footer: XSS | Andrew Kerr slidenumbers: true
- Fifth year Software Engineering @ UF
- Secretary of UFSIT for > 2yrs
- Full stack web developer
- Former security intern at Tumblr
- Former intern at BlockScore
Cross-Site Scripting (XSS) attacks are a type of injection, in which malicious scripts are injected into otherwise benign and trusted web sites. XSS attacks occur when an attacker uses a web application to send malicious code, generally in the form of a browser side script, to a different end user. -- OWASP
^ OWASP == Open Web Application Security Project; is a not-for-profit charitable organization focused on improving the security of software
^ OWASP Top 10 is a good place to start learning about web security
^ Let's say we have a form
^ How would we put JavaScript into this form?
^ Sure, let's put some JavaScript in there
^ Woah there! That's cool!
^ Now, keep in mind that there are mitigations to this which we'll talk about later.
- Browser is tricked into thinking the code is part of the site
- Backend server does not sanitize input correctly
- Poor client-side JavaScript executes given parameters
^ Browser is stupid and runs anything it's given!
^ Since browser is stupid, we need to sanitize input - we'll talk about how later
^ Site JavaScript allows for injectable JavaScript parameters
- Steal session cookies
- Steal logins by defacing
- Exploit the browser/plugins
- For the lulz
^ Session cookies keep track of login
^ Can inject a script to create a "login" page that sends creds to your server
^ BeEF hooks can turn your browser into a "zombie"
^ Forceful retweets!
- ❤️ emoji broke XSS sanitization on TweetDeck
- Auto-magically retweeted itself 70,000+ times
- Good thing it wasn't malicious!
^ Twitter and social media has real-world consequences now
- A TON of possible XSS payloads
<script>alert(1)</script><img src="x" onerror="alert(1)" /><a href="javascript: alert(1)">Click me!</a>- and more!
^ We'll come back to some of these later
- Reflected
- Stored
- DOM-based
- Ability to inject code and have the server return it back, unsanitized
- Not stored on the server/in a database!
- Normally hidden in the URL
- Don't click on random links!
- Example: search forms showing input on results page after submission
// www.site.com/search.php?q=search+query
$search_query = $_GET['q'];
echo '<h1>Search results for: ' . $search_query . '</h1>;
www.site.com/search.php?q=<script>alert(1)</script>
^ Reflected because it's a search result!
// www.site.com/search.php?q=search+query
$search_query = $_GET['q'];
echo '<h1>Search results for: ' . $search_query . '</h1>;
Q: What's wrong with this code? A: UNSANITIZED USER INPUT
- Ability to inject code and have the server store it and return it without sanitizing it in either case
- HOLY CRAP THIS IS HORRIBLE
- Only way for end user to protect themselves is to disable JS
- Example: form post storing XSS
^ Remember earlier?
^ https://twitter.com/dergeruhn/status/476764918763749376
- Posted 'but most of all, samy is my hero' to victims
- Fastest spreading virus of all time
- 1+ million runs in ~20hrs
^ Still there!
// Storing posts
$post = $_POST['post'];
$query = $mysql_conn->prepare("INSERT INTO posts VALUES ('" . $post . "')");
$query->execute();
// Fetching and outputting posts
$query = $mysql_conn->prepare("SELECT * FROM posts");
$query->execute();
$query->bind_result($post);
while($query->fetch()) {
echo '<p>' . $post . '</p>';
}Q: What's the issue? A: UNSANITIZED USER INPUT
^ Unsanitized on both input AND output
^ Ask: How would you exploit this?
- Similar to Reflected, but is not rendered from the server.
- Normally due to bad JavaScript code
- Also crafted by a URL
- Don't let users pass in JS via the URL!
// Pretend parse_get_params is imeplemented :)
var title = parse_get_params('title');
$('.page-header').html("<h1>" + title + "</h1>");Q: And, what's the issue here? A: UNSANITIZED USER INPUT
// Pretend parse_get_params is imeplemented :)
var title = parse_get_params('title');
$('.page-header').html("<h1>" + title + "</h1>");Q: How would we exploit this?
A: Craft a URL like:
www.site.com/page.html?title=<img src='x' onerror='alert(1)' />
// Pretend parse_get_params is imeplemented :)
var title = parse_get_params('title');
$('.page-header').html("<h1>" + title + "</h1>");- jQuery provides a
.htmlAND.text. - But, what's the difference?
Set the text contents of the matched elements. --
.text()
Set the HTML contents of each element in the set of matched elements. --
.html()
<html>
<head>
<title>Test Page</title>
<script type="text/javascript" src="jquery.min.js"></script>
<script type="text/javascript">
$(function(){
$("#div1").html('<a href="example.html">Link</a><b>hello</b>');
$("#div2").text('<a href="example.html">Link</a><b>hello</b>');
});
</script>
</head>
<body>
<div id="div1"></div>
<div id="div2"></div>
</body>
</html>- Know your framework/library/language!
- SANITIZE!
- Whitelist, not blacklist
- Headers
^ jQuery is popular and knowing these things means the difference between pwned and safe!
^ Sometimes, knowing your framework/language will help you sanitize
^ Whitelisting is a much better thing than blacklisting
^ Content-Security-Policy, X-XSS-Protection
- Or, ya know, read the NSA's recommendations.
- https://www.nsa.gov/ia/_files/factsheets/xss_iad_factsheet_final_web.pdf
^ Seriously. Do it. Automated scanners, by hand, who cares? JUST DO IT.
^ In order to test, we need to know how to...
- Wonderful cheatsheet by OWASP: https://www.owasp.org/index.php/XSS_Filter_Evasion_Cheat_Sheet
- Also, some guess work helps!
$input = $_POST['input'];
$sanitized = str_replace('script', '', $input);Q: How could we get by this? A: Think about it :)
^ Challenges!
- OWASP
- The Web Application Hackers Handbook
- Mutillidae Practice Application
- Server: 104.236.76.214
- Source: github.com/ufsit/xss-challenges
- Try not to use this!
- Cheatsheet: https://www.owasp.org/index.php/XSS_Filter_Evasion_Cheat_Sheet