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:

// 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():

// 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:

// 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.