SECURITY.md

Security Considerations

Url may point to system files

• Don't blindly accept urls from users as they may point to system files. Curl supports many protocols including FILE. The following would show the contents of file:///etc/passwd.

# Attacker.
$ curl https://www.example.com/display_webpage.php?url=file%3A%2F%2F%2Fetc%2Fpasswd

// display_webpage.php
$url = $_GET['url']; // DANGER!
$curl = new Curl();
$curl->get($url);
echo $curl->response;

Safer:

function is_website_url($url, $allowed_schemes = array('http', 'https')) {
    $validate_url = !(filter_var($url, FILTER_VALIDATE_URL) === false);
    $scheme = parse_url($url, PHP_URL_SCHEME);
    return $validate_url && in_array($scheme, $allowed_schemes, true);
}

$url = $_GET['url'];
if (!is_website_url($url)) {
    die('Unsafe url detected.');
}

Url may point to internal urls

• Url may point to internal urls including those behind a firewall (e.g. http://192.168.0.1/ or ftp://192.168.0.1/). Use a whitelist to allow certain urls rather than a blacklist.

Request data may refer to system files

• Request data prefixed with the @ character may have special interpretation and read from system files.

# Attacker.
$ curl https://www.example.com/upload_photo.php --data "photo=@/etc/passwd"

// upload_photo.php
$curl = new Curl();
$curl->post('http://www.anotherwebsite.com/', array(
    'photo' => $_POST['photo'], // DANGER!
));

Unsafe response with redirection enabled

• Requests with redirection enabled may return responses from unexpected sources. Downloading https://www.example.com/image.png may redirect and download https://www.evil.com/virus.exe

$curl = new Curl();
$curl->setOpt(CURLOPT_FOLLOWLOCATION, true); // DANGER!
$curl->download('https://www.example.com/image.png', 'my_image.png');

$curl = new Curl();
$curl->setOpt(CURLOPT_FOLLOWLOCATION, true); // DANGER!
$curl->get('https://www.example.com/image.png');

Keep SSL protections enabled

• Do not disable SSL protections.

curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, false); // DANGER!
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false); // DANGER!