The Apache Commons BCEL (Byte Code Engineering Library) is a crucial part of many Java applications. It provides comprehensive APIs to analyze, create and manipulate Java class files. However, security researchers have recently discovered an out-of-bounds writing issue in this library, specifically CVE-2022-42920, which exposes applications to potentially severe security vulnerabilities. In this post, we will walk you through the details of this vulnerability, its exploitation, and crucially, how to protect your applications by updating to the latest version of Apache Commons BCEL.

The Vulnerability Explained

In normal use cases, Apache Commons BCEL's APIs would only allow developers to modify specific, intended class characteristics, such as method and field names. However, due to the out-of-bounds writing issue (CVE-2022-42920), an attacker with the ability to pass data through these APIs can potentially produce arbitrary bytecode.

This means that the attacker could have more control over the resulting bytecode than expected, posing a severe risk to the integrity and security of applications that utilize the BCEL library.

Exploit Details

This vulnerability affects applications using Apache Commons BCEL versions prior to 6.6.. The main issue revolves around an out-of-bounds writing issue that could be exploited in applications that pass attacker-controllable data to BCEL's APIs. To demonstrate this, let's examine a simple code snippet:

import org.apache.commons.bcel.*;

public class BCELExploit {
    public static void main(String[] args) {
        // Attacker controlled input
        String unsafeInput = "someUnsafeInput";

        // Using vulnerable BCEL API
        // The attacker manages to manipulate the class data in a way that shouldn't be allowed
        ClassGen classGen = new ClassGen(unsafeInput, "java.lang.Object", "filename", Const.ACC_PUBLIC, null);
        // ...

In the above code snippet, you can see that the attacker is able to control the input (in this case, unsafeInput). This input goes through BCEL's ClassGen API, which is vulnerable to the out-of-bounds writing issue. As a result, the attacker can potentially manipulate the class data in unintended ways, producing arbitrary bytecode. The bytecode created in this scenario could potentially compromise the entire application by causing unexpected behavior, or worse yet, executing malicious code.

Mitigation: Update to Apache Commons BCEL 6.6.

The best way to protect your applications from this vulnerability is to update the Apache Commons BCEL library to version 6.6.. This version resolves the issue and ensures that your applications are no longer exposed to this out-of-bounds writing vulnerability. You can download the latest version of the library from the official Apache Commons BCEL website here.


In this post, we took a deep dive into CVE-2022-42920, a critical out-of-bounds writing issue affecting Apache Commons BCEL. By understanding the vulnerability and its exploitation, developers can take necessary security precautions to protect their applications. If you utilize this library in your Java applications, make sure to update to the latest version (6.6.) to secure your applications from potential attacks leveraging this vulnerability.


Published on: 11/07/2022 13:15:00 UTC
Last modified on: 11/08/2022 04:20:00 UTC