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 1 is copied into a JavaScript string which is encapsulated in single quotation marks. The payload 9f0f8'-alert(1)-'b9a0e02f466 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.
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 /user9f0f8'-alert(1)-'b9a0e02f466/freshface/portfolio HTTP/1.1 Host: themeforest.net 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 Server: nginx/0.7.65 Date: Sun, 09 Jan 2011 02:28:55 GMT Content-Type: text/html; charset=utf-8 Connection: close Status: 404 Not Found Content-Length: 20137 Set-Cookie: _fd_session=BAh7BjoPc2Vzc2lvbl9pZCIlZDcyYTU2NzhmYjAyMDIyZGUzNzBmZmFlYzk3OTFiMjk%3D--534caac76947ead77491853a9ba47b4217755cb6; path=/; expires=Tue, 08-Jan-2013 14:28:55 GMT; HttpOnly Cache-Control: no-cache
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> <head> <link href="h ...[SNIP]...
The value of REST URL parameter 2 is copied into a JavaScript string which is encapsulated in single quotation marks. The payload 3960e</script><script>alert(1)</script>3ad9a7ed78b 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.
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 /user/freshface3960e</script><script>alert(1)</script>3ad9a7ed78b/portfolio HTTP/1.1 Host: themeforest.net 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 Server: nginx/0.7.65 Date: Sun, 09 Jan 2011 02:29:00 GMT Content-Type: text/html; charset=utf-8 Connection: close Status: 404 Not Found Content-Length: 20159 Set-Cookie: _fd_session=BAh7BjoPc2Vzc2lvbl9pZCIlZTYwMDA5ZmYwMTEwNjA5Y2RmOGQ2NjE1N2U4ZDhlYWQ%3D--1d2e8164d05571132baa11a7ea7b5a052b5d5deb; path=/; expires=Tue, 08-Jan-2013 14:29:00 GMT; HttpOnly Cache-Control: no-cache
The page contains a form with the following action URL, which is submitted over clear-text HTTP:
http://themeforest.net/signin/authenticate
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 defense 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 /user/freshface/portfolio HTTP/1.1 Host: themeforest.net Accept: */* Accept-Language: en User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0) Connection: close
3. Password field with autocomplete enabledpreviousnext
Summary
Severity:
Low
Confidence:
Certain
Host:
http://themeforest.net
Path:
/user/freshface/portfolio
Issue detail
The page contains a form with the following action URL:
http://themeforest.net/signin/authenticate
The form contains the following password field with autocomplete enabled:
password
Issue background
Most browsers have a facility to remember user credentials that are entered into HTML forms. This function can be configured by the user and also by applications which employ user credentials. If the function is enabled, then credentials entered by the user are stored on their local computer and retrieved by the browser on future visits to the same application.
The stored credentials can be captured by an attacker who gains access to the computer, either locally or through some remote compromise. Further, methods have existed whereby a malicious web site can retrieve the stored credentials for other applications, by exploiting browser vulnerabilities or through application-level cross-domain attacks.
Issue remediation
To prevent browsers from storing credentials entered into HTML forms, you should include the attribute autocomplete="off" within the FORM tag (to protect all form fields) or within the relevant INPUT tags (to protect specific individual fields).
Request
GET /user/freshface/portfolio HTTP/1.1 Host: themeforest.net Accept: */* Accept-Language: en User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0) Connection: close
The page contains a form which POSTs data to the domain envato.us1.list-manage.com. The form contains the following fields:
FNAME
LNAME
EMAIL
subscribe
Issue background
The POSTing of data between domains does not necessarily constitute a security vulnerability. You should review the contents of the information that is being transmitted between domains, and determine whether the originating application should be trusting the receiving domain with this information.
Request
GET /user/freshface/portfolio HTTP/1.1 Host: themeforest.net Accept: */* Accept-Language: en User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0) Connection: close
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 /user/freshface/portfolio HTTP/1.1 Host: themeforest.net Accept: */* Accept-Language: en User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0) Connection: close