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SPRING FRAMEWORK RCE, EARLY ANNOUNCEMENT


Engineering
Rossen Stoyanchev

March 31, 2022

212 Comments

Updates

 * [04-04 10:45 BST] Updated Am I Impacted with improved description for
   deployment requirements
 * [04-01 16:35 BST] Updated Am I Impacted with additional notes
 * [04-01 13:05 BST] Updated Suggested Workarounds section for Apache Tomcat
   upgrades and Java 8 downgrades
 * [04-01 12:51 BST] Apache Tomcat releases versions 10.0.20, 9.0.62, and 8.5.78
   which close the attack vector on Tomcat’s side, see mitigation alternative
 * [03-31 15:40 BST] Spring Boot 2.6.6 is available
 * [03-31 14:38 BST] Spring Boot 2.5.12 is available
 * [03-31 14:00 BST] CVE-2022-22965 is published
 * [03-31 13:03 BST] Added section “Misconceptions”
 * [03-31 12:34 BST] Added section “Am I Impacted”
 * [03-31 12:11 BST] Fix minor issue in the workaround for adding
   disallowedFields
 * [03-31 11:59 BST] Spring Framework versions 5.3.18 and 5.2.20, which address
   the vulnerability, are now available. The release process for Spring Boot is
   in progress


TOC

 * Overview
 * Vulnerability
 * Am I Impacted
 * Status
 * Suggested Workarounds
 * Misconceptions


OVERVIEW

I would like to announce an RCE vulnerability in the Spring Framework that was
leaked out ahead of CVE publication. The issue was first reported to VMware late
on Tuesday evening, close to Midnight, GMT time by codeplutos, meizjm3i of
AntGroup FG. On Wednesday we worked through investigation, analysis, identifying
a fix, testing, while aiming for emergency releases on Thursday. In the mean
time, also on Wednesday, details were leaked in full detail online, which is why
we are providing this update ahead of the releases and the CVE report.


VULNERABILITY

The vulnerability impacts Spring MVC and Spring WebFlux applications running on
JDK 9+. The specific exploit requires the application to run on Tomcat as a WAR
deployment. If the application is deployed as a Spring Boot executable jar, i.e.
the default, it is not vulnerable to the exploit. However, the nature of the
vulnerability is more general, and there may be other ways to exploit it.


AM I IMPACTED?

These are the requirements for the specific scenario from the report:

 * Running on JDK 9 or higher
 * Apache Tomcat as the Servlet container.
 * Packaged as a traditional WAR and deployed in a standalone Tomcat instance.
   Typical Spring Boot deployments using an embedded Servlet container or
   reactive web server are not impacted.
 * spring-webmvc or spring-webflux dependency.
 * Spring Framework versions 5.3.0 to 5.3.17, 5.2.0 to 5.2.19, and older
   versions.

Additional notes:

 * The vulnerability involves ClassLoader access, and therefore in addition to
   the specific attack reported with a Tomcat specific ClassLoader, other
   attacks may be possible against a different custom ClassLoader.
 * The issue relates to data binding used to populate an object from request
   parameters (either query parameters or form data). Data binding is used for
   controller method parameters that are annotated with @ModelAttribute or
   optionally without it, and without any other Spring Web annotation.
 * The issues does not relate to @RequestBody controller method parameters (e.g.
   JSON deserialization). However, such methods may still be vulnerable if they
   have another method parameter populated via data binding from query
   parameters.


STATUS

 * Spring Framework 5.3.18 and 5.2.20, which contain the fixes, have been
   released.
 * Spring Boot 2.6.6 and 2.5.12 that depend on Spring Framework 5.3.18 have been
   released.
 * CVE-2022-22965 has been published.
 * Apache Tomcat has released versions 10.0.20, 9.0.62, and 8.5.78 which close
   the attack vector on Tomcat’s side, see Spring Framework RCE, Mitigation
   Alternative.


SUGGESTED WORKAROUNDS

The preferred response is to update to Spring Framework 5.3.18 and 5.2.20 or
greater. If you have done this, then no workarounds are necessary. However, some
may be in a position where upgrading is not possible to do quickly. For that
reason, we have provided some workarounds below.

 * Upgrading Tomcat
 * Downgrading to Java 8
 * Disallowed Fields

Please note that, workarounds are not necessarily mutually exclusive since
security is best done “in depth”.

UPGRADING TOMCAT

For older, unsupported Spring Framework versions, upgrading to Apache Tomcat
10.0.20, 9.0.62, or 8.5.78, provides adequate protection. However, this should
be seen as a tactical solution, and the main goal should be to upgrade to a
currently supported Spring Framework version as soon as possible. If you take
this approach, you should consider setting Disallowed Fields as well for defense
in depth approach.

DOWNGRADING TO JAVA 8

Downgrading to Java 8 is a viable workaround, if you can neither upgrade the
Spring Framework nor upgrade Apache Tomcat.

DISALLOWED FIELDS

Another viable workaround is to disable binding to particular fields by setting
disallowedFieldson WebDataBinder globally:


@ControllerAdvice
@Order(Ordered.LOWEST_PRECEDENCE)
public class BinderControllerAdvice {

    @InitBinder
    public void setAllowedFields(WebDataBinder dataBinder) {
         String[] denylist = new String[]{"class.*", "Class.*", "*.class.*", "*.Class.*"};
         dataBinder.setDisallowedFields(denylist);
    }

}Copy

This works generally, but as a centrally applied workaround fix, may leave some
loopholes, in particular if a controller sets disallowedFields locally through
its own @InitBinder method, which overrides the global setting.

To apply the workaround in a more fail-safe way, applications could extend
RequestMappingHandlerAdapter to update the WebDataBinder at the end after all
other initialization. In order to do that, a Spring Boot application can declare
a WebMvcRegistrations bean (Spring MVC) or a WebFluxRegistrations bean (Spring
WebFlux).

For example in Spring MVC (and similar in WebFlux):

package car.app;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.autoconfigure.web.servlet.WebMvcRegistrations;
import org.springframework.context.annotation.Bean;
import org.springframework.web.bind.ServletRequestDataBinder;
import org.springframework.web.context.request.NativeWebRequest;
import org.springframework.web.method.annotation.InitBinderDataBinderFactory;
import org.springframework.web.method.support.InvocableHandlerMethod;
import org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter;
import org.springframework.web.servlet.mvc.method.annotation.ServletRequestDataBinderFactory;


@SpringBootApplication
public class MyApp {


	public static void main(String[] args) {
		SpringApplication.run(CarApp.class, args);
	}


	@Bean
	public WebMvcRegistrations mvcRegistrations() {
		return new WebMvcRegistrations() {
			@Override
			public RequestMappingHandlerAdapter getRequestMappingHandlerAdapter() {
				return new ExtendedRequestMappingHandlerAdapter();
			}
		};
	}


	private static class ExtendedRequestMappingHandlerAdapter extends RequestMappingHandlerAdapter {

		@Override
		protected InitBinderDataBinderFactory createDataBinderFactory(List<InvocableHandlerMethod> methods) {

			return new ServletRequestDataBinderFactory(methods, getWebBindingInitializer()) {

				@Override
				protected ServletRequestDataBinder createBinderInstance(
						Object target, String name, NativeWebRequest request) throws Exception {
					
					ServletRequestDataBinder binder = super.createBinderInstance(target, name, request);
					String[] fields = binder.getDisallowedFields();
					List<String> fieldList = new ArrayList<>(fields != null ? Arrays.asList(fields) : Collections.emptyList());
					fieldList.addAll(Arrays.asList("class.*", "Class.*", "*.class.*", "*.Class.*"));
					binder.setDisallowedFields(fieldList.toArray(new String[] {}));
					return binder;
				}
			};
		}
	}
}
Copy

For Spring MVC without Spring Boot, an application can switch from @EnableWebMvc
to extending DelegatingWebMvcConfiguration directly as described in Advanced
Config section of the documentation, then overriding the
createRequestMappingHandlerAdapter method.


MISCONCEPTIONS

There was speculation surrounding the commit to deprecate SerializationUtils.
This class has only one usage within the framework and is not exposed to
external input. The deprecation is unrelated to this vulnerability.

There was confusion with a CVE for Spring Cloud Function which was released just
before the report for this vulnerability. It is also unrelated.


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