Vulnerable vehicles that are commonly used for Cross-site Scripting attacks are forums, message boards, and web pages that allow comments.Ī web page or web application is vulnerable to XSS if it uses unsanitized user input in the output that it generates. The web page or web application becomes a vehicle to deliver the malicious script to the user’s browser. The actual attack occurs when the victim visits the web page or web application that executes the malicious code. The attacker aims to execute malicious scripts in a web browser of the victim by including malicious code in a legitimate web page or web application. We could do this by injecting code which makes an asynchronous call to the API's super secure “delete user” endpoint.Cross-site Scripting (XSS) is a client-side code injection attack. We’ve already seen that an “alert()” works fine, but let’s see what happens if we try to use this attack vector to do something like delete a user account. Let’s go back to the previous example for a moment, as it introduced a scenario where you are forced to use a more complex payload than script tags. If you can’t overcome that complication, what threat does the XSS really present? How are you going to sell it to your client as something which is bad and needs fixing when all you can prove to them is that you can make a little box popup with a "1" in it? Odds are there will be some additional complication you took for granted. As it could not be further from the truth! XSS can be terrible, but what if you can’t actually leverage it to do anything meaningful? Well, I mean it's still pretty bad but just in a different way.īut the point is, next time you find an XSS, attempt to actually exploit it with relevance to the client. When you inject “alert(1)” into a page, see a popup, and write XSS into your report stating that it may bring the end of the world… This is something I call the “XSS façade”. B-b-b-basic requirements for bi-directional C2 using Cross-Origin Resource Sharing (CORS).HTTP/S mixed content - how to "cleanly" exfiltrate data.The XSS killer known as the Content Security Policy (CSP).Position matters, knowing when you need to wait.A common “gotcha” from dynamically created web pages. So I’m going to try and give a light touch on some really common issues I come across when developing full XSS PoCs against modern applications. I want to keep to a more "application" specific context, and write a post more about being creative with what we know, and not coming up with something brand new. But, for the purpose of this blog post I don’t want to talk about bypassing a load of different browser XSS controls. So I thought I’d put together a quick list of some common issues you might encounter on a typical job and how to get around these complications in order to achieve and demonstrate realistic XSS and real value.īut OK hang on everyone! Before I even begin, I’m just going to acknowledge that yes, many browsers have built in protections that attempt to prevent XSS, paired with their own set of specific weaknesses and bypasses. The idea of people only ever demonstrating XSS Proof of Concepts (PoCs) which completely disregard modern security controls keeps me up at night. Because of modern browser security controls and the increase in security awareness of application developers there are a number of things in our way that stop all our classic XSS attacks from working. The practicality of these methods for achieving Cross-Site Scripting (XSS) and exfiltrating/loading data are becoming less practical outside of your local host. The times of “alert(1)” and making use of “python –m SimpleHTTPServer” have well faded away.
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