Content-Security-Policy in ASP.NET WebForms

Question

I'm looking for a good way to implement a relatively strong Content-Security-Policy header for my ASP.NET WebForms application. I'm storing as much JavaScript as possible in files instead of inline, but by default, WebForms injects a lot of inline scripts—for things as simple as form submission and basic AJAX calls.

MVC has some simple ways to implement nonces, especially with the help of third party libraries like NWebsec, but I can't seem to find any methods of implementing them with WebForms. I wouldn't even have a problem using hashes if there were a way to predict and retrieve the hash for each .NET injected script tag.

I hate allowing the 'unsafe-inline' value. It feels wrong needing to turn off such a powerful security feature. Is there a reasonable way to implement it in WebForms?

Answer 1

I had the same problem. I'm sad to say this was the best we have done. We basically identified what we use and don't use. We even had to put unsafe-eval in some instructions because we were using third party controls that couldn't work without it. At least we avoid calls to external urls.

default-src 'self'; 
child-src 'self' 'unsafe-inline' 'unsafe-eval'; 
object-src 'none'; 
script-src 'self' 'unsafe-inline' 'unsafe-eval' https://www.google-analytics.com; 
img-src 'self' https://www.google-analytics.com; 
style-src 'self' 'unsafe-inline'

Answer 2

I have the same answer here re: what to do about all those injected scripts:

If you open up the dev tools in Chrome, you'll likely see a message like Refused to execute inline script because it violates the following Content Security Policy directive: "script-src 'self'. Either the 'unsafe-inline' keyword, a hash ('sha256-2NqnatcPqy5jjBXalTpZyJMO/0fUaYUb3ePlviUP4II='), or a nonce ('nonce-...') is required to enable inline execution.

If you look carefully at that message, it's telling you what the hash would be: sha256-2NqnatcPqy5jjBXalTpZyJMO/0fUaYUb3ePlviUP4II=

So if you don't want to go the nonce route, you can instead go the hash route and add

Content-Security-Policy: script-src 'self' 'sha256-2NqnatcPqy5jjBXalTpZyJMO/0fUaYUb3ePlviUP4II=' 'unsafe-eval';

You may have to add unsafe-eval in some cases as well for this to work.