ASM Patch for log4j exploit

MessagePatternConverterMixin.java

/*
 * Copyright 2021 Alex Thomson
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package io.github.lxgaming.license.agent.mixin.log4j.core.pattern;

import io.github.lxgaming.license.agent.mixin.Mixin;
import org.objectweb.asm.Opcodes;
import org.objectweb.asm.tree.ClassNode;
import org.objectweb.asm.tree.FieldInsnNode;
import org.objectweb.asm.tree.InsnList;
import org.objectweb.asm.tree.LdcInsnNode;
import org.objectweb.asm.tree.MethodNode;
import org.objectweb.asm.tree.VarInsnNode;

public class MessagePatternConverterMixin extends Mixin {
    
    @Override
    public boolean prepare() {
        putClass("org/apache/logging/log4j/core/pattern/MessagePatternConverter", true);
        return true;
    }
    
    @Override
    protected boolean handle(ClassLoader classLoader, ClassNode classNode, MethodNode methodNode) {
        if (methodNode.name.equals("<init>")) {
            InsnList insnList = new InsnList();
            insnList.add(new VarInsnNode(
                    Opcodes.ALOAD,
                    0
            ));
            insnList.add(new LdcInsnNode(
                    true
            ));
            insnList.add(new FieldInsnNode(
                    Opcodes.PUTFIELD,
                    "org/apache/logging/log4j/core/pattern/MessagePatternConverter",
                    "noLookups",
                    "Z"
            ));
            
            methodNode.instructions.insertBefore(methodNode.instructions.getLast(), insnList);
            
            return true;
        }
        
        return false;
    }
}