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 the admeld_adprovider_id request parameter is copied into a JavaScript string which is encapsulated in single quotation marks. The payload ae48c'-alert(1)-'49d3e5006f8 was submitted in the admeld_adprovider_id 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.
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 /usersync?calltype=admeld&admeld_user_id=6acccca4-d0e4-464e-a824-f67cb28d5556&admeld_adprovider_id=193ae48c'-alert(1)-'49d3e5006f8&admeld_call_type=js&admeld_callback=http://tag.admeld.com/match HTTP/1.1 Host: admeld.adnxs.com Proxy-Connection: keep-alive Referer: http://www.pandora.com/include/communityAdEmbed.html?genre=&artist=&webname=&sz=728x90&ord=1294536136217419152&clean=0&spgs=0&tile=1&_id=leaderboard_container Accept: */* User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.10 (KHTML, like Gecko) Chrome/8.0.552.224 Safari/534.10 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: anj=Kfu=8fG7]PCxrx)0s]#%2L_'x%SEV/hnJipx9oC)FXduyOWimI4KKhq.W^v=7v!+J; sess=1; uuid2=4760492999213801733
Response
HTTP/1.1 200 OK Cache-Control: no-store, no-cache, private Pragma: no-cache Expires: Sat, 15 Nov 2008 16:00:00 GMT P3P: CP="OTI DSP COR ADMo TAIo PSAo PSDo CONo OUR SAMo OTRo STP UNI PUR COM NAV INT DEM STA PRE LOC" Set-Cookie: sess=1; path=/; expires=Mon, 10-Jan-2011 02:02:34 GMT; domain=.adnxs.com; HttpOnly Set-Cookie: uuid2=4760492999213801733; path=/; expires=Sat, 09-Apr-2011 02:02:34 GMT; domain=.adnxs.com; HttpOnly Content-Type: application/x-javascript Date: Sun, 09 Jan 2011 02:02:34 GMT Content-Length: 183
The value of the admeld_callback request parameter is copied into a JavaScript string which is encapsulated in single quotation marks. The payload 16c38'-alert(1)-'3fc1cb53627 was submitted in the admeld_callback 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.
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 /usersync?calltype=admeld&admeld_user_id=6acccca4-d0e4-464e-a824-f67cb28d5556&admeld_adprovider_id=193&admeld_call_type=js&admeld_callback=http://tag.admeld.com/match16c38'-alert(1)-'3fc1cb53627 HTTP/1.1 Host: admeld.adnxs.com Proxy-Connection: keep-alive Referer: http://www.pandora.com/include/communityAdEmbed.html?genre=&artist=&webname=&sz=728x90&ord=1294536136217419152&clean=0&spgs=0&tile=1&_id=leaderboard_container Accept: */* User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.10 (KHTML, like Gecko) Chrome/8.0.552.224 Safari/534.10 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: anj=Kfu=8fG7]PCxrx)0s]#%2L_'x%SEV/hnJipx9oC)FXduyOWimI4KKhq.W^v=7v!+J; sess=1; uuid2=4760492999213801733
Response
HTTP/1.1 200 OK Cache-Control: no-store, no-cache, private Pragma: no-cache Expires: Sat, 15 Nov 2008 16:00:00 GMT P3P: CP="OTI DSP COR ADMo TAIo PSAo PSDo CONo OUR SAMo OTRo STP UNI PUR COM NAV INT DEM STA PRE LOC" Set-Cookie: sess=1; path=/; expires=Mon, 10-Jan-2011 02:03:03 GMT; domain=.adnxs.com; HttpOnly Set-Cookie: uuid2=4760492999213801733; path=/; expires=Sat, 09-Apr-2011 02:03:03 GMT; domain=.adnxs.com; HttpOnly Content-Type: application/x-javascript Date: Sun, 09 Jan 2011 02:03:03 GMT Content-Length: 183
The cookie does not appear to contain a session token, which may reduce the risk associated with this issue. You should review the contents of the cookie to determine its function.
Issue background
A cookie's domain attribute determines which domains can access the cookie. Browsers will automatically submit the cookie in requests to in-scope domains, and those domains will also be able to access the cookie via JavaScript. If a cookie is scoped to a parent domain, then that cookie will be accessible by the parent domain and also by any other subdomains of the parent domain. If the cookie contains sensitive data (such as a session token) then this data may be accessible by less trusted or less secure applications residing at those domains, leading to a security compromise.
Issue remediation
By default, cookies are scoped to the issuing domain and all subdomains. If you remove the explicit domain attribute from your Set-cookie directive, then the cookie will have this default scope, which is safe and appropriate in most situations. If you particularly need a cookie to be accessible by a parent domain, then you should thoroughly review the security of the applications residing on that domain and its subdomains, and confirm that you are willing to trust the people and systems which support those applications.
Request
GET /usersync?calltype=admeld&admeld_user_id=6acccca4-d0e4-464e-a824-f67cb28d5556&admeld_adprovider_id=193&admeld_call_type=js&admeld_callback=http://tag.admeld.com/match HTTP/1.1 Host: admeld.adnxs.com Proxy-Connection: keep-alive Referer: http://www.pandora.com/include/communityAdEmbed.html?genre=&artist=&webname=&sz=728x90&ord=1294536136217419152&clean=0&spgs=0&tile=1&_id=leaderboard_container Accept: */* User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.10 (KHTML, like Gecko) Chrome/8.0.552.224 Safari/534.10 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: anj=Kfu=8fG7]PCxrx)0s]#%2L_'x%SEV/hnJipx9oC)FXduyOWimI4KKhq.W^v=7v!+J; sess=1; uuid2=4760492999213801733
Response
HTTP/1.1 200 OK Cache-Control: no-store, no-cache, private Pragma: no-cache Expires: Sat, 15 Nov 2008 16:00:00 GMT P3P: CP="OTI DSP COR ADMo TAIo PSAo PSDo CONo OUR SAMo OTRo STP UNI PUR COM NAV INT DEM STA PRE LOC" Set-Cookie: sess=1; path=/; expires=Mon, 10-Jan-2011 02:01:48 GMT; domain=.adnxs.com; HttpOnly Set-Cookie: uuid2=4760492999213801733; path=/; expires=Sat, 09-Apr-2011 02:01:48 GMT; domain=.adnxs.com; HttpOnly Content-Type: application/x-javascript Date: Sun, 09 Jan 2011 02:01:48 GMT Content-Length: 155
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 /usersync?calltype=admeld&admeld_user_id=6acccca4-d0e4-464e-a824-f67cb28d5556&admeld_adprovider_id=193&admeld_call_type=js&admeld_callback=http://tag.admeld.com/match HTTP/1.1 Host: admeld.adnxs.com Proxy-Connection: keep-alive Referer: http://www.pandora.com/include/communityAdEmbed.html?genre=&artist=&webname=&sz=728x90&ord=1294536136217419152&clean=0&spgs=0&tile=1&_id=leaderboard_container Accept: */* User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.10 (KHTML, like Gecko) Chrome/8.0.552.224 Safari/534.10 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: anj=Kfu=8fG7]PCxrx)0s]#%2L_'x%SEV/hnJipx9oC)FXduyOWimI4KKhq.W^v=7v!+J; sess=1; uuid2=4760492999213801733
Response
HTTP/1.1 200 OK Cache-Control: no-store, no-cache, private Pragma: no-cache Expires: Sat, 15 Nov 2008 16:00:00 GMT P3P: CP="OTI DSP COR ADMo TAIo PSAo PSDo CONo OUR SAMo OTRo STP UNI PUR COM NAV INT DEM STA PRE LOC" Set-Cookie: sess=1; path=/; expires=Mon, 10-Jan-2011 02:01:48 GMT; domain=.adnxs.com; HttpOnly Set-Cookie: uuid2=4760492999213801733; path=/; expires=Sat, 09-Apr-2011 02:01:48 GMT; domain=.adnxs.com; HttpOnly Content-Type: application/x-javascript Date: Sun, 09 Jan 2011 02:01:48 GMT Content-Length: 155