1. Vulnerability Description

• CSRF is an attack that forces an Internet user to request an attacker’s intended behavior (modification, deletion, registration, etc.) to a particular website regardless of their will.
  If a user request is processed by the server without checking whether it is actually delivered through the service, the request generated by a well-written script is made with the user’s privileges.
• Cross Site Request Forgery is based on the following scenarios:
  These attacks allow you to automatically write bulletin boards, subscribe automatically, and steal other users’ rights to execute important functions.
  Also, it is possible to write advertising comments on other people’s blogs.

• CSRF attacks are possible in environments where the following conditions are met:
  • Services that have content manipulative functions such as write/change (Reading function is not subject to CSRF vulnerability)
  • Victim is logged in to the service that sends forged requests
  • Victim accesses pages with attack codes written by the attacker

2. How to check vulnerability

• A CSRF attack usually generates an HTTP request using the following tags, which are triggered by Action through the GET Method:

<img src=‘CRSF attack code />
<script src=…. />
<iframe src=…… />
<frame src=…… />
<meta ….. />

• In addition to this, CSRF attack that generates POST method through Javascript technique is possible as follows.

<form name="frm" method="POST" … > …</form>      <script>frm.submit();</script>
<script> var req = new ActiveXObject("Microsoft.XMLHTTP");     req.open("POST",  …);     req.send(null); </script>

3. Vulnerability Countermeasure

3.1. CSRF Countermeasures

• Only image extensions such as jpg, gif, png, and bmp are allowed for values entered in the tag’s src property.
• When writing, create Token in the requested file and check Token validity in the Action file.
• All action files, such as writing bulletin boards and changing settings, use POST Method and check Referer.
• When checking Referer, check whether to start http://www.target.com/, not including www.target.com.

3.2. CSRF solutions and its problems

Countermeasure	Purpose	Vulnerability / Problem	Note
Using POST Method	Defend against an attack code by GET query	<form id=‘frm’ method=‘POST’ action=‘CSRF attack code’ ></form>	- Unable to defend attacks using form.submit() - Unable to defend AJAX queries where requests occur
Referrer Verification	POST Method Vulnerability Alternative	Most attack codes can be defended only by referrer check	- Problems exist in browsers that do not deliver referrer values in Web plugin such as flash. - It can be occured the web browser dependencies and Mobile program compatibility problem.
Using Security Token	Referrer Problem Alternative	Requires review of vulnerabilities resulting from the creation/issuance/verification/disposal of Security Token	- Creating/issuing/verifying/disabling security Token is key - Using security Token on the Web - Using security Token in web plugin such as flash - Using security Token in mobile(independent) programs
Secondary Verification (CAPCHA, Password Authentication)	Inserts a secondary validation page for user awareness	–	–

3.3. Countermeasures for each use environment

• Among the defense techniques against CSRF attacks, the recommended countermeasures are arranged in order of safety.

4. Sample Code

4.1. Creating a CSRF token

// Any type of request may have a basically CSRF vulnerability.
 
// generate a CSRF token when logging in or requesting a work screen and save it to the session.
session.setAttribute("CSRF_TOKEN",UUID.randomUUID().toString());
 
// When requesting work screen, attach CSRF token to the screen and send
<input type="hidden" id="csrf" name="csrf" value="${CSRF_TOKEN}" />

4.2. Check CSRF token value

// check the CSRF token value using intercept for POST method request.
if ( ! request.getMethod().equalsIgnoreCase("post")) {
    return true;
}
else {
    if( request instanceof MultipartHttpServletRequest) {
                Enumeration<String> names = request.getParameterNames();
                while( names.hasMoreElements()) {
                    String paramName=names.nextElement();
                    if( paramName.equals("csrf")) {
                            String paramValue=request.getParameter(paramName);
                            // If the CSRF value passed to the parameter and the CSRF_TOKEN value stored in the session are the same,
                            if ( paramValue.equals( request.getSession().getAttribute("CSRF_TOKEN"))) {
                                    // forward the request to the Controller.
                                    return true;
                            }
                            // If the CSRF_TOKEN value does not match, it is recognized as a CSRF attack and processed, or moved to the page used for the original request.
                            response.sendRedirect("write.do");
                            return false;
                    }
                }
        }
}