Echoing user-controllable data within a script context is inherently dangerous and can make XSS attacks difficult to prevent. If at all possible, the application should avoid echoing user data within this context.
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.
GET /?62ebe'%3balert(1)//27734d0d8b2=1 HTTP/1.1 Host: autocad.autodesk.com Accept: */* Accept-Language: en User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0; CloudScan Vuln Crawler http://cloudscan.me) Connection: close
HTTP/1.1 200 OK connection: close content-type: text/html; charset=UTF-8 date: Fri, 12 Nov 2010 04:14:55 GMT p3p: CP="NON CUR OTPi OUR NOR UNI" server: Apache/2.2.3 (Red Hat) vary: Accept-Encoding Set-Cookie: uuid=12716910351316; Path=/; Expires=Fri, 11 Nov 2011 20:14:55 UTC Set-Cookie: day=1; Path=/ Set-Cookie: agent=local; Path=/; Expires=Sun, 26 Dec 2010 20:14:55 UTC Set-Cookie: LB-INFO=1669538570.20480.0000; Path=/ Set-Cookie: LB-INFO-WS=1229005578.20480.0000; path=/
The following cookies were issued by the application and do not have the HttpOnly flag set:
uuid=12716910351316; Path=/; Expires=Fri, 11 Nov 2011 20:14:49 UTC
agent=local; Path=/; Expires=Sun, 26 Dec 2010 20:14:49 UTC
The cookies do not appear to contain session tokens, which may reduce the risk associated with this issue. You should review the contents of the cookies to determine their function.
There is usually no good reason not to set the HttpOnly flag on all cookies. Unless you specifically require legitimate client-side scripts within your application to read or set a cookie's value, you should set the HttpOnly flag by including this attribute within the relevant Set-cookie directive.
You should be aware that the restrictions imposed by the HttpOnly flag can potentially be circumvented in some circumstances, and that numerous other serious attacks can be delivered by client-side script injection, aside from simple cookie stealing.
GET / HTTP/1.1 Host: autocad.autodesk.com Accept: */* Accept-Language: en User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0; CloudScan Vuln Crawler http://cloudscan.me) Connection: close
HTTP/1.1 200 OK connection: close content-type: text/html; charset=UTF-8 date: Fri, 12 Nov 2010 04:14:49 GMT p3p: CP="NON CUR OTPi OUR NOR UNI" server: Apache/2.2.3 (Red Hat) vary: Accept-Encoding Set-Cookie: uuid=12716910351316; Path=/; Expires=Fri, 11 Nov 2011 20:14:49 UTC Set-Cookie: day=1; Path=/ Set-Cookie: agent=local; Path=/; Expires=Sun, 26 Dec 2010 20:14:49 UTC Set-Cookie: LB-INFO=1686315786.20480.0000; Path=/ Set-Cookie: LB-INFO-WS=1195451146.20480.0000; path=/
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="C ...[SNIP]...
Report generated by XSS.CX at Fri Nov 12 07:36:06 EST 2010.