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 defences:
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 value of an HTML tag attribute which is encapsulated in double quotation marks. The payload 718ff"><img%20src%3da%20onerror%3dalert(1)>e4f57d7fb52 was submitted in the REST URL parameter 1. This input was echoed as 718ff"><img src=a onerror=alert(1)>e4f57d7fb52 in the application's response.
This proof-of-concept attack demonstrates that it is possible to inject arbitrary JavaScript into the application's response. The PoC attack demonstrated uses an event handler to introduce arbitrary JavaScript into the document.
Request
GET /content718ff"><img%20src%3da%20onerror%3dalert(1)>e4f57d7fb52/career-opportunities HTTP/1.1 Host: about-monster.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 Date: Thu, 18 Aug 2011 18:15:47 GMT Server: Apache/2.2.17 (EL) X-Powered-By: PHP/5.2.16 Expires: Sun, 19 Nov 1978 05:00:00 GMT Last-Modified: Thu, 18 Aug 2011 18:15:47 GMT Cache-Control: store, no-cache, must-revalidate Cache-Control: post-check=0, pre-check=0 Connection: close Content-Type: text/html; charset=utf-8 Content-Length: 23053
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en">
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 64f73"><img%20src%3da%20onerror%3dalert(1)>9442e8f030d was submitted in the REST URL parameter 1. This input was echoed as 64f73"><img src=a onerror=alert(1)>9442e8f030d in the application's response.
This proof-of-concept attack demonstrates that it is possible to inject arbitrary JavaScript into the application's response. The PoC attack demonstrated uses an event handler to introduce arbitrary JavaScript into the document.
Request
GET /content64f73"><img%20src%3da%20onerror%3dalert(1)>9442e8f030d/monster-employment-index-annual-growth-rate-13-november HTTP/1.1 Host: about-monster.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 Date: Thu, 18 Aug 2011 18:15:49 GMT Server: Apache/2.2.17 (EL) X-Powered-By: PHP/5.2.16 Expires: Sun, 19 Nov 1978 05:00:00 GMT Last-Modified: Thu, 18 Aug 2011 18:15:49 GMT Cache-Control: store, no-cache, must-revalidate Cache-Control: post-check=0, pre-check=0 Connection: close Content-Type: text/html; charset=utf-8 Content-Length: 23333
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en">
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.
GET /content/career-opportunities HTTP/1.1 Host: about-monster.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 Date: Thu, 18 Aug 2011 18:15:33 GMT Server: Apache/2.2.17 (EL) X-Powered-By: PHP/5.2.16 Last-Modified: Thu, 18 Aug 2011 18:03:05 GMT ETag: "db3326e603d2d4a35068c88f8db96543" Expires: Sun, 19 Nov 1978 05:00:00 GMT Cache-Control: must-revalidate Connection: close Content-Type: text/html; charset=utf-8 Content-Length: 40792
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en">
GET /content/monster-employment-index-annual-growth-rate-13-november HTTP/1.1 Host: about-monster.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 Date: Thu, 18 Aug 2011 18:15:33 GMT Server: Apache/2.2.17 (EL) X-Powered-By: PHP/5.2.16 Last-Modified: Thu, 18 Aug 2011 18:02:38 GMT ETag: "bc1d29cc40fcc210f4bec6efc0d1bde7" Expires: Sun, 19 Nov 1978 05:00:00 GMT Cache-Control: must-revalidate Connection: close Content-Type: text/html; charset=utf-8 Content-Length: 78810
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en">
The following email addresses were disclosed in the response:
Deepika.Murty@monster.com
Matthew.Henson@monster.com
Issue background
The presence of email addresses within application responses does not necessarily constitute a security vulnerability. Email addresses may appear intentionally within contact information, and many applications (such as web mail) include arbitrary third-party email addresses within their core content.
However, email addresses of developers and other individuals (whether appearing on-screen or hidden within page source) may disclose information that is useful to an attacker; for example, they may represent usernames that can be used at the application's login, and they may be used in social engineering attacks against the organisation's personnel. Unnecessary or excessive disclosure of email addresses may also lead to an increase in the volume of spam email received.
Issue remediation
You should review the email addresses being disclosed by the application, and consider removing any that are unnecessary, or replacing personal addresses with anonymous mailbox addresses (such as helpdesk@example.com).
Request
GET /content/monster-employment-index-annual-growth-rate-13-november HTTP/1.1 Host: about-monster.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 Date: Thu, 18 Aug 2011 18:15:33 GMT Server: Apache/2.2.17 (EL) X-Powered-By: PHP/5.2.16 Last-Modified: Thu, 18 Aug 2011 18:02:38 GMT ETag: "bc1d29cc40fcc210f4bec6efc0d1bde7" Expires: Sun, 19 Nov 1978 05:00:00 GMT Cache-Control: must-revalidate Connection: close Content-Type: text/html; charset=utf-8 Content-Length: 78810
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en">
