Restricting JavaScript from accessing secrets #536

craigfrancis · 2017-11-14T12:50:59Z

All modern browsers block JavaScript from accessing httpOnly cookies; and Chrome blocks JavaScript from accessing the nonce attribute in <script nonce="abc123"> (details).

Would it be possible for websites to also block (potentially malicious) JavaScript from accessing other values from the DOM?

For example, I'd like to stop any JavaScript/CSS from accessing the CSRF token from a hidden input field (used in addition to SameSite cookies).

Two features I'd find useful would include:

An attribute on the <input> so it's value (or the element) can't be accessed via the DOM, or via CSS selectors.
A header to block XMLHttpRequest (or fetch) from accessing the content from a same-site request.

The first might be possible to introduce via the inert attribute, which is currently being specced as a simple flag to stop interaction events, text search, selecting text, and hiding from assistive technology.

The second could be as simple as recognising Access-Control-Allow-Origin: 'none' to stop all origins from being allowed to read the contents of the response (effectively causing same-site requests to be treated like a cross-origin request).

In Google Chrome, this example shows how the <script nonce="abc123"> can't be accessed, whereas the hidden input field named "csrf" can be.

Content-Security-Policy:
    default-src 'none';
    script-src 'nonce-123js';
    style-src 'nonce-123css';
    connect-src 'self';

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8" />
    <title>Sensitive Page</title>
</head>
<body>

    <form action="./?id=5" method="post">
        <input type="hidden" name="csrf" value="123csrf" />
        <input type="submit" value="Delete" />
    </form>

    <script nonce="123js">

        var script = document.getElementsByTagName('script')[0],
            input = document.getElementsByTagName('input')[0];

        console.log(script.getAttribute('nonce'));
        console.log(script.outerHTML); // Returns <script nonce="">...
        console.log(script.nonce); // Works, but not needed for this case.
        console.log(input.getAttribute('value'));

        fetch(window.location).then(function(response) {
            return response.text();
        }).then(function(text) {
            console.log(text);
        });

    </script>

    <style nonce="123css">
        input[value^="123"] + input {
            border: 1px solid red;
        }
    </style>

</body>
</html>

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mikewest · 2017-11-15T09:34:10Z

This sounds somewhat similar to https://mikewest.github.io/credentialmanagement/writeonly/, which basically no one but me was interested in. :)

I don't think it's a terrible idea, but it needs a lot more work to be a robust defense. Some of that work we've done for nonces, so reusing it wouldn't be terrible. shrug

craigfrancis · 2017-11-15T10:21:49Z

That would be ideal for this case as well - while I was thinking of read and write protection, if some JS decides to replace the CSRF value (writeonly), it would simply break the validation (as it should).

I suspect most developers weren’t interested because they use JS for everything, but there will be some websites (e.g. banking) where this type of protection would be very useful.

Any thoughts on the second part? Where a response from the server (typically HTML) could include a header to block JS from reading the content.

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Restricting JavaScript from accessing secrets #536

Restricting JavaScript from accessing secrets #536

craigfrancis commented Nov 14, 2017 •

edited

mikewest commented Nov 15, 2017

craigfrancis commented Nov 15, 2017

Restricting JavaScript from accessing secrets #536

Restricting JavaScript from accessing secrets #536

Comments

craigfrancis commented Nov 14, 2017 • edited

mikewest commented Nov 15, 2017

craigfrancis commented Nov 15, 2017

craigfrancis commented Nov 14, 2017 •

edited