956236",
+                "shasum": ""
+            },
+            "require": {
+                "php": "^8.1"
+            },
+            "require-dev": {
+                "php-parallel-lint/php-parallel-lint": "^1.4",
+                "phpstan/phpstan": "^2",
+                "phpunit/phpunit": "^10",
+                "squizlabs/php_codesniffer": "^3.2"
+            },
+            "type": "library",
+            "autoload": {
+                "files": [
+                    "lib/special_cases.php",
+                    "generated/apache.php",
+                    "generated/apcu.php",
+                    "generated/array.php",
+                    "generated/bzip2.php",
+                    "generated/calendar.php",
+                    "generated/classobj.php",
+                    "generated/com.php",
+                    "generated/cubrid.php",
+                    "generated/curl.php",
+                    "generated/datetime.php",
+                    "generated/dir.php",
+                    "generated/eio.php",
+                    "generated/errorfunc.php",
+                    "generated/exec.php",
+                    "generated/fileinfo.php",
+                    "generated/filesystem.php",
+                    "generated/filter.php",
+                    "generated/fpm.php",
+                    "generated/ftp.php",
+                    "generated/funchand.php",
+                    "generated/gettext.php",
+                    "generated/gmp.php",
+                    "generated/gnupg.php",
+                    "generated/hash.php",
+                    "generated/ibase.php",
+                    "generated/ibmDb2.php",
+                    "generated/iconv.php",
+                    "generated/image.php",
+                    "generated/imap.php",
+                    "generated/info.php",
+                    "generated/inotify.php",
+                    "generated/json.php",
+                    "generated/ldap.php",
+                    "generated/libxml.php",
+                    "generated/lzf.php",
+                    "generated/mailparse.php",
+                    "generated/mbstring.php",
+                    "generated/misc.php",
+                    "generated/mysql.php",
+                    "generated/mysqli.php",
+                    "generated/network.php",
+                    "generated/oci8.php",
+                    "generated/opcache.php",
+                    "generated/openssl.php",
+                    "generated/outcontrol.php",
+                    "generated/pcntl.php",
+                    "generated/pcre.php",
+                    "generated/pgsql.php",
+                    "generated/posix.php",
+                    "generated/ps.php",
+                    "generated/pspell.php",
+                    "generated/readline.php",
+                    "generated/rnp.php",
+                    "generated/rpminfo.php",
+                    "generated/rrd.php",
+                    "generated/sem.php",
+                    "generated/session.php",
+                    "generated/shmop.php",
+                    "generated/sockets.php",
+                    "generated/sodium.php",
+                    "generated/solr.php",
+                    "generated/spl.php",
+                    "generated/sqlsrv.php",
+                    "generated/ssdeep.php",
+                    "generated/ssh2.php",
+                    "generated/stream.php",
+                    "generated/strings.php",
+                    "generated/swoole.php",
+                    "generated/uodbc.php",
+                    "generated/uopz.php",
+                    "generated/url.php",
+                    "generated/var.php",
+                    "generated/xdiff.php",
+                    "generated/xml.php",
+                    "generated/xmlrpc.php",
+                    "generated/yaml.php",
+                    "generated/yaz.php",
+                    "generated/zip.php",
+                    "generated/zlib.php"
+                ],
+                "classmap": [
+                    "lib/DateTime.php",
+                    "lib/DateTimeImmutable.php",
+                    "lib/Exceptions/",
+                    "generated/Exceptions/"
+                ]
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "MIT"
+            ],
+            "description": "PHP core functions that throw exceptions instead of returning FALSE on error",
+            "support": {
+                "issues": "https://github.com/thecodingmachine/safe/issues",
+                "source": "https://github.com/thecodingmachine/safe/tree/v3.3.0"
+            },
+            "funding": [
+                {
+                    "url": "https://github.com/OskarStark",
+                    "type": "github"
+                },
+                {
+                    "url": "https://github.com/shish",
+                    "type": "github"
+                },
+                {
+                    "url": "https://github.com/staabm",
+                    "type": "github"
+                }
+            ],
+            "time": "2025-05-14T06:15:44+00:00"
+        },
         {
             "name": "tijsverkoyen/css-to-inline-styles",
             "version": "v2.3.0",
diff --git a/resources/schemas/v3.json b/resources/schemas/v3.json
index cda80c6e..7e23f513 100644
--- a/resources/schemas/v3.json
+++ b/resources/schemas/v3.json
@@ -26,7 +26,7 @@
         },
         "default_account": {
             "type": "integer",
-            "minimum": 1
+            "minimum": 0
         },
         "delimiter": {
             "type": "string",
@@ -55,14 +55,27 @@
             }
         },
         "do_mapping": {
-            "type": "array",
-            "items":
+            "anyOf": [
                 {
-                    "type": "boolean"
+                    "type": "object"
+                },
+                {
+                    "type": "array",
+                    "items": {
+                        "type": "boolean"
+                    }
                 }
+            ]
         },
         "mapping": {
-            "type": "object"
+            "anyOf": [
+                {
+                    "type": "object"
+                },
+                {
+                    "type": "array"
+                }
+            ]
         },
         "duplicate_detection_method": {
             "type": "string",
@@ -107,7 +120,14 @@
             "type": "boolean"
         },
         "accounts": {
-            "type": "object"
+            "anyOf": [
+                {
+                    "type": "object"
+                },
+                {
+                    "type": "array"
+                }
+            ]
         },
         "date_range": {
             "type": "string"
@@ -131,7 +151,14 @@
             "type": "string"
         },
         "nordigen_requisitions": {
-            "type": "object"
+            "anyOf": [
+                {
+                    "type": "object"
+                },
+                {
+                    "type": "array"
+                }
+            ]
         },
         "conversion": {
             "type": "boolean"