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 mode request parameter is copied into a JavaScript expression which is not encapsulated in any quotation marks. The payload a3c3c%3balert(1)//9068c272512 was submitted in the mode parameter. This input was echoed as a3c3c;alert(1)//9068c272512 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.
The value of the mode request parameter is copied into a JavaScript string which is encapsulated in single quotation marks. The payload db776'%3balert(1)//fd860d356c6 was submitted in the mode parameter. This input was echoed as db776';alert(1)//fd860d356c6 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.
The value of the c0-param0 request parameter is copied into the HTML document as plain text between tags. The payload 84d45<script>alert(1)</script>345b521cb4c was submitted in the c0-param0 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.
HTTP/1.1 200 OK Date: Tue, 23 Aug 2011 19:41:09 GMT Server: Apache-Coyote/1.1 X-JAL: 2 Content-Type: text/javascript Vary: Accept-Encoding,User-Agent X-JSL: D=209321 t=1314128468997584 Content-Length: 269 Connection: close
//#DWR-INSERT //#DWR-REPLY dwr.engine._remoteHandleException('0','0',{javaClassName:"org.directwebremoting.extend.MarshallException",message:"Error marshalling int: Format error converting 3884d45<script>alert(1)</script>345b521cb4c. See the logs for more details."});
The value of the c0-param1 request parameter is copied into the HTML document as plain text between tags. The payload c203a<script>alert(1)</script>2079d88aab5 was submitted in the c0-param1 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.
HTTP/1.1 200 OK Date: Tue, 23 Aug 2011 19:41:09 GMT Server: Apache-Coyote/1.1 X-JAL: 2 Content-Type: text/javascript Vary: Accept-Encoding,User-Agent X-JSL: D=66601 t=1314128469556963 Content-Length: 272 Connection: close
//#DWR-INSERT //#DWR-REPLY dwr.engine._remoteHandleException('0','0',{javaClassName:"org.directwebremoting.extend.MarshallException",message:"Error marshalling long: Format error converting 3776c203a<script>alert(1)</script>2079d88aab5. See the logs for more details."});
The page contains a form with the following action URL, which is submitted over clear-text HTTP:
http://community.jboss.org/cs_login
The form contains the following password field:
password
Issue background
Passwords submitted over an unencrypted connection are vulnerable to capture by an attacker who is suitably positioned on the network. This includes any malicious party located on the user's own network, within their ISP, within the ISP used by the application, and within the application's hosting infrastructure. Even if switched networks are employed at some of these locations, techniques exist to circumvent this defence and monitor the traffic passing through switches.
Issue remediation
The application should use transport-level encryption (SSL or TLS) to protect all sensitive communications passing between the client and the server. Communications that should be protected include the login mechanism and related functionality, and any functions where sensitive data can be accessed or privileged actions can be performed. These areas of the application should employ their own session handling mechanism, and the session tokens used should never be transmitted over unencrypted communications. If HTTP cookies are used for transmitting session tokens, then the secure flag should be set to prevent transmission over clear-text HTTP.
Request
GET /blogs/marek-novotny/2011/04/27/seam-222final-security-release HTTP/1.1 Host: community.jboss.org Proxy-Connection: keep-alive Referer: http://seamframework.org/ User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/535.1 (KHTML, like Gecko) Chrome/13.0.782.112 Safari/535.1 Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8 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
Server-side source code may contain sensitive information which can help an attacker formulate attacks against the application.
Issue remediation
Server-side source code is normally disclosed to clients as a result of typographical errors in scripts or because of misconfiguration, such as failing to grant executable permissions to a script or directory. You should review the cause of the code disclosure and prevent it from happening.
The response dynamically includes the following scripts from other domains:
http://w.sharethis.com/button/buttons.js
http://www.redhat.com/j/elqNow/elqCfg.js
http://www.redhat.com/j/elqNow/elqImg.js
http://www.redhat.com/j/s_code.js
Issue background
When an application includes a script from an external domain, this script is executed by the browser within the security context of the invoking application. The script can therefore do anything that the application's own scripts can do, such as accessing application data and performing actions within the context of the current user.
If you include a script from an external domain, then you are trusting that domain with the data and functionality of your application, and you are trusting the domain's own security to prevent an attacker from modifying the script to perform malicious actions within your application.
Issue remediation
Scripts should not be included from untrusted domains. If you have a requirement which a third-party script appears to fulfil, then you should ideally copy the contents of that script onto your own domain and include it from there. If that is not possible (e.g. for licensing reasons) then you should consider reimplementing the script's functionality within your own code.
Request
GET /blogs/marek-novotny/2011/04/27/seam-222final-security-release HTTP/1.1 Host: community.jboss.org Proxy-Connection: keep-alive Referer: http://seamframework.org/ User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/535.1 (KHTML, like Gecko) Chrome/13.0.782.112 Safari/535.1 Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8 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
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 /blogs/marek-novotny/2011/04/27/seam-222final-security-release HTTP/1.1 Host: community.jboss.org Proxy-Connection: keep-alive Referer: http://seamframework.org/ User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/535.1 (KHTML, like Gecko) Chrome/13.0.782.112 Safari/535.1 Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8 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
The response contains multiple Content-type statements which are incompatible with one another. The following statements were received:
Content-Type: application/x-javascript
text/html; charset=UTF-8
Issue background
If a web response specifies multiple incompatible content types, then the browser will usually analyse the response and attempt to determine the actual MIME type of its content. This can have 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 multiple incompatible content type statements 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.
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 XML.
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.
