Revision as of 22:36, 13 December 2010

Overview

The most important aspect of deploying OWASP CSRFGuard is configuration of the Owasp.CsrfGuard.properties file. There are a minimum number of configuration settings that users should review and specify before running an instance of OWASP CSRFGuard. Such configurations include specifying the new token landing page, enabling Ajax support for applications making use of XMLHttpRequest, capturing pages that should not be protected, as well as configuring one or more actions that should be invoked when a CSRF attack is identified. The purpose of this article is to provide an overview of key OWASP CSRFGuard configuration settings.

Logger

The logger property (org.owasp.csrfguard.Logger) defines the qualified class name of the object responsible for processing all log messages produced by CSRFGuard. The default CSRFGuard logger is org.owasp.csrfguard.log.ConsoleLogger. This class logs all messages to System.out which JavaEE application servers redirect to a vendor specific log file. Developers can customize the logging behavior of CSRFGuard by implementing the org.owasp.csrfguard.log.ILogger interface and setting the logger property to the new logger's qualified class name. The following configuration snippet instructs OWASP CSRFGuard to capture all log messages to the console:

org.owasp.csrfguard.Logger=org.owasp.csrfguard.log.ConsoleLogger

New Token Landing Page

The new token landing page property (org.owasp.csrfguard.NewTokenLandingPage) defines where to send a user if the token is being generated for the first time. CSRFGuard will redirect the user to the current page without any parameters if the property is not specified. Preventing the protected application from consuming a request whose session does not yet have a CSRF token through the use of a redirect prevents the execution of a one-time CSRF attack. The following configuration snippet instructs OWASP CSRFGuard to redirect the user to /Owasp.CsrfGuard.Test/index.html when they visit a protected resource without having a corresponding CSRF token present in the HttpSession object:

org.owasp.csrfguard.NewTokenLandingPage=/Owasp.CsrfGuard.Test/index.html

Unique Token Per Page

The unique token per-page property (org.owasp.csrfguard.TokenPerPage) is a boolean value that determines if CSRFGuard should make use of unique per-page (i.e. URI) prevention tokens as opposed to unique per-session prevention tokens. When a user requests a protected resource, CSRFGuard will determine if a page specific token has been previously generated. If a page specific token has not yet been previously generated, CSRFGuard will verify the request was submitted with the per-session token intact. After verifying the presence of the per-session token, CSRFGuard will create a page specific token that is required for all subsequent requests to the associated resource. The per-session CSRF token can only be used when requesting a resource for the first time. All subsequent requests must have the per-page token intact or the request will be treated as a CSRF attack. Use of the unique token per page property is currently experimental but provides a significant amount of improved security. Consider the exposure of a CSRF token using the legacy unique per-session model. Exposure of this token facilitates the attacker's ability to carry out a CSRF attack against the victim's active session for any resource exposed by the web application. Now consider the exposure of a CSRF token using the experimental unique token per-page model. Exposure of this token would only allow the attacker to carry out a CSRF attack against the victim's active session for a small subset of resources exposed by the web application. Use of the unique token per-page property is a strong defense in depth strategy significantly reducing the impact of exposed CSRF prevention tokens. The following configuration snippet instructs OWASP CSRFGuard to utilize the unique token per-page model:

org.owasp.csrfguard.TokenPerPage=true

Token Rotation

The rotate token property (org.owasp.csrfguard.Rotate) is a boolean value that determines if CSRFGuard should generate and utilize a new token after verifying the previous token. Rotation helps minimize the window of opportunity an attacker has to leverage the victim's stolen token in a targeted CSRF attack. However, this functionality generally causes navigation problems in most applications. Specifically, the 'Back' button in the browser will often cease to function properly. When a user hits the 'Back' button and interacts with the HTML, the browser may submit an old token causing CSRFGuard to incorrectly believe this request is a CSRF attack in progress (i.e. a 'false positive'). Users can prevent this scenario by preventing the caching of HTML pages containing FORM submissions using the cache-control header. However, this may also introduce performance problems as the browser will have to request HTML on a more frequent basis. The following configuration snippet disables token rotation:

org.owasp.csrfguard.Rotate=false

Ajax and XMLHttpRequest Support

The Ajax property (org.owasp.csrfguard.Ajax) is a boolean value that indicates whether or not OWASP CSRFGuard should support the injection and verification of unique per-session prevention tokens for XMLHttpRequests. To leverage Ajax support, the user must not only set this property to true but must also reference the JavaScript DOM Manipulation code using a script element. This dynamic script will override the send method of the XMLHttpRequest object to ensure the submission of an X-Requested-With header name value pair coupled with the submission of a custom header name value pair for each request. The name of the custom header is the value of the token name property and the value of the header is always the unique per-session token value. This custom header is analogous to the HTTP parameter name value pairs submitted via traditional GET and POST requests. If the X-Requested-With header was sent in the HTTP request, then CSRFGuard will look for the presence and ensure the validity of the unique per-session token in the custom header name value pair. Note that verification of these headers takes precedence over verification of the CSRF token supplied as an HTTP parameter. More specifically, CSRFGuard does not verify the presence of the CSRF token if the Ajax support property is enabled and the corresponding X-Requested-With and custom headers are embedded within the request. The following configuration snippet instructs OWASP CSRFGuard to support Ajax requests by verifying the presence and correctness of the X-Requested-With and custom headers:

org.owasp.csrfguard.Ajax=true

Referrer Checking

The check referrer header property (org.owasp.csrfguard.Referrer) verifies that the supplied referrer header matches a regular expression. When enabled, verification of the referrer header will take precedence over verification of the CSRF token in the HTTP request. Referrer headers are consistently sent by browsers for pure HTTPS communications. Verification of the referrer header is a strong defense against CSRF attacks when consistently using HTTPS as a CSRF attack will not include this header in the request(s) unless the malicious HTML is embedded within the site for which the attack is targeted. Inclusion of the referrer header is less consistent when using traditional HTTP traffic across browsers. CSRFGuard will fall back to verifying the presence of the CSRF token if the referrer header was not included in the HTTP request. The following code snippet instructs OWASP CSRFGuard to verify that the referrer header matches the regular expression .*localhost.*, ensuring the request came from a local resource:

org.owasp.csrfguard.Referrer=.*localhost.*

Unprotected Pages

The unprotected pages property (org.owasp.csrfguard.unprotected.*) defines a series of pages that should not be protected by CSRFGuard. Such configurations are useful when the CsrfGuardFilter is aggressively mapped (ex: /*). The syntax of the property name is org.owasp.csrfguard.unprotected.[PageName], where PageName is some arbitrary identifier that can be used to reference a resource. The syntax of defining the uri of unprotected pages is the same as the syntax used by the JavaEE container for uri mapping. Specifically, CSRFGuard will identify the first match (if any) between the requested uri and an unprotected page in order of declaration. Match criteria is as follows:

Case 1: exact match between request uri and unprotected page

Case 2: longest path prefix match, beginning / and ending /*

Case 3: extension match, beginning *.

Default: requested resource must be validated by CSRFGuard

The following code snippet illustrates the three use cases over four examples. The first two examples (Tag and JavaScriptServlet) look for direct URI matches. The third example (Html) looks for all resources ending in a .html extension. The last example (Public) looks for all resources prefixed with the URI path /MySite/Public/*.

org.owasp.csrfguard.unprotected.Tag=/Owasp.CsrfGuard.Test/tag.jsp
org.owasp.csrfguard.unprotected.JavaScriptServlet=/Owasp.CsrfGuard.Test/JavaScriptServlet
org.owasp.csrfguard.unprotected.Html=*.html
org.owasp.csrfguard.unprotected.Public=/MySite/Public/*

Actions: Responding to Attacks

The actions directive (org.owasp.csrfguard.action.*) gives the user the ability to specify one or more actions that should be invoked when a CSRF attack is detected. Every action must implement the org.owasp.csrfguard.action.IAction interface either directly or indirectly through the org.owasp.csrfguard.action.AbstractAction helper class. Many actions accept parameters that can be specified along with the action class declaration. These parameters are consumed at runtime and impact the behavior of the associated action.

The syntax for defining and configuring CSRFGuard actions is relatively straight forward. Let us assume we wish to redirect the user to a default page when a CSRF attack is detected. A redirect action already exists within the CSRFGuard bundle and is available via the class name org.owasp.csrfguard.actions.Redirect. In order to enable this action, we capture the following declaration in the Owasp.CsrfGuard.properties file:

syntax: org.owasp.csrfguard.action.[actionName]=[className]
example: org.owasp.csrfguard.action.class.Redirect=org.owasp.csrfguard.actions.Redirect

The aforementioned directive declares an action called "Redirect" (i.e. [actionName]) referencing the Java class "org.owasp.csrfguard.actions.Redirect" (i.e. [className]). Anytime a CSRF attack is detected, the Redirect action will be executed. You may be asking yourself, "but how do I specify where the user is redirected?"; this is where action parameters come into play. In order to specify the redirect location, we capture the following declaration in the Owasp.CsrfGuard.properties file:

syntax: org.owasp.csrfguard.action.[actionName].[parameterName]=[parameterValue]
example: org.owasp.csrfguard.action.Redirect.ErrorPage=/Owasp.CsrfGuard.Test/error.html

The aforementioned directive declares an action parameter called "ErrorPage" (i.e. [parameterName]) with the value of "/Owasp.CsrfGuard.Test/error.html" (i.e. [parameterValue]) for the action "Redirect" (i.e. [actionName]). The Redirect action expects the "ErrorPage" parameter to be defined and will redirect the user to this location when an attack is detected.

There are several "out of the box" actions made available with the OWASP CSRFGuard distributable.

org.owasp.csrfguard.action.Log

Creates a customized log message whenever a CSRF attack is detected by OWASP CSRFGuard. The log message is captured using the logger mechanism defined by the org.owasp.csrfguard.Logger directive in the Owasp.CsrfGuard.properties file. Log accepts the following action parameters:

org.owasp.csrfguard.action.Log.Message

Allows the user to define the format of the log message to be recorded. The Message parameter supports several formatting options to aide in the customization of the message to the particular attack. The following formatting options are supported:

%exception - String representation of the CsrfGuardException thrown by the CsrfGuard class.
%exception_message - Localized exception message of the class CsrfGuardException thrown by the CsrfGuard class.
%remote_ip - Remote IP address of the client that sent the CSRF attack to the server.
%remote_host - Remote host name of the client that sent the CSRF attack to the server.
%remote_port - Remote port of the client that sent the CSRF attack to the server.
%local_ip - Local IP address of the server that detected the CSRF attack.
%local_host - Local host name of the server that detected the CSRF attack.
%local_port - Local port of the server that detected the CSRF attack.
%request_uri - Request URI for which the CSRF attack was targeting.
%request_url - Request URL for which the CSRF attack was targeting.
%user - User information as made available by the javax.servlet.http.HttpServletRequest.getRemoteUser() method invocation.

org.owasp.csrfguard.action.Invalidate

Invalidate any existing HttpSession associated with the current HttpServletRequest. Note that this is the session of the victim for which the CSRF attack was targeting. Invalidating the JavaEE container's session generally results in the user having to re-authenticate. There are no action parameters associated with this action.

org.owasp.csrfguard.action.Redirect

Redirects the user to the URI specified in the Page action parameter. Attempting to utilize this action in combination with Forward will generally result in the JavaEE container throwing IllegalStateException. The action accepts the following action parameters:

org.owasp.csrfguard.action.Redirect.Page

Defines the URI for which the user is redirected when a CSRF attack is detected.

org.owasp.csrfguard.action.Forward

Forwards the user to the URI specified in the Page action parameter. Attempting to utilize this action in combination with Redirect will generally result in the JavaEE container throwing IllegalStateException. The action accepts the following action parameters:

org.owasp.csrfguard.action.Forward.Page

Defines the URI for which the user is forwarded when a CSRF attack is detected.

org.owasp.csrfguard.action.RequestAttribute

Makes the CsrfGuardException thrown by the CsrfGuard class when an attack is detected programmatically available as an attribute in the HttpServletRequest object. The action accepts the following action parameters:

org.owasp.csrfguard.action.RequestAttribute.AttributeName

Defines the attribute name that should be used when placing the CsrfGuardException in the HttpServletRequest attributes collection.

org.owasp.csrfguard.action.SessionAttribute

Makes the CsrfGuardException thrown by the CsrfGuard class when an attack is detected programmatically available as an attribute in the HttpSession object. The action accepts the following action parameters:

org.owasp.csrfguard.action.SessionAttribute.AttributeName

Defines the attribute name that should be used when placing the CsrfGuardException in the HttpSession attributes collection.

Miscellaneous Configurations

Token Name

The token name property (org.owasp.csrfguard.TokenName) defines the name of the HTTP parameter to contain the value of the OWASP CSRFGuard token for each request. The following configuration snippet sets the CSRFGuard token parameter name to the value OWASP_CSRFTOKEN:

org.owasp.csrfguard.TokenName=OWASP_CSRFTOKEN

Session Key

The session key property (org.owasp.csrfguard.SessionKey) defines the string literal used to save and lookup the CSRFGuard token from the session. This value is used by the filter and the tag libraries to retrieve and set the token value in the session. Developers can use this key to programmatically lookup the token within their own code. The following configuration snippet sets the session key to the value OWASP_CSRFTOKEN:

org.owasp.csrfguard.SessionKey=OWASP_CSRFTOKEN

Token Length

The token length property (org.owasp.csrfguard.TokenLength) defines the number of characters that should be found within the CSRFGuard token. Note that characters are delimited by dashes (-) in groups of four. For cosmetic reasons, users are encourage to ensure the token length is divisible by four. The following configuration snippet sets the token length property to 32 characters:

org.owasp.csrfguard.TokenLength=32

Pseudo-Random Number Generator

The pseudo-random number generator property (org.owasp.csrfguard.PRNG) defines what PRNG should be used to generate the OWASP CSRFGuard token. Always ensure this value references a cryptographically strong pseudo-random number generator algorithm. The following configuration snippet sets the pseudo-random number generator to SHA1PRNG:

org.owasp.csrfguard.PRNG=SHA1PRNG