mitigations.md

Mitigation Strategies Analysis for spartnernl/laravel-excel

Mitigation Strategy: Strict Input Validation and Sanitization (with Laravel Excel Specifics)

Description:

1. Identify Input Sources: Determine all points where user-supplied data is passed to Laravel Excel for spreadsheet generation (e.g., form fields, API parameters, data used in export classes).
2. Define Allowed Data (Whitelist): For each input field that will be used in a cell, define precisely what is allowed (characters, data types, length, regex).
3. Laravel Validation: Use Laravel's validation rules (Validator, Form Requests) to enforce these rules before passing data to Laravel Excel.
4. Sanitize for Formula Injection: Create a helper function (or use an existing one) to sanitize cell values specifically for formula injection. This function should:
   • Check if the value is a string.
   • Check if the value starts with =, +, -, @, \t, or \r.
   • If it does, prepend a single quote (') to the value.

   function sanitizeCellValue($value) {
       if (is_string($value) && in_array(substr($value, 0, 1), ['=', '+', '-', '@', "\t", "\r"])) {
           return "'" . $value;
       }
       return $value;
   }

5. Apply Sanitization: Consistently apply this sanitizeCellValue function (or equivalent) to all user-supplied data before it is passed to any Laravel Excel method that sets cell values (e.g., setCellValue, fromArray, fromCollection). This is the critical step for preventing formula injection within the context of Laravel Excel.
6. setCellValueExplicit(): Use $sheet->setCellValueExplicit($cell, $sanitizedValue, DataType::TYPE_STRING); instead of setCellValue(). This forces the value to be treated as a string, preventing Excel from interpreting it as a formula.
7. Test with Malicious Payloads: Specifically test your export functionality with known formula injection payloads to ensure your sanitization is effective.

• Threats Mitigated:

  • Formula Injection (CSV Injection): High Severity. Prevents attackers from injecting malicious formulas.
  • Command Injection (Indirect): Medium Severity. Reduces the risk if the generated file is processed by another vulnerable system.
  • Data Validation Errors: Low Severity. Ensures data integrity.

• Impact:

  • Formula Injection: Risk significantly reduced (High to Low/Negligible).
  • Command Injection: Risk reduced (Medium to Low).
  • Data Validation Errors: Risk eliminated (Low to Negligible).

• Currently Implemented:

  • Some basic Laravel validation exists.
  • setCellValueExplicit is used in some places.

• Missing Implementation:

  • Comprehensive whitelisting and regex validation are incomplete.
  • The sanitizeCellValue function (or equivalent) is not consistently applied.
  • setCellValueExplicit is not used universally.
  • Targeted testing for formula injection is lacking.

Mitigation Strategy: Limit Spreadsheet Size and Data Input (within Export Classes)

Description:

1. Determine Limits: Based on server resources, define maximum row and column limits for each export class.
2. Implement Limits in Export Classes: Within your app/Exports classes, enforce these limits. For example:

   // app/Exports/MyExport.php
   public function collection()
   {
       $maxRows = 1000; // Example: Limit to 1000 rows
       return Data::query()->take($maxRows)->get();
   }
   
   public function headings(): array
   {
       $maxColumns = 20; // Example: Limit to 20 columns
       $headings = ['Column 1', 'Column 2', /* ... */];
       return array_slice($headings, 0, $maxColumns);
   }

3. Limit Query Results: If data comes from a database, always use ->take($maxRows) or pagination to limit the retrieved data within the export class.
4. Validate Input for Size Control: If user input can influence the size or scope of the exported data (e.g., a date range, a filter), validate these input parameters strictly using Laravel's validation rules before they are used in the export class.
5. Queue Large Exports: For any export that might exceed the defined limits or take a significant amount of time, use Laravel's queue system to process the generation asynchronously. This prevents blocking the web server. This involves creating a job class and dispatching it.

• Threats Mitigated:

  • Denial of Service (DoS) via Resource Exhaustion: High Severity. Prevents attackers from causing DoS by requesting excessively large spreadsheets.

• Impact:

  • DoS: Risk significantly reduced (High to Low/Negligible).

• Currently Implemented:

  • Some ->take() limits are used in some export classes.
  • A queue system exists, but not all large exports are queued.

• Missing Implementation:

  • Consistent and strict row/column limits are not enforced across all export classes.
  • Not all potentially long-running exports are dispatched to the queue.
  • Input validation for parameters controlling export size is not comprehensive.

Mitigation Strategy: Secure Temporary File Handling (with Laravel Excel Focus)

Description:

1. Explicit unlink(): While Laravel Excel should handle temporary file cleanup, add explicit unlink() calls immediately after the file has been downloaded or processed. This is a defense-in-depth measure within your Laravel code.

   $filePath = Excel::download(new MyExport, 'report.xlsx')->getFile()->getPathname();
   // ... (send the file to the user, e.g., using a response) ...
   unlink($filePath); // Explicitly delete the temporary file

   Place this code immediately after the Excel::download or Excel::store call, ensuring the file is no longer needed.
2. Review Data Written: Carefully review your export classes to identify any sensitive data that might be written to the temporary file. If possible, refactor the code to avoid writing sensitive data directly. If unavoidable, consider encrypting the data before writing it to the temporary file and decrypting it after reading. This is a more advanced technique and requires careful key management.

• Threats Mitigated:

  • Unauthorized Access to Temporary Files: Medium Severity.
  • Information Disclosure: Medium Severity.
  • Temporary File Tampering: Low Severity.

• Impact:

  • Unauthorized Access: Risk reduced (Medium to Low).
  • Information Disclosure: Risk reduced (Medium to Low).
  • Tampering: Risk reduced (Low to Negligible).

• Currently Implemented:

  • Laravel Excel's default behavior likely handles unique file names and attempts cleanup.

• Missing Implementation:

  • Explicit unlink() calls are not consistently used after file download/processing within the Laravel application code.
  • A review of whether sensitive data is unnecessarily written to temporary files is needed.