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 Referer HTTP header is copied into an HTML comment. The payload f0c0c--><script>alert(1)</script>ab07c28887 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.
Note that a redirection occurred between the attack request and the response containing the echoed input. It is necessary to follow this redirection for the attack to succeed. When the attack is carried out via a browser, the redirection will be followed automatically.
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 HTML comment tags does not prevent XSS attacks if the user is able to close the comment or use other techniques to introduce scripts within the comment context.
Request
GET /applications/SearchTools/search.asp HTTP/1.1 Host: www.compusa.com Accept: */* Accept-Language: en User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0) Connection: close Referer: http://www.google.com/search?hl=en&q=f0c0c--><script>alert(1)</script>ab07c28887
The value of the Referer HTTP header is copied into an HTML comment. The payload 24a87--><script>alert(1)</script>ec22dac0bbf 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.
Note that a redirection occurred between the attack request and the response containing the echoed input. It is necessary to follow this redirection for the attack to succeed. When the attack is carried out via a browser, the redirection will be followed automatically.
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 HTML comment tags does not prevent XSS attacks if the user is able to close the comment or use other techniques to introduce scripts within the comment context.
Request
GET /cgi-bin/order.asp HTTP/1.1 Host: www.compusa.com Accept: */* Accept-Language: en User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0) Connection: close Referer: http://www.google.com/search?hl=en&q=24a87--><script>alert(1)</script>ec22dac0bbf
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.
The highlighted cookie appears to contain a session token, which may increase the risk associated with this issue. You should review the contents of the cookies to determine their function.
Request
GET /applications/SearchTools/search.asp HTTP/1.1 Host: www.compusa.com Accept: */* Accept-Language: en User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0) Connection: close
The cookies do not appear to contain session tokens, which may reduce the risk associated with this issue. You should review the contents of the cookies to determine their function.
Request
GET /cgi-bin/icart.asp?p=upsell HTTP/1.1 Host: www.compusa.com Proxy-Connection: keep-alive Referer: http://www.compusa.com/applications/searchtools/item_upsell.asp?EdpNo=&msg= 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 Cookie: pop%5Fcheck=active; Warranty=POPPED; DB=msImageSC=%2Fmicrosoft%2FMSelasticity%2Dbnr%5F620x150B%2Ejpg&Sidenav=B&Surveyflag=1&msImageID=%2Fmicrosoft%2FMSelasticity%2Dbnr%5F430x150B%2Ejpg&msProduct=1782290&msRandX=61; beta=Y; SessionId=3547262620110417105004173193214243; Cart=Landing=http%3A%2F%2Fwww%2Ecompusa%2Ecom%2Fapplications%2Fsearchtools%2Fitem%5Fupsell%2Easp%3FEdpNo%3D%26msg%3D&Referer=http%3A%2F%2Fwww%2Egoogle%2Ecom%2Fsearch%3Fhl%3Den%26q%3D24a87%2D%2D%3E%3Cscript%3Ealert%28%22IDIOT%22%29%3C%2Fscript%3Eec22dac0bbf&PHRoutine=10; SRCCODE=COMPGOOSFS; SRVR=WEBX23%2D03B
The cookies do not appear to contain session tokens, which may reduce the risk associated with this issue. You should review the contents of the cookies to determine their function.
Request
GET /cgi-bin/order.asp HTTP/1.1 Host: www.compusa.com Accept: */* Accept-Language: en User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0) Connection: close
<head><title>Object moved</title></head> <body><h1>Object Moved</h1>This object may be found <a HREF="/applications/searchtools/item_upsell.asp?EdpNo=&msg=">here</a>.</body>
If a web response states that it contains HTML content but does not specify a character set, then the browser may analyse the HTML and attempt to determine which character set it appears to be using. Even if the majority of the HTML actually employs a standard character set such as UTF-8, the presence of non-standard characters anywhere in the response may cause the browser to interpret the content using a different character set. This can have unexpected results, and can lead to cross-site scripting vulnerabilities in which non-standard encodings like UTF-7 can be used to bypass the application's defensive filters.
In most cases, the absence of a charset directive 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 HTML content, the application should include within the Content-type header a directive specifying a standard recognised character set, for example charset=ISO-8859-1.
Request
GET /cgi-bin/icart.asp?p=upsell HTTP/1.1 Host: www.compusa.com Proxy-Connection: keep-alive Referer: http://www.compusa.com/applications/searchtools/item_upsell.asp?EdpNo=&msg= 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 Cookie: pop%5Fcheck=active; Warranty=POPPED; DB=msImageSC=%2Fmicrosoft%2FMSelasticity%2Dbnr%5F620x150B%2Ejpg&Sidenav=B&Surveyflag=1&msImageID=%2Fmicrosoft%2FMSelasticity%2Dbnr%5F430x150B%2Ejpg&msProduct=1782290&msRandX=61; beta=Y; SessionId=3547262620110417105004173193214243; Cart=Landing=http%3A%2F%2Fwww%2Ecompusa%2Ecom%2Fapplications%2Fsearchtools%2Fitem%5Fupsell%2Easp%3FEdpNo%3D%26msg%3D&Referer=http%3A%2F%2Fwww%2Egoogle%2Ecom%2Fsearch%3Fhl%3Den%26q%3D24a87%2D%2D%3E%3Cscript%3Ealert%28%22IDIOT%22%29%3C%2Fscript%3Eec22dac0bbf&PHRoutine=10; SRCCODE=COMPGOOSFS; SRVR=WEBX23%2D03B