hash with CSP
hash of a script or style can be used to allow it .
Suppose we have the following script on our page:
doSomething(); you will get the value:
Finally we can add the hash to our script-src directive to allow it to execute via our Content-Security-Policy header:
The CSP Level 2 specification allows
The easiest way to generate it is to just open the developer tools console and it will output what the expected hash of your script was in the console error message.
You can also use tools such as openssl to generate it, whitespace is not ignored.
Here is an example using openssl, which will be installed by default on most mac or linux systems:
echo -n 'doSomething();' | openssl sha256 -binary | openssl base64
You can use a CSP hash on external scripts or stylesheets to allow them to execute. For example if we have a CSP policy similar to the following:
Content-Security-Policy: default-src 'none';script-src 'sha256-/JqT3SQfawRcv/BIHPThkBvs0OEvtFFmqPF/lYI/Cxo='
Now we can load jquery-3.7.1.min.js based on it's hash alone, without adding code.jquery.com to the CSP policy.
<script src="https://code.jquery.com/jquery-3.7.1.min.js" integrity="sha256-/JqT3SQfawRcv/BIHPThkBvs0OEvtFFmqPF/lYI/Cxo=" crossorigin="anonymous"></script>
Note that the above example is also using subresource integrity or SRI to enforce the hash. You might find it duplicating to have it in both places, but it can serve as a good way to document which hash is for which script.
On the topic of SRI, it is worth noting that CSP did implement a directive called
require-sri-for, which could be used to enforce SRI for script tags like this:
require-sri-for script. This feature was experimental and was removed for a few different reasons. One of the reasons it was not as useful as it sounds is that it also required CORS access in order to verify the the integrity from the policy. The
require-sri-for directive was implemented in Chrome and Firefox around 2017-2019
There are a three common reasons your CSP hash might not be working:
script-src sha256-abc123; you need to wrap it in single quotes, for example:
hash source list directive was added to CSP Level 2. This means that support has existed since 2015 in Chrome and Firefox, Safari 10+ or Edge 15+.
It is not supported at all in Internet Explorer, you need to use the Edge browser instead.
Advisory Week is a weekly roundup of all the security advisories published by the major software vendors.