Skip to main content

Bypassing XSS filter in alert msg box

Chintan, Ronnie and I were discussing about one issue regarding how to bypass a specific XSS filter in alert message box. We were unable to breakout.
Chintan's problem statement was:
I am running into one issue with XSS and was interested if there is any way I can bypass it.
Following the response code where user supplied input is embedded. Input is taken via a text box.
<script type="text/javascript">alert('No Information is found for the card 1');script>

User supplied input 1 is highlighted in red. I am trying to break out of this alert box, however when a single quote is given as input, the output is escaped using a backslash. It is as follows:

Input:     1'
Output: <script type="text/javascript">alert('No Information is found for the card 1\'');script>

I am using IE 8 and tried using back ticks just to check if I can get around this limitation, however it did not work.
Any suggestion on how to break out of this would be much helpful.

All characters except the single quote, <!-- and </script> are working.
I tried the following vector to escape out:

Input:     1`);alert(1);(`');
Output: <script type="text/javascript">alert('No Information is found for the card 1`);alert(1);(`');script>

So, after a serires of discussion we came out with the solution:
;/\');alert(1);//  or   \');alert(\'XSS

And now this was successful bypass:
<script type="text/javascript">alert('No Information is found for the card ;/\\');alert("XSS");//');</script>
So, we successfully bypassed it

Comments

*Adi* said…
Interesting article on XSS Filter Bypassing. Helped a lot. And the best way to bypass filters is by analyzing the outputs for a given input, and then modifying the queries. Like you did.
Cool job! :)
June 9, 2012 at 11:00 AM
Post a Comment

Popular posts from this blog

File Upload through Null Byte Injection

Sometimes, during file upload we come across situation wherein there would be check on the file extension at the client side as well as server side too. If the application does allow only .jpeg extension to be uploaded, the client side java script checks for the extension of the file before passing the request. We all know that how easily this can be defeated. Some applications, checks for the extension at the server side also. That's not easy to bypass. However there are some ways with which it still can be bypassed. Most of server side scripts are written in high level languages such as Php, Java etc who still use some C/C++ libraries to read the file name and contents. That leads to the problem. In C/C++ a line ends with /00 or which is called Null Byte. So whenever the interpreter sees a null byte at the end of the a string, it stops reading thinking it has reached at the end of the string. This can be used for the bypass. It works for many servers, specially php servers. T...
1 comment
Read more

'Information Leakage-Improper Error Handling' dropped

From Owasp Top 10 2010 List, the issue 'Information Leakage-Improper Error Handling' has been dropped. But it's not the final list,its child release actually. Bu I feel it shouldn't be set aside because its still the one of the prevalent issues these days. That's why I mailed to Dave Wicher: Hi Dave, Excellent work, Congrats! Just one little query- Don't you think that Information Leakage & Improper Error Handling still deserves to be in Top 10? Dave replied: This topic is clearly a very prevalent issue that deserves attention by most organizations. However, the typical impact of such a flaw is usually very low. Therefore, the overall risk of this type of flaw is lower than the other items in the top 10, which is why it was replaced in this update with one of the 2 new items. Regarding dropping Info Leak/Error handling - It is incredibly prevalent, no question. But their impact is typically very low, so the overall risk is low, which is why it fell out of t...
3 comments
Read more

jtool - an alternative to otool

jtool comes with a capability of running on Linux environment. Some ipa scanning tools are created to run on Linux environment where mac environment is not available. In such cases tools such as otool and class-dump-z will not work. So jtool can be an alternative to otool. For more information on jtool please refer to http://www.newosxbook.com/tools/jtool.html . It lists down various commands which have same output as otool or a equivalent. There are several commands mentioned in link. But for our customized requirements and basis checks I have listed down the below ones after running on many binaries. The outputs are similar or equivalent to otool and class-dump-z: Commands for checking PIE flag (ASLR) in jTool jtool -d -v -arch | grep stack ·           Automatic Reference Counting (ARC) protection: jtool -d -v -arch | grep _objc_release ·           To check if the devic...
1 comment
Read more