How do I send a server postback safely with PHP?

Question

I'm building an application that will send an http request to a url (I hope..) provided by a user.

Probably most of you will know this as a postback, callback or webhook.

However, I'm concerned about security, because the other server will send a response. That response might contain code or who knows what.

I've considered the following functions so far:

• Curl()
• file_get_contents()

What is the most secure way of doing this, without opening up a security vulnerability?

Answer 1

You doesn't have a security problem in any case if you don't process the response of the server.

For example, when you use:

$url = 'http://www.example.com/testaddr';
$result = file_get_contents($url);
unset($result);

You have a variable with the data. But these data aren't processed yet.

With cURL, you can get the same approach with these options:

$url = 'http://www.example.com/testaddr';
$curl = curl_init();                
curl_setopt ($curl, CURLOPT_URL, $url);
curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);

$result = curl_exec($curl);

//If you need to check result, use this:
if (!curl_errno($curl)) {
  $http_code = curl_getinfo($curl, CURLINFO_HTTP_CODE);
  if ($http_code === 200) {
    echo "OK";
  } else {
    echo 'Unexpected HTTP code: ', $http_code, "\n";
  }
}
curl_close($curl);
unset($result);

In that case it's the same, you get the response on $result var, but, you didn't use it, in that case, it isn't a security failure.

Also, in both cases, for security reasons and prevent excessive memory usage, I delete the $result variable after finish the process.

As you can see on PHP doc:

CURLOPT_RETURNTRANSFER TRUE to return the transfer as a string of the return value of curl_exec() instead of outputting it out directly.