XSS using JSF**k
This works for me:
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See the fiddle http://jsfiddle.net/34jz07ae/
Taken straight from http://jscrew.it/
The JSF*** code should work too at least in Chrome if you choose the right parameters. Note that you need to generate executable code with Eval Source, not just a string.
An obvious scenario, as @schroeder pointed out in one of the comments, is when the server inspects strings for blacklisted strings like alert(1). Apart from that, in order for the payload to work, the application must meet the following conditions:
- There should be no restriction on the input length (as you can see, the payload can be enormous length).
- The application must not JavaScript encode any of these characters: ! ( ) + [ ].
- The payload must be evaluated (unless the user input is being directly fed into an
eval()function). The reason why you need this is because of the way such esoteric subsets work.
So how and why does JSF**k works? The complete concept has been wonderfully explained on this page:
It works because every JavaScript program can be written as a string that gets evaluated. For example, this normal JavaScript code:
alert("Hello, world!")can be written as
Function('alert("Hello, world!")')
So, basically, JSF##k allows you to write any JavaScript code as a string, which need to be evaluated for the execution. For the example mentioned in the question, the browser treats the alert(1) in the onerror event handler as a valid JavaScript function, however, its JSF##k equivalent is going to be a string which then needs to be evaluated.