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CSRF Guard
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Overview
Just when developers are starting to run in circles over Cross Site Scripting Flaw, the 'sleeping giant' awakes for yet another web-catastrophe. Cross-Site Request Forgery (CSRF) is an attack whereby the victim is tricked into loading information from or submitting information to a web application for which they are currently authenticated. The problem is that the web application has no means of verifying the integrity of the request.
Recommended Prevention Measure: Unique Request Tokens
The core issue with CSRF attacks is that form submission can be imitated with forged requests. The application must be able to differentiate between legal requests and forged requests. Since all headers, cookies, and credentials will be submitted with both legal and forged requests, the only method of truly verifying the integrity of the request is with a uniquely identifiable token in the form of an HTTP parameter. When the user first visits the site, the application will generate and store a session specific unique request token. This session specific unique request token is then placed in each form and link of the HTML response, ensuring that this value will be submitted with the next request. For each subsequent request, the application must verify the existence of the unique token parameter and compare its value to that of the value stored in the user's session. The security of the approach is based on the fact that this unique token value is specific to a user's session and is hard to guess. Therefore, it is imperative that this value is large and cryptographically secure.
- UPDATE: There have been discussions suggesting that the unique request token can be compromised using JavaScript. This attack implies that the application also contains a Cross-Site scripting vulnerability, which is a more severe issue than Cross-Site request forgery. The first Myspace worm worked in this manner where it used a Cross Site Scripting vulnerability to forge requests to update a user's profile, where the user profile update mechanism was protected with an antiCSRF defense mechanism similar to what is provided by this filter. You can mitigate this risk somewhat by making your form key AND value a CSRF Guard-type token.
Example: The Java CSRF Guard
Java EE Filter
Java EE filters provide the ability to intercept, view, and modify both the request and associated response for the requesting client. Filters are inserted and executed by the Java EE container's deployment descriptor (web.xml) file. For example, if an HTTP request for a JSP page hits our Apache web server, the request is sent to Tomcat for processing. Before Tomcat executes the code inside of the JSP, the request must be passed along a chain of Java EE filters. The following snippet illustrates how to declare and map a filter to a particular URI-space in web.xml:
<filter>
<filter-name>CSRFGuard</filter-name>
<filter-class>org.owasp.csrf.CSRFGuard</filter-class>
<init-param>
<param-name>error-page</param-name>
<param-value>error.jsp</param-value>
</init-param>
</filter>
<filter-mapping>
<filter-name>CSRFGuard</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
Implementing the CSRF Guard as a Java EE Filter gives us the ability to verify the integrity of the request before it ever hits our web application.
Secure Random
When implementing the CSRF Guard, we must ensure that the unique request token is cryptographically strong. After all, our implementation relies on the principle that the unique token is difficult to guess. If the unique request token can be easily guessed then a CSRF attack can be executed.
The following code snippet generates a BASE64 encoded string of 'size' bytes:
private String generateCSRFToken(int size) {
SecureRandom sr = null;
byte[] random = new byte[size];
BASE64Encoder encoder = new BASE64Encoder();
String digest = null;
try {
sr = SecureRandom.getInstance("SHA1PRNG");
sr.nextBytes(random);
digest = encoder.encode(random);
} catch (Exception e) {
e.printStackTrace();
}
return digest;
}
Implementation
The following Java EE Filter will attempt to verify the integrity of the request by comparing the OWASP_CSRFTOKEN HTTP parameter value with that of the OWASP_CSRFTOKEN session attribute. If the two values do not match, then the request is forged and we invoked the doError() method. This method will invalidate the existing session and redirect the user to a page specified by the filter init-parameter "error-page". If the parameter value equals the corresponding session attribute value, then we call doChain() and pass the request to the webapplication. Once the web application is finished processing the request, the buildLinkParameters() method will search the HTML response for forms and links and insert the appropriate OWASP_CSRFTOKEN parameter value. Unfortunately, the dependency on an HTML response means that the current filter may not work with some Web 2.0 applications. Furthermore, the filter only modifies response objects whose "Content-Type" header is text based. If your web application neglects to set the appropriate value for the "Content-Type" header, then the CSRF Guard will not make the appropriate changes to the HTML response and each subsequent request will be considered a violation.
/*
* CSRFGuard.java
*
* Created on January 2, 2007, 11:35 AM
*
* Copyright (C) 2007 Eric Sheridan
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation; either
* version 2.1 of the License, or (at your option) any later version.
*
* This library is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this library; if not, write to the Free Software
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
*/
package org.owasp.csrf;
import java.io.OutputStream;
import java.io.IOException;
import java.security.SecureRandom;
import java.util.regex.Pattern;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpSession;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import sun.misc.BASE64Encoder;
import org.owasp.csrf.http.MutableHttpResponse;
/**
*
* @author esheridan
*/
public class CSRFGuard implements Filter {
public final static String OWASP_CSRFTOKEN = "OWASP_CSRFTOKEN";
public final static Pattern FORM_PATTERN = Pattern.compile("(?i)</form>");
public final static Pattern SKIPPABLE_PATTERN = Pattern.compile(".*\\.(gif|jpg|png|css|js|ico|swf|axd.*)$");
private String errorPage = null;
public void init(FilterConfig config) {
errorPage = config.getInitParameter("error-page");
}
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) {
if(request instanceof HttpServletRequest && response instanceof HttpServletResponse) {
try {
HttpServletRequest hRequest = (HttpServletRequest)request;
MutableHttpResponse mResponse = new MutableHttpResponse((HttpServletResponse)response);
HttpSession session = hRequest.getSession(false);
if(session != null) {
if(!hasCSRFToken(session)) {
setCSRFToken(session);
doChain(hRequest, mResponse, response, chain);
} else if(isSkippable(hRequest) || isValidRequest(session, hRequest)) {
doChain(hRequest, mResponse, response, chain);
} else {
doError(session, (HttpServletResponse)response);
}
} else {
chain.doFilter(request, response);
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
public void destroy() {
}
private void doError(HttpSession session, HttpServletResponse response) throws IOException {
session.invalidate();
response.sendRedirect(errorPage);
}
private void doChain(HttpServletRequest request, MutableHttpResponse mResponse, ServletResponse response, FilterChain chain) throws IOException, ServletException {
OutputStream out = response.getOutputStream();
chain.doFilter(request, mResponse);
String contentType = mResponse.getContentType();
String token = getCSRFToken(request.getSession());
if(contentType != null && contentType.startsWith("text")) {
String content = new String(mResponse.getContent());
content = getModifiedResponse(token, content);
byte[] result = content.getBytes();
response.setContentLength(result.length);
out.write(result);
} else {
out.write(mResponse.getContent());
}
out.close();
}
private String getModifiedResponse(String token, String content) {
content = buildFormParameters(content, token);
content = buildLinkParameters(content, token);
return content;
}
private String getCSRFToken(HttpSession session) {
return (String)session.getAttribute(OWASP_CSRFTOKEN);
}
private boolean hasCSRFToken(HttpSession session) {
return getCSRFToken(session) != null;
}
private void setCSRFToken(HttpSession session) {
session.setAttribute(OWASP_CSRFTOKEN, generateCSRFToken());
}
private String generateCSRFToken() {
return generateCSRFToken(32);
}
private String generateCSRFToken(int size) {
SecureRandom sr = null;
byte[] random = new byte[size];
BASE64Encoder encoder = new BASE64Encoder();
String digest = null;
try {
sr = SecureRandom.getInstance("SHA1PRNG");
sr.nextBytes(random);
digest = encoder.encode(random).replace('+', '_');
} catch (Exception e) {
e.printStackTrace();
}
return digest;
}
private boolean isSkippable(HttpServletRequest request) {
return SKIPPABLE_PATTERN.matcher(request.getRequestURI()).matches();
}
private boolean isValidRequest(HttpSession session, HttpServletRequest request) {
String original = (String)session.getAttribute(OWASP_CSRFTOKEN);
String now = (String)request.getParameter(OWASP_CSRFTOKEN);
boolean result = false;
if(now != null && now.equals(original)) {
result = true;
}
return result;
}
private String buildFormParameters(String content, String token) {
return FORM_PATTERN.matcher(content).replaceAll("<input type=\"hidden\" name=\"" + OWASP_CSRFTOKEN + "\" value=\"" + token + "\">\n</form>");
}
private String buildLinkParameters(String content, String token) {
content = buildLinkParameters(content, token, "href=\"", '"');
content = buildLinkParameters(content, token, "src=\"", '"');
return content;
}
private String buildLinkParameters(String content, String token, String pattern, char termChar) {
StringBuffer buffer = new StringBuffer();
int i=0;
int index = 0;
int length = content.length();
while(i < length) {
index = content.toLowerCase().indexOf(pattern, i);
if(index != -1) {
int offset = 0;
int n = 0;
boolean parameters = false;
String tokenString = null;
buffer.append(content.substring(i, index+pattern.length()));
for(n=index+pattern.length(), offset=n; n<=length && content.charAt(n) != termChar; n++) {
buffer.append(content.charAt(n));
if(content.charAt(n) == '?') {
parameters = true;
}
}
if(parameters) {
tokenString = "&" + OWASP_CSRFTOKEN + "=" + token;
} else {
tokenString = "?" + OWASP_CSRFTOKEN + "=" + token;
}
buffer.append(tokenString);
i = index + pattern.length() + (n - offset);
} else {
buffer.append(content.substring(i, length));
i = length;
}
}
return buffer.toString();
}
}
Download
The Proof-of-Concept implementation of the Cross-Site Request Forgery Guard discussed in this article can be downloaded here: CSRF_Guard.zip
Related Projects
A Proof-of-Concept implementation of the CSRF Guard for the PHP platform is under development: PHP CSRF Guard
A Proof-of-Concept implementation of the CSRF Guard for the ASP.Net platform (as an ASP.Net Module/Filter) is under development as part of the OWASP .Net project toolset. .Net CSRF Guard
