Fuzzing Java in OSS-Fuzz

What Does Jazzer Support?

Jazzer supports all languages that compile to JVM bytecode, since instrumentation is done on the bytecode level. This includes:

• Java
• Kotlin
• Scala
• Clojure

Jazzer can also provide coverage feedback from native code that is executed through JNI. This can uncover interesting memory corruption vulnerabilities in memory unsafe native code.

Why Fuzz Java/JVM-based Code?

As discussed in our post on Atheris, fuzzing code written in memory safe languages, such as JVM-based languages, is useful for finding bugs where code behaves incorrectly or crashes. Incorrect behavior can be just as dangerous as memory corruption. For example, Jazzer was used to find CVE-2021-23899 in json-sanitizer which could be exploited for cross-site scripting (XSS). Bugs causing crashes or incorrect exceptions can sometimes be used for denial of service. For example, OSS-Fuzz recently found a denial of service issue that could have been used to take “a major part of the ethereum network offline”.

When fuzzing memory safe code, you can use the same classic approach for fuzzing memory unsafe code: passing mutated input to code and waiting for crashes. Or you can take a more unit test like approach where your fuzzer verifies that the code is behaving correctly (example).

Another way fuzzing can find interesting bugs in JVM-based code is through differential fuzzing. With differential fuzzing, your fuzzer passes mutated input from the fuzzer to multiple library implementations that should have the same functionality. Then it compares the results from each library to find differences.
Check out our documentation to get started. We will explore this more during our OSS-Fuzz talk at FuzzCon Europe.