Some questions about xss



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Post Wed Dec 28, 2011 8:35 pm

Some questions about xss

I have been digging with xss in the recent times(i just know the very basics of it),
as usual i had ended up with some questions so tought of asking here

the following are the xss bugs discovered by a pen tester in google before a few months(but now these bugs are fixed)


www.google.com/search?q=google&hl=en&gbv=2&biw=1024&bih=664&noj=1&tbs=isz:ex,iszw:"><script>alert("XSS") ;</script>,iszh:"><script>alert("XSS") ;</script>&tbm=isch&source=lnt&sa=X&ei=9NSxTpLhEIGaiQfDrsDLAg&ved=0CBIQpwUoBQ

--->earned 500$

www.google.com/fusiontables/DataSource?dsrcid=2257355"><script>alert("XSS") ;</script> ---->  earned 1000$ reward


0$ reward as it is a self xss

1)i just got  wondered with these bugs,i know bugs were common and exists every where,
i heard googles application are pretty secured and i am assuming they are developed by secure programmers,
how it is vulnerable to such
"><script>alert("XSS") ;</script>

simple attacks ? I wonder how they missed to filter such a simple form of xss ,especially the 2nd one?

2)Also i would like to know does google rewards people for a non-persistent xss ?

3)Assume we are trying to penetrate  a web-application for xss injection,and say our xss injectio queries are not working,
how can we determine that it is blocked by the apache or some kind of server or WAF  or it is blocked by the application it self ?

4)And i am beginner,i would like to ask you guys that
do you consider non-persistent xss as a threat ?

5)And i had seen some web-sites having their search bars vulnerable to non-persistent xss,
is there a way to read or write server files through such non-persistent bugs ?

6)does non - persistent xss bug is limited only to client side ?

7)is it possible to inject sql queries in to the server  with non-persistent xss?

8)assume your self as a attacker,
how  would you try to exploit  the non-persistent xss in the web-site?
Last edited by manoj9372 on Wed Dec 28, 2011 8:39 pm, edited 1 time in total.



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Post Wed Dec 28, 2011 9:54 pm

Re: Some questions about xss

1. They're probably better than most, but that doesn't make them perfect. There may have been a weird circumstance that resulted in a vulnerability scanner not detecting it, it could have been overlooked by human error, etc.

2. http://www.google.com/about/corporate/c ... ogram.html

3. If you could get some attacks to work while others fail, you may be able to fingerprint what controls are in place (or at the minimum, the types of checks the application is performing). Perhaps you could identify other discrepancies in functionality or error messages. I'm not sure how much this ultimately matters though.

4. Absolutely. All you have to do is get someone to click on a link (email, IM, forum posts, blog comments, etc.)

5. Persistency is not relevant here. If the vulnerability allows you to obtain an administrative session for the application, this could be a possibility.

6. The client browser is going to be targeted regardless of persistency.

7. Again, persistency is not relevant here. You could do this, but I'm not sure what the benefit of going this route over just exploiting the SQLi vulnerability directly. Maybe if SQLi could only be performed on a per-user basis in some weird circumstance…

8. See #4
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Post Fri Dec 30, 2011 7:51 pm

Re: Some questions about xss

The reason why they fail to sanitize user-input globally, is because of more than one developer, stress on the developers, and thereby an insecure development lifecycle.

Initially, the script may have been secure, but over time when it has been extended and extended, reaching ten-thousand lines of code (in several different files) or more, without proper quality control, it may eventually become insecure.

Often, a "catch and sanitize all" function, is not the best thing to do, as it may break the visual formating of input and output, or not function correctly. But just like simple stack overflows or "Remote File Inclusions" are hardly seen anymore in well known apps, while "off-by-one" and XSS is often hard to completely secure against in huge applications, that weren't secure from the start and hasn't had a completely safe development lifecycle.

There are of course, occasionally new attacks, that suddenly spawns and affects pretty much all applications, such as Click Jacking, Cross-Site Request Forgery, and more. Vulnerabilities, or "features" that most, didn't consider a vulnerability until they were perhaps, widely abused or realised to be a risk.

This depends on the case. But as the examples above, were non-persistent GET-request based XSS items, most likely.

By knowing different WAF's, webservers, and common applications, you can tell if it's most likely "this" or "that". But there's no way to be 100% sure, as the way a WAF filters input / output, can easily be integrated into a web application as a custom function, or for that sake, a webserver.

It's often not important to find out where the filter / sanitization is done, only if you know there's a publicly available copy of the source code somewhere, so you can take a look at the function preventing the XSS.

Yes. Even though it requires user-interaction / social engineering, but I've been successful with that several times (for legal harmless purposes of course). So it is possible, it just requires the right attack vector, where the human element plays a role. Of course, with persistent XSS, there's also a human role, but if there's no protection against the XSS attack in the browser, then it's a very limited role except that the attacker waits for the user to trigger the exploit / malicious piece of code.

Not with JavaScript or HTML. You need to crawl outside that box and think there, instead of thinking inside the box :) What can do be done? Here's an example: http://www.exploit-db.com/vbseo-from-xs ... php-shell/
That's custom work of course, but it works, and it was fun to create. (It took some research and time of course.)

XSS is always executed client-side. It may force a user, to execute PHP code somehow via a feature on the target website, or compromise the user's browser and execute system commands, but it (the XSS payload) is still, executed client-side.

If it's persistent, all it means is that it's stored on the server, just like this reply is. If this server had a XSS vulnerability, and it was saved. Then, whenever a user requests that "file" (or record, e.g., in a mysql database), the XSS will be served to the user. The server will just grab the XSS along with the other data, and send it in (most likely) a 200 OK response, with the "text/html" header specifying HTML, Javascript code, etc. should be executed by the browser. There's of course, more header than just these.

I think it would be a good idea, if you researched a bit about how a webserver / HTTP server functions, not as in how it serves data in detail. More like: Which ports, which processes are used (php, mysql, apache, as an example), how do these work together, what happens when you request e.g. index.php on a webserver (you should read the raw http request to see it with your own eyes), and so forth. This is "simple" (for me), but may sound advanced even though it is not, once you've tried it out yourself.

The best way to learn, is:
1. Set up a webserver, such as XAMPP on Windows where everything is done for you, or a LAMP yourself. LAMP means Linux Apache Mysql PHP. The last, can also be Python or Perl of course.

2. Write a few small scripts like <?php echo "Hello!"; ?> and test if they work. (Make sure to place them in the right directories.

3. Access these scripts, in your webbrowser. Do they work? Yes? If so, install an intercepting proxy like Burp Suite Free, and redirect traffic from e.g., FireFox into that, and then hit F5 on the page where your own script is that is served from your own webserver, and then, view how it looks like.

It's raw text! And if you take a look at most of the headers, it will make sense, at least, somewhat to begin with. Some things may not make sense. Over time, you'll suddenly find out, that it's a lot easier than it appears to be.

7) Yes, with XSSQLI. http://www.xssed.com/article/31/The_Beg ... de_to_XSS/ (MUST READ!) You also need to understand in depth, how all this plays together and if you have to use XSS to perform SQL Injection or not. For example, if a SQL Injection is only located in an administration panel, and there's an XSS or CSRF issue in that panel, you should use these (XSS and / or CSRF) vulnerabilities, to trick a user into performing the SQL Injection for you, which you're not allowed to as you don't have access to the administration panel. That's just an example of course.

Send a targeted user a maliciously crafted link, via e-mail. (If HTML is used, the link can be masked somewhat, including the use of a few "tricks".)

A (soon) happy new years!  ;D
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