Subresource Integrity: How to show only warning but not block resource?
Secure approach
If you need some kind of flexibility, then you should use a fallback mechanism - loading required resource from another URL. Probability that two different URL's will be hacked at the same time is a lot smaller compared to hacking just one resource. Fallback doesn't violate site security, because you must trust your known-good sources which you use in your code. If your resource is a Javascript - you can use a noncanonical-src attribute for a fallback too.
Insecure approach
Now, if you really, really want a user to break server and/or client security by forcing compromised resource load - at least ask a user if he/she takes responsibility by doing so. Of course this will still be a stupid thing, it's like asking "Would you like to run a virus in your computer ?". I bet nobody would like to say YES. Anyway, here is the code, which does asking these type of questions:
<script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/crypto-js/3.1.9-1/crypto-js.min.js"></script>
<script>
function loadResource(path) {
var xhttp = new XMLHttpRequest();
xhttp.onreadystatechange = function() {
if (this.readyState == 4 && this.status == 200) {
var cs = CryptoJS.SHA256(this.responseText);
if (btoa(cs) == 'NjBiMTllNWRhNmE5MjM0ZmY5MjIwNjY4YTVlYzExMjVjMTU3YTI2ODUxMzI1NjE4OGVlODBmMmQyYzhkOGQzNg==' ||
confirm('Bootstrap is NOT the latest version 4.3.1, load anyway ?')
) {
var link = document.createElement('link');
link.rel = "stylesheet";
link.href = path;
document.head.appendChild(link);
}
else {
var err = document.getElementById('error');
err.title = "Component version error !";
err.innerHTML = ' ⚠️';
}
}
};
xhttp.open("GET", path, true);
xhttp.send();
}
loadResource(
//'https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css' // newest boostrap
'https://stackpath.bootstrapcdn.com/twitter-bootstrap/2.0.4/css/bootstrap-combined.min.css' // old legacy
);
</script>
DEMO