Add additional security considerations

README.md

@@ -282,7 +282,7 @@ MultiCurl::verbose($on = true, $output = STDERR)
 
 ### Security
 
-See [SECURITY.md](https://github.com/php-curl-class/php-curl-class/blob/master/SECURITY.md).
+See [SECURITY](https://github.com/php-curl-class/php-curl-class/blob/master/SECURITY.md) for security considerations.
 
 ### Contribute
 1. Check for open issues or open a new issue to start a discussion around a bug or feature.

SECURITY.md

@@ -1,11 +1,66 @@
-### Security
+### Security Considerations
 
-* Don't blindly accept arbitrary urls. Curl supports many protocols including `FILE`. The following would show the contents of `file:///etc/passwd`.
+* Url may point to system files. Don't blindly accept arbitrary urls from users. Curl supports many protocols including
+  `FILE`. The following would show the contents of `file:///etc/passwd`.
+
+```bash
+# Attacker.
+$ curl https://www.example.com/display_webpage.php?url=file%3A%2F%2F%2Fetc%2Fpasswd
+``
 
 ```php
-// https://www.example.com/fetch_page.php?url=file%3A%2F%2F%2Fetc%2Fpasswd
-$unsafe_url = $_GET['url']; // DANGER!
+// display_webpage.php
+$url = $_GET['url']; // DANGER!
 $curl = new Curl();
-$curl->get($unsafe_url);
+$curl->get($url);
 echo $curl->response;
 ```
+
+Safer:
+
+```php
+function is_website_url($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 behind firewall (e.g. http://192.168.0.1/ or ftp://192.168.0.1/). Use a whitelist to
+  allow certain urls. Definitely don't use a blacklist.
+
+* Request data prefixed with the @ character may have special interpretation and read from system files.
+
+```bash
+# Attacker.
+$ curl https://www.example.com/upload_photo.php --data "photo=@/etc/password"
+``
+
+```php
+// upload_photo.php
+$curl = new Curl();
+$curl->post('http://www.anotherwebsite.com/', array(
+    'photo' => $_POST['photo'], // DANGER!
+));
+```
+
+* Requests with redirection enabled may return responses from unexpected sources.
+  Downloading https://www.example.com/image.png may redirect and download https://www.evil.com/virus.exe
+
+```php
+$curl = new Curl();
+$curl->setOpt(CURLOPT_FOLLOWLOCATION, true); // DANGER!
+$curl->download('http://www.example.com/image.png', 'my_image.png');
+```
+
+* Do not disable SSL protections.
+
+```php
+curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, FALSE); // DANGER!
+curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, FALSE); // DANGER!
+```