Double URL-encoded XSS

Right after the quarantine ended I had a mini burn out for more than a month, where I didn’t even bother to try and hunt for bugs. After those one and a half months of trying to find my motivation again, I found a XSS that required a bit more thinking than the typical javascript:alert(1) cases.

Revisiting my old notes from a BB program, I observed an endpoint that I tried to exploit the last time, before I go through that burn out. Back then I wasn’t able to exploit it, so now I thought I’d give it a second chance. The endpoint was There was a button, where you would get redirected to the endpoint the paramteter redir was pointing. Analyzing the source code, I could see that the website was redirecting the user this way:


At first, I thought that this should be easy. I tried escaping the parenthesis by injecting ') at the redir parameter. The result was that the illegal characters would be removed. Actually, if there was any illegal characters, the app would redirect you, by default, to /.

I tried to inject a null byte (%00), just to see how the app would react. The app translated the null byte into NULL. So the payload redir=/%00/ would get reflected into the source code as javascript:redirect('/NULL/'). I thought great, the url encoded characters are getting translated. I clicked on the button and… I got redirected to /NULL/. Also, any other illegal url encoded character would be removed, just as I described previously.

So, apparently, the URL was getting decoded and sent to javascript:redirect() which was redirecting the user after they pressed the button. But I wanted to inject some “illegal” characters to escape the parenthesis. The only way to send illegal characters to javascript:redirect() was in a URL-encoded way, after they have been decoded from the URL bar. Hence, double URL-encoding! I sent the second payload, where %2500 is the double URL-encoded null byte. The result was for javascript:redirect('/%00/test') to get reflected. After hitting the button, I got redirected to, meaning that the null byte was actually getting decoded. So I could actually send double URL-encoded payloads and they would get rendered.

Having this on my mind, I crafted a payload for this endpoint (and by crafting, I mean copying and pasting the XSS payload from @brutelogic in PayloadAllTheThing and adding a ') in front). I double-URL-encoded it and injected it into the parameter. And after hitting the button, the XSS was getting triggered, right before the user get’s redirected to /.

Final exploit:



Written on June 2, 2020