Creating a CSRF protection with Spring 3.1

CSRF Attacks still seems to be a problem, a pity that there is no standard solution in the Spring 3.1 framework. Although not probably, i wanted to protect my projects by malicious crafted links.

I didn’t want to use an extra library but something which is already available in the Spring framework. Here is what i come up with:

I choose the token protection mechanism for my implementation.

The core of my solution is the CSRFToken Service:

public interface CSRFTokenService {
	public final static String TOKEN_PARAMETER_NAME = "_tk";
 
	public final static String TOKEN_ATTRIBUTE_NAME = "CSRFToken";
 
	public final static List<String> METHODS_TO_CHECK = Collections.unmodifiableList(Arrays.asList("POST", "PUT", "DELETE"));
 
	/** Generates a new CSRF Protection token */
	public String generateToken();
 
	/** Obtains the token from the session. If there is no token, a new one will be generated */
	public String getTokenFromSession(final HttpServletRequest request);
 
	/** This method tests, if a token is acceptable when a user is logged in */
	public boolean acceptsTokenIn(HttpServletRequest request);
}

import java.security.SecureRandom;
 
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpSession;
 
import org.apache.commons.codec.binary.Base64;
import org.apache.commons.lang.StringUtils;
import org.springframework.stereotype.Service;
 
import de.dailyfratze.services.CSRFTokenService;
 
@Service("csrfTokenService")
public class CSRFTokenServiceImpl implements CSRFTokenService {
	private final SecureRandom random = new SecureRandom();
 
	@Override
	public String generateToken() {
		final byte[] bytes = new byte[32];
		random.nextBytes(bytes);
		return Base64.encodeBase64URLSafeString(bytes);
	}
 
	@Override
	public String getTokenFromSession(final HttpServletRequest request) {
		return request.getUserPrincipal() == null ? null : this.getTokenFromSessionImpl(request.getSession(false));
	}
 
	private String getTokenFromSessionImpl(final HttpSession session) {
		String token = null;
 
		if(session != null) {
			token = (String) session.getAttribute(TOKEN_ATTRIBUTE_NAME);
			if(StringUtils.isBlank(token))
				session.setAttribute(TOKEN_ATTRIBUTE_NAME, (token = generateToken()));
		}
		return token;
	}
 
	@Override
	public boolean acceptsTokenIn(HttpServletRequest request) {
		boolean rv = false;
 
		// Token is only verified if principal is not null
		if(request.getUserPrincipal() == null) 
			rv = true;
		else {
			final HttpSession session = request.getSession(false);
			rv = session != null && this.getTokenFromSessionImpl(session).equals(request.getParameter(TOKEN_PARAMETER_NAME));
		}
		return rv;
	}
}

“getTokenFromSession” is called right after a user logs in, so that the token gets stored into his session.

As you can see in the implementation of “acceptsTokenIn”, the token is only needed and verified when the principal is not null, meaning when a user is authenticated.

The interface contains some constants: The name of the token in forms and requests and the name of the attribute under which the token is stored in the session. The token itself is just a base64 of some random bytes.

I only want the token to be checked in writing methods: METHODS_TO_CHECK, meaning only in put, delete and posts requests. My applications don’t change state based on get requests.

So where to check for the token? I use a pretty simple Spring “HandlerInterceptor”:

package de.dailyfratze.controller;
 
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
 
import org.apache.commons.lang.StringUtils;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.servlet.HandlerInterceptor;
import org.springframework.web.servlet.ModelAndView;
 
import de.dailyfratze.services.CSRFTokenService;
 
public class CSRFInterceptor implements HandlerInterceptor { 
	@Autowired
	private CSRFTokenService csrfTokenService;
 
	@Override
	public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
		boolean rv = true;
		if(CSRFTokenService.METHODS_TO_CHECK.contains(StringUtils.defaultIfBlank(request.getMethod(), "").toUpperCase()) && !csrfTokenService.acceptsTokenIn(request)) {
			response.addHeader("X-DailyFratze-InvalidCSRFToken", Boolean.toString(true));
			response.sendError(HttpServletResponse.SC_FORBIDDEN);
			rv = false;
		} 		
		return rv;
	}
 
	@Override
	public void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView) throws Exception {	
	}
 
	@Override
	public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception {		
	}		
}

This interceptor stops the chain if the request method should be checked and the token is not acceptable by sending a HTTP forbidden error. The additional response header is used by Ajax calls to present a dialog that the session is invalidated.

How to get the token into forms? I wanted to be able to change the token name in only one place so i came up with the following custom tag:

import java.io.IOException;
 
import javax.servlet.http.HttpServletRequest;
import javax.servlet.jsp.JspException;
import javax.servlet.jsp.tagext.TagSupport;
 
import org.apache.commons.lang.StringUtils;
 
import de.dailyfratze.services.CSRFTokenService;
import de.dailyfratze.utils.HelperRegistry;
 
/**
 * Creates a hidden input field with the CSRF Token
 * @author michael.simons, 2011-09-20
 */
public class CSRFTokenTag extends TagSupport {
	private static final long serialVersionUID = 745177955805541350L;
 
	private boolean plainToken = false;
 
	@Override
	public int doStartTag() throws JspException {
		final CSRFTokenService csrfTokenService = HelperRegistry.getHelper(super.pageContext.getServletContext(), super.pageContext.getRequest(), CSRFTokenService.class, "csrfTokenService");
		final String token = csrfTokenService.getTokenFromSession((HttpServletRequest) super.pageContext.getRequest());
		if(!StringUtils.isBlank(token))
			try {
				if(plainToken)
					pageContext.getOut().write(token);
				else
					pageContext.getOut().write(String.format("<input type=\"hidden\" name=\"%1$s\" id=\"%1$s\" value=\"%2$s\" />", CSRFTokenService.TOKEN_PARAMETER_NAME, token));
			} catch (IOException e) {
			}
		return SKIP_BODY;
	}
 
	@Override
	public int doEndTag() throws JspException {
		return EVAL_PAGE;
	}
 
	public boolean isPlainToken() {
		return plainToken;
	}
 
	public void setPlainToken(boolean plainToken) {
		this.plainToken = plainToken;
	}
 
	public static String getTokenParameterName() {
		return CSRFTokenService.TOKEN_PARAMETER_NAME;
	}
}

with the corresponding mapping:

<?xml version="1.0" encoding="UTF-8" ?>
<taglib xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-jsptaglibrary_2_1.xsd"
	version="2.1">
	<tlib-version>1.0</tlib-version>
	<short-name>df</short-name>
	<uri>http://michael-simons.eu/taglibs/df</uri>
	<tag>
		<name>csrfToken</name>
		<tag-class>de.dailyfratze.tags.CSRFTokenTag</tag-class>
		<body-content>empty</body-content>
		<attribute>
			<name>plainToken</name>
			<required>false</required>	
		</attribute>
	</tag>	
	<function>
		<name>csrfTokenParameter</name>
		<function-class>de.dailyfratze.tags.CSRFTokenTag</function-class>
		<function-signature>java.lang.String getTokenParameterName()</function-signature>
	</function>
</taglib>

I can use this tag in forms like so:

<form method="post" action="foobar">
   <df:csrfToken />
</form>

Or for generating url parameters for example for ajax calls like so:

<c:url value="/foobar">
  <c:param name="${df:csrfTokenParameter()}">
    <df:csrfToken plainToken="true"/>
  </c:param>
</c:url>

So if a token is invalid, the user is either redirect to an error page if it is a normal post, ajax calls through jQuery can be handled like so:

function isInvalidCSRFToken = function(xhr) {
	var rv = false;
	if(xhr.status == 403 && xhr.getResponseHeader('X-DailyFratze-InvalidCSRFToken') == 'true') {			
		alert($('Session is invalid').text());
		rv = true;
	}
	return rv;
}
 
$.ajax({
	type: 'post',
	url: theUrl,	
	dataType: 'text',		
	complete: function(xhr, status) {
		if(isInvalidCSRFToken(xhr))	    				
			return;	    				
		// handle the result
	} 	        
});

The code snippets are all taken from a running project. If you want to use them, use them. The package names are missing and must be added. Also the JavaScript code isn’t complete.

Feel free to comment, if you have suggestions, remarks or anything else. Also, if you can use this, i’d be happy to hear from you.

info.michael-simons.eu

Creating a CSRF protection with Spring 3.1

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