prevent xss attack via url( PHP) - php

Question

I am trying to avoid XSS attack via url
url :http://example.com/onlineArcNew/html/terms_conditions_1.php/%22ns=%22alert%280x0000DC%29
I have tried
var_dump(filter_var('http://10.0.4.2/onlineArcNew/html/terms_conditions_1.php/%22ns=%22alert%280x0000DC%29', FILTER_VALIDATE_URL));
and other url_validation using regex but not worked at all.
above link shows all the information but my css and some java script function doesn't work.
please suggest the best possible solution...

Answer 1

Try using FILTER_SANITIZE_SPECIAL_CHARS Instead
$url = 'http://10.0.4.2/onlineArcNew/html/terms_conditions_1.php/%22ns=%22alert%280x0000DC%29';
// Original
echo $url, PHP_EOL;
// Sanitise
echo sanitiseURL($url), PHP_EOL;
// Satitise + URL encode
echo sanitiseURL($url, true), PHP_EOL;
Output
http://10.0.4.2/onlineArcNew/html/terms_conditions_1.php/%22ns=%22alert%280x0000DC%29
http://10.0.4.2/onlineArcNew/html/terms_conditions_1.php/"ns="alert(0x0000DC)
http%3A%2F%2F10.0.4.2%2FonlineArcNew%2Fhtml%2Fterms_conditions_1.php%2F%26%2334%3Bns%3D%26%2334%3Balert%280x0000DC%29
Function Used
function sanitiseURL($url, $encode = false) {
$url = filter_var(urldecode($url), FILTER_SANITIZE_SPECIAL_CHARS);
if (! filter_var($url, FILTER_VALIDATE_URL))
return false;
return $encode ? urlencode($url) : $url;
}

Answer 2

If you're using MVC, then try to decode all ofthe values before routing, and use stript_tags() to get rid of these nasties. And as the docs say, case should not impact anything.
If not, create a utility function and do the same while retrieveing the variables from the URI. But I am by no means an XSS expert, so this might be just a part of the trick.
From Janis Peisenieks

Answer 3

Step 1: Escape Output Provided by Users
If you want to include data within a page that’s been provided by users, escape the output. And, in this simplified list, we’re going to stick with one simple escape operation: HTML encode any <, >, &, ‘, “. For example, PHP provides the htmlspecialchars() function to accomplish this common task.
Step 2: Always Use XHTML
Read through OWASP’s XSS prevention strategies, and it becomes apparent that protecting against injection requires much more effort if you use unquoted attributes in your HTML. In contrast, in quoted attributes, escaping data becomes the same process needed to escape data for content within tags, the escape operation we already outlined above. That’s because the only troublemaker in terms of sneaking in structurally significant content within the context of a quoted attribute is the closing quote.
Obviously, your markup doesn’t have to be XHTML in order to contain quoted attributes. However, shooting for and validating against XHTML makes it easy to test if all of the attributes are quoted.
Step 3: Only Allow Alphanumeric Data Values in CSS and JavaScript
We need to limit the data you allow from users that will be output within CSS and Javascript sections of the page to alphanumeric (e.g., a regex like [a-zA-Z0-9]+) types, and make sure they are used in a context in which they truly represent values. In Javascript this means user data should only be output within quoted strings assigned to variables (e.g., var userId = “ALPHANUMERIC_USER_ID_HERE”;.) In CSS this means that user data should only be output within the context for a property value (e.g., p { color: #ALPHANUMERIC_USER_COLOR_HERE;}.) This might seem Draconian, but, hey, this is supposed to be a simple XSS tutorial
Now, to be clear, you should always validate user data to make sure it meets your expectations, even for data that’s output within tags or attributes, as in the earlier examples. However, it’s especially important for CSS and JavaScript regions, as the complexity of the possible data structures makes it exceedingly difficult to prevent XSS attacks.
Common data you might want users to be able supply to your JavaScript such as Facebook, Youtube, and Twitter ID’s can all be used whilst accommodating this restriction. And, CSS color attributes and other styles can be integrated, too.
Step 4: URL-Encode URL Query String Parameters
If user data is output within a URL parameter of a link query string, make sure to URL-encode the data. Again, using PHP as example, you can simply use the urlencode() function. Now, let’s be clear on this and work through a couple examples, as I’ve seen much confusion concerning this particular point.
Must URL-encode
The following example outputs user data that must be URL-encoded because it is used as a value in the query string.
http://site.com?id=USER_DATA_HERE_MUST_BE_URL_ENCODED”>
Must Not URL-Encode
The following example outputs the user-supplied data for the entire URL. In this case, the user data should be escaped with the standard escape function (HTML encode any <, >, &, ‘, “), not URL-encoded. URL-encoding this example would lead to malformed links.