Prevent formula injection in spreadsheet exports

Answer/candidate text starting with =, +, -, or @ was written to XLSX
exports as a live formula rather than plain text. Since seasons can
have multiple owners, a co-owner could plant a formula that runs (and
can exfiltrate data) when another owner opens the export in Excel.

src/Helpers/FormulaInjectionSafeValueBinder.php

@@ -0,0 +1,31 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tvdt\Helpers;
+
+use PhpOffice\PhpSpreadsheet\Cell\Cell;
+use PhpOffice\PhpSpreadsheet\Cell\DefaultValueBinder;
+
+/**
+ * Stores any string that would otherwise be auto-detected as a spreadsheet formula (or that
+ * spreadsheet software re-interprets as one on open/paste) as plain text instead, to prevent
+ * formula injection via user-controlled export data (e.g. a candidate or answer named
+ * `=WEBSERVICE(...)`).
+ */
+final class FormulaInjectionSafeValueBinder extends DefaultValueBinder
+{
+    private const string DANGEROUS_PREFIXES = "=+-@\t\r";
+
+    #[\Override]
+    public function bindValue(Cell $cell, mixed $value): bool
+    {
+        if (\is_string($value) && '' !== $value && str_contains(self::DANGEROUS_PREFIXES, $value[0])) {
+            $cell->setValueExplicit($value);
+
+            return true;
+        }
+
+        return parent::bindValue($cell, $value);
+    }
+}

src/Service/DataExportService.php

@@ -5,6 +5,7 @@ declare(strict_types=1);
 namespace Tvdt\Service;
 
 use Doctrine\ORM\EntityManagerInterface;
+use PhpOffice\PhpSpreadsheet\Cell\Cell;
 use PhpOffice\PhpSpreadsheet\Cell\Coordinate;
 use PhpOffice\PhpSpreadsheet\Spreadsheet;
 use PhpOffice\PhpSpreadsheet\Worksheet\Worksheet;
@@ -19,6 +20,7 @@ use Tvdt\Entity\Quiz;
 use Tvdt\Entity\Season;
 use Tvdt\Entity\User;
 use Tvdt\Helpers\FilenameSanitizer;
+use Tvdt\Helpers\FormulaInjectionSafeValueBinder;
 use Tvdt\Repository\QuizRepository;
 
 use function Safe\tempnam;
@@ -31,7 +33,9 @@ class DataExportService
         private readonly EntityManagerInterface $entityManager,
         private readonly QuizSpreadsheetService $quizSpreadsheetService,
         private readonly QuizRepository $quizRepository,
-    ) {}
+    ) {
+        Cell::setValueBinder(new FormulaInjectionSafeValueBinder());
+    }
 
     /** @throws FilesystemException @return string path to a temp zip file; caller is responsible for removing it */
     public function exportForUser(User $user): string

src/Service/QuizSpreadsheetService.php

@@ -4,6 +4,7 @@ declare(strict_types=1);
 
 namespace Tvdt\Service;
 
+use PhpOffice\PhpSpreadsheet\Cell\Cell;
 use PhpOffice\PhpSpreadsheet\Cell\Coordinate;
 use PhpOffice\PhpSpreadsheet\Reader;
 use PhpOffice\PhpSpreadsheet\Spreadsheet;
@@ -14,9 +15,15 @@ use Tvdt\Entity\Answer;
 use Tvdt\Entity\Question;
 use Tvdt\Entity\Quiz;
 use Tvdt\Exception\SpreadsheetDataException;
+use Tvdt\Helpers\FormulaInjectionSafeValueBinder;
 
 class QuizSpreadsheetService
 {
+    public function __construct()
+    {
+        Cell::setValueBinder(new FormulaInjectionSafeValueBinder());
+    }
+
     public function generateTemplate(bool $fillExample = true): \Closure
     {
         $quiz = new Quiz();