Refused to execute a JavaScript script. Source code of script found within request

In WebKit I get the following error on my JavaScript:

Refused to execute a JavaScript script. The source code of script found within request.

The code is for a JavaScript spinner, see ASCII Art.

The code used to work OK and is still working correctly in Camino and Firefox. The error only seems to be thrown when the page is saved via a POST and then retrieved via a GET. It happens in both Chrome/Mac and Safari/Mac.

Anyone know what this means, and how to fix this?

Answers:

Answer

It's a security measure to prevent XSS (cross-site scripting) attacks.

This happens when some JavaScript code is sent to the server via an HTTP POST request, and the same code comes back via the HTTP response. If Chrome detects this situation, the script is refused to run, and you get the error message Refused to execute a JavaScript script. Source code of script found within request.

Also see this blogpost about Security in Depth: New Security Features.

Answer

This "feature" can be disabled by sending the non-standard HTTP header X-XSS-Protection on the affected page.

X-XSS-Protection: 0
Answer

I used this hacky PHP trick just after I commit to database, but before the script is rendered from my _GET request.:

if(!empty($_POST['contains_script'])) { 
    echo "<script>document.location='template.php';</script>";
}

This was the cheapest solution for me.

Answer

Short answer: refresh the page after making your initial submission of the javascript, or hit the URL that will display the page you're editing.

Long answer: because the text you filled into the form includes javascript, and the browser doesn't necessarily know that you are the source of the javascript, it is safer for the browser to assume that you are not the source of this JS, and not run it.

An example: Suppose I gave you a link your email or facebook with some javascript in it. And imagine that the javascript would message all your friends my cool link. So, the game of getting that link to be invoked becomes simply, find a place to send the javascript such that it will be included in the page.

Chrome and other WebKit browsers try to mitigate this risk by not executing any javascript that is in the response, if it was present in the request. My nefarious attack would be thwarted because your browser would never run that JS.

In your case, you're submitting it into a form field. The Post of the form field will cause a render of the page that will display the Javascript, causing the browser to worry. If your javascript is truly saved, however, hitting that same page without submitting the form will allow it to execute.

Answer

As others have said, this happens when an HTTP response contains a JavaScript and/or HTML string that was also in the request. This is usually caused by entering JS or HTML into a form field, but can also be triggered in other ways such as manually tweaking the URL's parameters.

The problem with this is that someone with bad intentions could put whatever JS they want as the value, link to that URL with the malicious JS value, and cause your users trouble.

In almost every case, this can be fixed by HTML encoding the response, though there are exceptions. For example, this will not be safe for content inside a <script> tag. Other specific cases can be handled differently - for example, injecting input into a URL is better served by URL encoding.

As Kendall Hopkins mentioned, there may be a few cases when you actually want JavaScript from form inputs to be executed, such as creating an application like JSFiddle. In those cases, I'd recommend that you you at least scrub through the input in your backend code before blindly writing it back. After that, you can use the method he mentioned to prevent the XSS blockage (at least in Chrome), but be aware that it is opening you to attackers.

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