DEV Community


Discussion on: What is CSP? Why & How to Add it to Your Website.

aidantwoods profile image
Aidan Woods

The biggest hurdle I am currently finding is getting CSP to work with large open source frameworks that change JavaScript on build or inject JavaScript & CSS styles.

For scripts, can 'strict-dynamic' help? It'll permit even non-external scripts being added into the document by nonced/hashed scripts so-long as they are not "parser inserted". e.g. a nonced script would be permitted to insert a script into the DOM via something like document.head.appendChild (but not via document.write).

For styles (and perhaps some particular scripts) 'unsafe-hashed-attributes' in scripts and style may be worth looking into (once it's finished). The idea is to allow things like:

<div style="color:red" onclick="foobar()"></div>

to be compatible with CSP (provided you know ahead of time what the attribute will be). I believe the current proposal is to hash the content of the attribute, so something like <img onerror="foobar()"> would have the same script hash as above (even though the attribute and element is different). For this reason it'll be possible to abuse these in certain situations e.g. consider if the following were legitimate code on the page, whitelisted by attribute

<a onclick="deleteAccount()">Delete account</a>

An attacker could then inject

<img src=# onerror="deleteAccount()" />

and have it execute on pageload.

That said, having to "be careful" with 'unsafe-hashed-attributes' is certainly a preferable approach to 'unsafe-inline', which essentially says "run all the things" :)

Thread Thread
mattferderer profile image
Matt Ferderer Author

Using strict-dynamic is an excellent choice when possible. It is something I should investigate closer. A lot of my front ends are static sites, so that brings some challenges there.