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Find your first bug in Java

This guide covers the basics of using cifuzz with a Java project. CI Fuzz directly supports Maven and Gradle build systems.

This guide was created using Ubuntu 20.04 x64. If you are on MacOS or Windows, you should be able to follow along without any issues.

If you haven't yet installed CI Fuzz, then first go to the installation section

Maven​

1.Set up​

Download or clone the following repository: https://github.com/CodeIntelligenceTesting/ci-fuzz-cli-tutorials. This repository contains several projects. For this guide, you need to use the project in the java/maven directory.

note

Try to run all cifuzz commands listed in this section from the java/maven directory.

2.Initialize the project​

The first step for any project is to initialize it. Initialization means creating a configuration file, cifuzz.yaml, in the project directory and modifying your top level pom.xml file. Run the following command:

cifuzz init

When you run cifuzz init it recognizes the project as a Maven project and provide two dependencies that you should add to your pom.xml:

  • jazzer-junit - this dependency enables cifuzz integration with your project
  • junit-jupiter-engine - this dependency ensures easy integration between your IDE and cifuzz

After adding both dependencies, your pom.xml should look similar to:

<?xml version="1.0" encoding="UTF-8"?>

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.github.CodeIntelligenceTesting.cifuzz</groupId>
    <artifactId>maven-example</artifactId>
    <version>1.0-SNAPSHOT</version>

    <name>maven-example</name>
    <description>A simple maven-example for cifuzz</description>

    <properties>
        <maven.compiler.source>8</maven.compiler.source>
        <maven.compiler.target>8</maven.compiler.target>
    </properties>

    <dependencies>
        <dependency>
            <groupId>com.code-intelligence</groupId>
            <artifactId>jazzer-junit</artifactId>
            <version>0.13.0</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.junit.jupiter</groupId>
            <artifactId>junit-jupiter-engine</artifactId>
            <version>5.9.0</version>
            <scope>test</scope>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.jacoco</groupId>
                <artifactId>jacoco-maven-plugin</artifactId>
                <version>0.8.8</version>
                <executions>
                    <execution>
                        <goals>
                            <goal>prepare-agent</goal>
                        </goals>
                    </execution>
                    <execution>
                        <id>report</id>
                        <phase>test</phase>
                        <goals>
                            <goal>report</goal>
                        </goals>
                    </execution>
                </executions>
            </plugin>
            <plugin>
                <artifactId>maven-clean-plugin</artifactId>
                <version>3.1.0</version>
            </plugin>
            <plugin>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.8.0</version>
            </plugin>
            <plugin>
                <artifactId>maven-surefire-plugin</artifactId>
                <version>2.22.1</version>
            </plugin>
            <plugin>
                <artifactId>maven-jar-plugin</artifactId>
                <version>3.0.2</version>
            </plugin>
        </plugins>
    </build>
</project>

3.Create a fuzz test​

The next step is to create a java fuzz test template. With one of the main goals of cifuzz being to make fuzz testing as easy as unit testing, you create and then place the fuzz test in the src/test/java/com/example directory, exactly as you would a standard unit test. Run the following commands:

mkdir -p src/test/java/com/example
cifuzz create java -o src/test/java/com/example/MyFuzzTest.java

If you open src/test/java/com/example/MyFuzzTest.java, you should see a fuzz test template.

Before you write the fuzz test, take a look at the target method that you want to fuzz. It's located in src/main/java/com/example/ExploreMe.java:

package com.example;

public class ExploreMe {
    public static void exploreMe(int a, int b, String c) {
        if (a >= 20000) {
            if (b >= 2000000) {
                if (b - a < 100000) {
                    // Create reflective call
                    if (c.startsWith("@")) {
                        String className = c.substring(1);
                        try {
                            Class.forName(className).newInstance();
                        } catch (ClassNotFoundException | InstantiationException | IllegalAccessException ignored) {
                        }
                    }
                }
            }
        }
    }
}

The main parts to focus on here is the parameters that the exploreMe method requires, int a, int b, string c. As long as you can pass the correct data types to the function, the fuzzer takes care of the rest.

Now you write the fuzz test. You can write or copy/paste the following into src/test/java/com/example/MyFuzzTest.java:

package com.example;

import com.code_intelligence.jazzer.api.FuzzedDataProvider;
import com.code_intelligence.jazzer.junit.FuzzTest;

public class MyFuzzTest {
    @FuzzTest
    void myFuzzTest(FuzzedDataProvider data) {
        int a = data.consumeInt();
        int b = data.consumeInt();
        String c = data.consumeRemainingAsString();

        ExploreMe.exploreMe(a, b, c);
    }
}

A few notes about this fuzz test:

  • The fuzz test is part of the same package as the target class/method.
  • The @FuzzTest annotation is what enables you to write fuzz tests similar to how you'd write JUnit tests.
  • The fuzz test must import com.code_intelligence.jazzer.junit.FuzzTest
  • This fuzz test uses the FuzzedDataProvider class. This isn't required, but it's a convenient way to split the fuzzing input in the data variable into different data types. Here is a link to the FuzzedDataProvider class documentation if you want to view it's other methods.
  • Once you have created the appropriate variables (a, b, and c) using data from the fuzzer, the fuzz test has to call the target method (ExploreMe.exploreMe) with the fuzz data.

4.Run the fuzz test​

Everything is configured and the fuzz test is created. Run the fuzz test using:

cifuzz run com.example.MyFuzzTest

After a moment you get a notification that cifuzz discovered a potential Remote Code Execution in exploreMe. Here is a snippet of the output:

<snip>
💥 [vibrant_cow] Security Issue: Remote Code Execution in exploreMe (com.example.ExploreMe:12)
<snip>

5.Examine the findings​

When cifuzz discovers a finding, it stores the output from the finding and the input that caused it. You can list all findings discovered so far by running cifuzz findings. If you want to see the details of a specific finding just provide it's name, for example cifuzz finding vibrant_cow. This provides the stack trace and other details about the finding that helps you debug and fix the issue. Examining the output from cifuzz finding vibrant_cow below shows that there was an unrestricted class loading based on externally controlled data. This triggered at line 12 in the com.example.ExploreMe.exploreMe.

<snip>
== Java Exception: com.code_intelligence.jazzer.api.FuzzerSecurityIssueHigh: Remote Code Execution
Unrestricted class loading based on externally controlled data may allow
remote code execution depending on available classes on the classpath.
      at jaz.Zer.<clinit>(Zer.java:54)
      at java.base/java.lang.Class.forName0(Native Method)
      at java.base/java.lang.Class.forName(Class.java:375)
      at com.example.ExploreMe.exploreMe(ExploreMe.java:12)
      at com.example.MyFuzzTest.myFuzzTest(MyFuzzTest.java:13)
<snip>

If you examine line 12 in src/main/java/com/example/ExploreMe.java, you can see this is where it attempts to load a class based off of input data.

cifuzz also stores the crashing input in a resources directory. In this example, that would be src/test/resources/com/example/MyFuzzTestInputs/vibrant_cow. This can be helpful when debugging your app.

Gradle​

1.Set up​

Download or clone the following repository: https://github.com/CodeIntelligenceTesting/ci-fuzz-cli-tutorials. This repository contains several projects. For this guide, you need to use the project in the java/gradle directory.

note

Try to run all cifuzz commands listed in this section from the java/gradle directory.

2.Initialize the project​

The first step for any project is to initialize it. Initialization means creating a configuration file, cifuzz.yaml, in the project directory and modifying your build.gradle file. Run the following command:

cifuzz init

When you run cifuzz init it recognizes the project as a Gradle project and provide two dependencies that you should add to your build.gradle:

  • com.code-intelligence:jazzer-junit - this dependency enables cifuzz integration with your project
  • org.junit.jupiter:junit-jupiter - this dependency ensures easy integration between your IDE and cifuzz

After adding both dependencies, your build.gradle should look similar to:

plugins {
    // Apply the application plugin to add support for building a CLI application in Java.
    id 'application'
    id 'jacoco'
}

repositories {
    // Use Maven Central for resolving dependencies.
    mavenCentral()
}

dependencies {
    testImplementation 'org.junit.jupiter:junit-jupiter:5.9.0'
    testImplementation 'com.code-intelligence:jazzer-junit:0.13.0'

    // This dependency is used by the application.
    implementation 'com.google.guava:guava:31.1-jre'
}

application {
    // Define the main class for the application.
    mainClass = 'com.github.CodeIntelligenceTesting.cifuzz.App'
}

tasks.named('test') {
    // Use JUnit Platform for unit tests.
    useJUnitPlatform()
}

3.Create a fuzz test​

The next step is to create a java fuzz test template. With one of the main goals of cifuzz being to make fuzz testing as easy as unit testing, you'll create and then place the fuzz test in the src/test/java/com/example directory, exactly as you would a standard unit test. Run the following commands:

mkdir -p src/test/java/com/example
cifuzz create java -o src/test/java/com/example/MyFuzzTest.java

If you open src/test/java/com/example/MyFuzzTest.java, you should see a fuzz test template.

Before you write the fuzz test, take a look at the target method that you want to fuzz. It's located in src/main/java/com/example/ExploreMe.java:

package com.example;

public class ExploreMe {
    public static void exploreMe(int a, int b, String c) {
        if (a >= 20000) {
            if (b >= 2000000) {
                if (b - a < 100000) {
                    // Create reflective call
                    if (c.startsWith("@")) {
                        String className = c.substring(1);
                        try {
                            Class.forName(className).newInstance();
                        } catch (ClassNotFoundException | InstantiationException | IllegalAccessException ignored) {
                        }
                    }
                }
            }
        }
    }
}

The main parts to focus on here is the parameters that the exploreMe method requires, int a, int b, string c. As long as you can pass the correct data types to the function, the fuzzer takes care of the rest.

Now you'll write the fuzz test. You can write or copy/paste the following into src/test/java/com/example/MyFuzzTest.java:

package com.example;

import com.code_intelligence.jazzer.api.FuzzedDataProvider;
import com.code_intelligence.jazzer.junit.FuzzTest;

public class MyFuzzTest {
    @FuzzTest
    void myFuzzTest(FuzzedDataProvider data) {
        int a = data.consumeInt();
        int b = data.consumeInt();
        String c = data.consumeRemainingAsString();

        ExploreMe.exploreMe(a, b, c);
    }
}

A few notes about this fuzz test:

  • The fuzz test is part of the same package as the target class/method.
  • The @FuzzTest annotation is what enables you to write fuzz tests similar to how you'd write JUnit tests.
  • The fuzz test must import com.code_intelligence.jazzer.junit.FuzzTest
  • This fuzz test uses the FuzzedDataProvider class. This isn't required, but it's a convenient way to split the fuzzing input in the data variable into different data types. Here is a link to the FuzzedDataProvider class documentation if you want to view it's other methods.
  • Once you have created the appropriate variables (a, b, and c) using data from the fuzzer, the fuzz test has to call the target method (ExploreMe.exploreMe) with the fuzz data.

4.Run the fuzz test​

Everything is configured and the fuzz test is created. Run the fuzz test using:

cifuzz run com.example.MyFuzzTest

After a moment you get a notification that cifuzz discovered a potential Remote Code Execution in exploreMe. Here is a snippet of the output:

<snip>
💥 [vibrant_cow] Security Issue: Remote Code Execution in exploreMe (com.example.ExploreMe:12)
<snip>

5.Examine the findings​

When cifuzz discovers a finding, it stores the output from the finding and the input that caused it. You can list all findings discovered so far by running cifuzz findings. If you want to see the details of a specific finding just provide it's name, for example cifuzz finding vibrant_cow. This provides the stack trace and other details about the finding that helps you debug and fix the issue. Examining the output from cifuzz finding vibrant_cow below shows that there was an unrestricted class loading based on externally controlled data. This triggered at line 12 in the com.example.ExploreMe.exploreMe.

<snip>
== Java Exception: com.code_intelligence.jazzer.api.FuzzerSecurityIssueHigh: Remote Code Execution
Unrestricted class loading based on externally controlled data may allow
remote code execution depending on available classes on the classpath.
      at jaz.Zer.<clinit>(Zer.java:54)
      at java.base/java.lang.Class.forName0(Native Method)
      at java.base/java.lang.Class.forName(Class.java:375)
      at com.example.ExploreMe.exploreMe(ExploreMe.java:12)
      at com.example.MyFuzzTest.myFuzzTest(MyFuzzTest.java:13)
<snip>

If you examine line 12 in src/main/java/com/example/ExploreMe.java, you can see this is where it attempts to load a class based off of input data.

cifuzz also stores the crashing input in a resources directory. In this example, that would be src/test/resources/com/example/MyFuzzTestInputs/vibrant_cow. This can be helpful when debugging your app.