re: What is CSP? Why & How to Add it to Your Website. VIEW POST

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re: CSP fan here :) Some additional notes: Shameless plug to a library that'll help with CSP and other security headers if you use PHP :) SecureHeade...
 

Thanks for reading & taking time to give an excellent reply. Those are all excellent points & I hope people give them a try after getting a basic CSP setup & working. I personally did not know about using two CSPs. I will for sure give that a test.

We need a lot more a lot more CSP fans so fellow developers & frameworks start coding with CSP & security in mind. 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.

 

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" :)

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.

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