Is this safe for providing JSONP?

Question

<?php header('content-type: application/json');

$json = json_encode($data);

echo isset($_GET['callback'])
    ? "{$_GET['callback']}($json)"
    : $json;

Or should I for example filter the $_GET['callback'] variable so that it only contains a valid JavaScript function name? If so, what are valid JavaScript function names?

Or is not filtering that variable a bit of the point with JSONP?

---

Current solution: Blogged about my current solution at http://www.geekality.net/?p=1021. In short, for now, I have the following code, which hopefully should be pretty safe:

<?php header('content-type: application/json; charset=utf-8');

function is_valid_callback($subject)
{
     $identifier_syntax
       = '/^[$_\p{L}][$_\p{L}\p{Mn}\p{Mc}\p{Nd}\p{Pc}\x{200C}\x{200D}]*+$/u';

     $reserved_words = array('break', 'do', 'instanceof', 'typeof', 'case',
       'else', 'new', 'var', 'catch', 'finally', 'return', 'void', 'continue', 
       'for', 'switch', 'while', 'debugger', 'function', 'this', 'with', 
       'default', 'if', 'throw', 'delete', 'in', 'try', 'class', 'enum', 
       'extends', 'super', 'const', 'export', 'import', 'implements', 'let', 
       'private', 'public', 'yield', 'interface', 'package', 'protected', 
       'static', 'null', 'true', 'false');

     return preg_match($identifier_syntax, $subject)
         && ! in_array(mb_strtolower($subject, 'UTF-8'), $reserved_words);
}

$data = array(1, 2, 3, 4, 5, 6, 7, 8, 9);
$json = json_encode($data);

# JSON if no callback
if( ! isset($_GET['callback']))
     exit( $json );

# JSONP if valid callback
if(is_valid_callback($_GET['callback']))
     exit( "{$_GET['callback']}($json)" );

# Otherwise, bad request
header('Status: 400 Bad Request', true, 400);

Answer 1

No, if you intend to limit the JSONP to select domains. Specify the encoding too or people who shouldn't be able to access the JSON can possibly do UTF-7 injection attacks. Use this header instead:

header('Content-Type: application/json; charset=utf-8');

If it's supposed to be a public JSONP service, then yes it is safe, and also use application/javascript instead of application/json.