SafeURL for PHP

Originally contributed by Jack Whitton @fin1te

Overview

SafeURL is a library that aids developers in protecting against a class of vulnerabilities known as Server Side Request Forgery. It does this by validating each part of the URL against a configurable white or black list before making an HTTP request. SafeURL is open-source and licensed under MIT.

Note that for mitigating SSRF vulnerabilities, we first recommend routing outbound requests from your infrastructure through a proxy such as Smokescreen. Alternately, ensure that all services which can make outbound requests to potentially user-controlled URLs are firewalled from talking to other internal hosts. Application-layer defences such as this library should only be used if those options are not practical. Please see our blog post for further information.

Installation

Clone this repository and import it into your project.

Implementation

SafeURL intends to be a wrapper replacement for the curl_exec() method from libcurl in PHP. It can simply be replaced with SafeURL::execute() wrapped in a try {} catch {} block.

  try {
      $url = $_GET['url']; // User controlled input

      $curlHandle = curl_init();
      //Your usual cURL options
      curl_setopt($ch, CURLOPT_USERAGENT, 'Mozilla/5.0 (SafeURL)');

      //Execute using SafeURL
      $response = SafeURL::execute($url, $curlHandle);
  } catch (Exception $e) {
      //URL wasn't safe
  }

Configuration

Options such as white and black lists can be modified. For example:

//Create an Options object
$options = new Options();
//Works with regex
$options->addToList('blacklist', 'domain', '([\w\.\-]+\.)?evil.com');
$options->addToList('whitelist', 'scheme', 'ftp');

//This will now throw an InvalidDomainException
$response = SafeURL::execute('http://evil.com', $curlHandle, $options);

//Whilst this will be allowed, and return the response
$response = SafeURL::execute('ftp://example.com', $curlHandle, $options);