I’ve been reading about XSS and I made a simple form with a text and submit input, but when I execute <script>alert();</script> on it, nothing happens, the server gets that string and that’s all.
What do I have to do for make it vulnerable?? (then I’ll learn what I shouldn’t do hehe)
Cheers.
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Answer
Indeed just let the server output it so that the input string effectively get embedded in HTML source which get returned to the client.
PHP example:
<!doctype html>
<html lang="en">
<head><title>XSS test</title></head>
<body>
<form><input type="text" name="xss"><input type="submit"></form>
<p>Result: <?= $_GET['xss'] ?></p>
</body>
</html>
JSP example:
<!doctype html>
<html lang="en">
<head><title>XSS test</title></head>
<body>
<form><input type="text" name="xss"><input type="submit"></form>
<p>Result: ${param.xss}</p>
</body>
</html>
Alternatively you can redisplay the value in the input elements, that’s also often seen:
<input type="text" name="xss" value="<?= $_GET['xss'] ?>">
resp.
<input type="text" name="xss" value="${param.xss}">
This way “weird” attack strings like "/><script>alert('xss')</script><br class=" will work because the server will render it after all as
<input type="text" name="xss" value=""/><script>alert('xss')</script><br class="">
XSS-prevention solutions are among others htmlspecialchars() and fn:escapeXml() for PHP and JSP respectively. Those will replace among others <, > and " by <, > and " so that enduser input doesn’t end up to be literally embedded in HTML source but instead just got displayed as it was entered.