Reflected cross-site scripting vulnerabilities arise when data is copied from a request and echoed into the application's immediate response in an unsafe way. An attacker can use the vulnerability to construct a request which, if issued by another application user, will cause JavaScript code supplied by the attacker to execute within the user's browser in the context of that user's session with the application.
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.
Issue remediation
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 REST URL parameter 1 is copied into the HTML document as plain text between tags. The payload 8e5e2<a>074b39b533a was submitted in the REST URL parameter 1. This input was echoed unmodified in the application's response.
This behaviour demonstrates that it is possible to inject new HTML tags into the returned document. An attempt was made to identify a full proof-of-concept attack for injecting arbitrary JavaScript but this was not successful. You should manually examine the application's behaviour and attempt to identify any unusual input validation or other obstacles that may be in place.
Request
GET /p-aasG6JkxVvmNA8e5e2<a>074b39b533a HTTP/1.1 Host: www.quantcast.com Accept: */* Accept-Language: en User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0) Connection: close
Response
HTTP/1.1 404 Not Found Server: Apache-Coyote/1.1 Content-Type: text/html Content-Language: en Date: Thu, 03 Feb 2011 22:03:07 GMT Connection: close
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; cha ...[SNIP]... <em> p-aasG6JkxVvmNA8e5e2<a>074b39b533a</em> ...[SNIP]...
The value of REST URL parameter 1 is copied into the value of an HTML tag attribute which is encapsulated in double quotation marks. The payload aabbe"><a>fe7c65bf24b was submitted in the REST URL parameter 1. This input was echoed unmodified in the application's response.
This behaviour demonstrates that it is possible to inject new HTML tags into the returned document. An attempt was made to identify a full proof-of-concept attack for injecting arbitrary JavaScript but this was not successful. You should manually examine the application's behaviour and attempt to identify any unusual input validation or other obstacles that may be in place.
Request
GET /p-aasG6JkxVvmNAaabbe"><a>fe7c65bf24b HTTP/1.1 Host: www.quantcast.com Accept: */* Accept-Language: en User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0) Connection: close
Response
HTTP/1.1 404 Not Found Server: Apache-Coyote/1.1 Content-Type: text/html Content-Language: en Date: Thu, 03 Feb 2011 22:03:07 GMT Connection: close
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
The value of the Referer HTTP header is copied into the value of an HTML tag attribute which is encapsulated in double quotation marks. The payload ff882"><script>alert(1)</script>9cbb95e5ed5 was submitted in the Referer HTTP header. This input was echoed unmodified in the application's response.
This proof-of-concept attack demonstrates that it is possible to inject arbitrary JavaScript into 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.
Request
GET /p-aasG6JkxVvmNA HTTP/1.1 Host: www.quantcast.com Accept: */* Accept-Language: en User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0) Connection: close Referer: http://www.google.com/search?hl=en&q=ff882"><script>alert(1)</script>9cbb95e5ed5
Response
HTTP/1.1 200 OK Server: Apache-Coyote/1.1 Date: Thu, 03 Feb 2011 22:03:05 GMT Expires: Wed, 02 Feb 2011 10:03:06 GMT Cache-control: private, max-age=0 Set-Cookie: qcVisitor=0|82|1296770586032|0|NOTSET; Expires=Sat, 26-Jan-2041 22:03:06 GMT; Path=/ Set-Cookie: JSESSIONID=44D9028629FFF99EE86CE90A605E6EC0; Path=/ Content-Type: text/html;charset=UTF-8 Content-Language: en-US Connection: close
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
The highlighted cookie appears to contain a session token, which may increase the risk associated with this issue. You should review the contents of the cookies to determine their function.
Issue background
If the HttpOnly attribute is set on a cookie, then the cookie's value cannot be read or set by client-side JavaScript. This measure can prevent certain client-side attacks, such as cross-site scripting, from trivially capturing the cookie's value via an injected script.
Issue remediation
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.
Request
GET /p-aasG6JkxVvmNA HTTP/1.1 Host: www.quantcast.com Accept: */* Accept-Language: en User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0) Connection: close
Response
HTTP/1.1 200 OK Server: Apache-Coyote/1.1 Date: Thu, 03 Feb 2011 22:03:02 GMT Expires: Wed, 02 Feb 2011 10:03:02 GMT Cache-control: private, max-age=0 Set-Cookie: qcVisitor=1|69|1296770582348|0|NOTSET; Expires=Sat, 26-Jan-2041 22:03:02 GMT; Path=/ Set-Cookie: JSESSIONID=64B221D97727744347E466E5663A8A8D; Path=/ Content-Type: text/html;charset=UTF-8 Content-Language: en-US Connection: close
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
When an application includes a script from an external domain, this script is executed by the browser within the security context of the invoking application. The script can therefore do anything that the application's own scripts can do, such as accessing application data and performing actions within the context of the current user.
If you include a script from an external domain, then you are trusting that domain with the data and functionality of your application, and you are trusting the domain's own security to prevent an attacker from modifying the script to perform malicious actions within your application.
Issue remediation
Scripts should not be included from untrusted domains. If you have a requirement which a third-party script appears to fulfil, then you should ideally copy the contents of that script onto your own domain and include it from there. If that is not possible (e.g. for licensing reasons) then you should consider reimplementing the script's functionality within your own code.
Request
GET /p-aasG6JkxVvmNA HTTP/1.1 Host: www.quantcast.com Accept: */* Accept-Language: en User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0) Connection: close
Response
HTTP/1.1 200 OK Server: Apache-Coyote/1.1 Date: Thu, 03 Feb 2011 22:03:02 GMT Expires: Wed, 02 Feb 2011 10:03:02 GMT Cache-control: private, max-age=0 Set-Cookie: qcVisitor=1|69|1296770582348|0|NOTSET; Expires=Sat, 26-Jan-2041 22:03:02 GMT; Path=/ Set-Cookie: JSESSIONID=64B221D97727744347E466E5663A8A8D; Path=/ Content-Type: text/html;charset=UTF-8 Content-Language: en-US Connection: close
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">