it's very common to find XSS in search functions on web apps where the text a user enters into the form is reflected onto the page after the form is submitted. so you hit an app and search for "foo" and on the search results page you get back the search form is populated with "foo" which you just searched for. well if someone constructs a malicious link like:
http://someapp.somedomain.edu/search.htm?q=foo"><script>evil code here...
you end up w/ an xss attack assuming the app is poorly written...
typically during web app assessments you've gotta go smack the developer and tell them to validate their inputs and encode their outputs, but this time it took me a minute to figure out what was going on... sooooo here's the resulting html src of a little PoC i put together and tested w/ google app engine and ff3.x:
<input type="submit" name="btnTest" id="btnTest" value="testfoo" onclick="" />
so wtf is that? ok, this was based on a search form on an ajax-ish web app. there was more to the real app, but this includes all the relevant bits. when i searched on the app, i saw my inputs were reflecting in my browser so i went to check if they were html encoding them server side... but the value i was inputting in the search field never showed up in the page src... ermm, wot?
well, here's what i think is happening:
note that the "value=" tag is missing above. that makes the value attribute null when the server first serves it. when you use the form the app acted on your inputs using stuff like onkeyup/onkeydown, but when the user data needs to be read, it's done using the object oriented "this." convention which allows an object to refer to itself.
when you submitted the form the app would process your inputs, but the actual value you enter is never written to the page by the server. it exists only in memory on your client machine and is never written into html src. when the page refreshes your client browser renders the input element and snags the 'value=' value from memory and thus seems to avoid those pesky output encoding issues...?
anywho, it looks legit to me, but it's not a game changer or anything. kinda limited in it's application, and doesn't do anything for sql injection, csrf, etc.
but still kinda nifty mb ;)