Admidio is an open-source user management solution. Prior to version 5.0.9, the add mode in modules/documents-files.php accepts a name parameter validated only as 'string' type (HTML encoding), allowing path traversal characters (../) to pass through unfiltered. Combined with the absence of CSRF protection on this endpoint and SameSite=Lax session cookies, a low-privileged attacker can trick a documents administrator into clicking a crafted link that registers an arbitrary server file (e.g., install/config.php containing database credentials) into a documents folder accessible to the attacker. This issue has been patched in version 5.0.9.

Affected: Packagist: admidio/admidio

## Summary

The `add` mode in `modules/documents-files.php` accepts a `name` parameter validated only as `'string'` type (HTML encoding), allowing path traversal characters (`../`) to pass through unfiltered. Combined with the absence of CSRF protection on this endpoint and `SameSite=Lax` session cookies, a low-privileged attacker can trick a documents administrator into clicking a crafted link that registers an arbitrary server file (e.g., `install/config.php` containing database credentials) into a documents folder accessible to the attacker.

## Details

**Root cause — incorrect input validation type (modules/documents-files.php:222):**

```php case 'add': $getName = admFuncVariableIsValid($_GET, 'name', 'string'); ```

The `'string'` type in `admFuncVariableIsValid()` only applies `SecurityUtils::encodeHTML(StringUtils::strStripTags($value))` (system/bootstrap/function.php:414-416). Since `../` contains no HTML special characters (`<`, `>`, `&`, `"`, `'`), path traversal sequences pass through unchanged.

The correct type would be `'file'`, which calls `StringUtils::strIsValidFileName()` (src/Infrastructure/Utils/StringUtils.php:217-236). This function checks `basename($filename) !== $filename` at line 228, which would reject any path containing directory separators.

**Missing CSRF protection (modules/documents-files.php:221-238):**

```php case 'add': $getName = admFuncVariableIsValid($_GET, 'name', 'string');

if (!$gCurrentUser->isAdministratorDocumentsFi…