examples/src/main/java/com/example/Log4jFuzzer.java

// Copyright 2021 Code Intelligence GmbH
//
// 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 com.example;

import com.code_intelligence.jazzer.api.FuzzedDataProvider;
import org.apache.logging.log4j.Level;
import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import org.apache.logging.log4j.core.Appender;
import org.apache.logging.log4j.core.Core;
import org.apache.logging.log4j.core.LogEvent;
import org.apache.logging.log4j.core.appender.AbstractAppender;
import org.apache.logging.log4j.core.config.Configurator;
import org.apache.logging.log4j.core.config.builder.api.AppenderComponentBuilder;
import org.apache.logging.log4j.core.config.builder.api.RootLoggerComponentBuilder;
import org.apache.logging.log4j.core.config.builder.impl.DefaultConfigurationBuilder;
import org.apache.logging.log4j.core.config.plugins.Plugin;
import org.apache.logging.log4j.core.config.plugins.PluginAttribute;
import org.apache.logging.log4j.core.config.plugins.PluginFactory;
import org.apache.logging.log4j.core.layout.PatternLayout;
import org.apache.logging.log4j.status.StatusLogger;

// This fuzzer reproduces the log4j RCE vulnerability CVE-2021-44228.
public class Log4jFuzzer {
  private final static Logger log = LogManager.getLogger(Log4jFuzzer.class.getName());

  public static void fuzzerTestOneInput(FuzzedDataProvider data) {
    log.error(data.consumeRemainingAsString());
  }

  public static void fuzzerInitialize() {
    // Install a logger that constructs the log message, but never prints it.
    // This noticeably increases the fuzzing performance
    DefaultConfigurationBuilder configBuilder = new DefaultConfigurationBuilder();
    configBuilder.setPackages(FuzzingAppender.class.getPackage().getName());
    AppenderComponentBuilder fuzzingAppender =
        configBuilder.newAppender("nullAppender", "FuzzingAppender");
    configBuilder.add(fuzzingAppender);
    RootLoggerComponentBuilder rootLogger = configBuilder.newRootLogger();
    rootLogger.add(configBuilder.newAppenderRef("nullAppender"));
    configBuilder.add(rootLogger);
    Configurator.reconfigure(configBuilder.build());

    // Disable logging of exceptions caught in log4j itself.
    StatusLogger.getLogger().reset();
    StatusLogger.getLogger().setLevel(Level.OFF);
  }

  @Plugin(
      name = "FuzzingAppender", category = Core.CATEGORY_NAME, elementType = Appender.ELEMENT_TYPE)
  public static class FuzzingAppender extends AbstractAppender {
    protected FuzzingAppender(String name) {
      super(name, null, PatternLayout.createDefaultLayout(), true);
    }

    @PluginFactory
    public static FuzzingAppender createAppender(@PluginAttribute("name") String name) {
      return new FuzzingAppender(name);
    }

    @Override
    public void append(LogEvent event) {
      try {
        getLayout().toByteArray(event);
      } catch (Exception ignored) {
        // Prevent exceptions from being logged to stderr.
      }
    }
  }
}