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Table of contents
What is Vulnerability?
What is Log4j?
Log4j Library
Need for Log4j
Features of Log4j
Components of Log4j
What is Log4J Vulnerability?
How the Log4j Vulnerability Works
How to Mitigate Log4j Vulnerability
What is Logging?
Pros and Cons of Logging 
Frequently Asked Questions
What is Log4j's primary goal?
What is the impact of Log4j?
Why is Log4j so popular?
Is it safe to use log4j?
When was Log4j discovered?
How risky is Log4j?
Where is log4j stored?
Last Updated: Mar 27, 2024


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Zero-day vulnerability has brought the Log4j logging framework into the limelight. It helps to identify and mitigate potential security risks in Java-based applications and systems. 


In this blog, we will discuss everything about Log4j. But before discussing Log4j, you must know about vulnerability. In simple language, a vulnerability is a system flaw that hackers can exploit to launch an attack. We will see all about that in this blog. Let’s get started.

Also see, Duck Number in Java and Hashcode Method in Java

What is Vulnerability?

vulnerability is a kind of flaw in a system that hackers can use to launch an attack; it can be present in a network, a hardware system, or, in the case of Log4j, the software, notably a Java library. 


A vulnerability is the weakest point in a system from a security standpoint, not the assault itself or the prospect of being attacked. The biggest concern in such a situation is that unauthorized users could readily access a crucial component. 

Computer software vulnerabilities are classified as zero-days (sometimes referred to as 0-days) if they are either unknown to people who should be concerned about mitigating them (such as the vendor of the target software) or known, but a fix has not been created. 


Let’s move on to the blog's next section to learn about the Log4j library.

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What is Log4j?

Log4j is a generally used open-source logging library for Java-based applications. It allows developers to add logging capabilities to their code, enabling the application to produce log messages during runtime. These log messages can provide valuable insights into the application's behavior, errors, and performance, which aids in debugging and troubleshooting issues. 

Log4j provides different logging levels like debug, info, error, etc, to categorize the importance of log messages. Developers can configure Log4j to control the log output destination, such as console, file, or database, and set log levels to filter the information. With its flexibility, Log4j has become a standard choice for logging in Java applications. It helps developers to maintain and monitor their software effectively.

Log4j Library

Log4j Library

The Apache-developed Log4j Java-based logging library was created by Ceki Gülcü and primarily used to log system information as tasks were carried out. The library offers the user multiple levels of logging, including Off, Fatal, Error, Warn, Info, Debug, and Trace. Each group provides more information on the operating system, the instructions, the variables, and the system's state.

The logged data may be stored in log files as text or shown on the status console. 

To make debugging simpler for developers, Log4j also enables customized configurations using the XMLYAMLJSON, or properties file formats. These configurations allow users to adjust the loggers, appenders, layouts, and filters.

Need for Log4j

Need for Log4j

In this section, we will see why we need Log4j. 

  • It uses open-source software.
  • We can use Log4j to store the Selenium Automation flow details in a file or database.
  • Both large and small projects use Log4j.
  • To know the status of the project while it is running in Log4j, we utilize log statements rather than SOPL statements in the code.
  • Developers use Log4j to monitor activity in their software programs or web services.
  • Developers use Log4j to keep track of system or application activities in order to spot issues that can affect clients.
  • You can use a log level in Log4j to record debugging data, which will subsequently be used for filtering purposes. You can turn off messages that pertain to a specific log level if, for example, seeing WARN messages in production worries you more than DEBUG ones.

Features of Log4j

Features of Log4j

As we have discussed the need for Log4j, we will now see some of its important features.

  • It is thread-safe.
  • It is geared towards speed.
  • It is founded on a hierarchy of named loggers.
  • It supports multiple output appenders.
  • It encourages globalisation.
  • It is not limited to the particular group of facilities.
  • Using a configuration file, logging behavior can be configured at runtime.
  • It is built from the bottom up to handle Java Exceptions.

Components of Log4j

There are three primary parts to Log4j:

  • Loggers: It is in charge of recording logging data. The Logger class offers numerous ways to manage logging operations. It offers two static methods, Public static Logger getRootLogger() and Public static Logger getLogger(String name), for acquiring a Logger Object.
  • Appenders: It is employed to transmit LogEvents to their intended location. It makes decisions regarding what will be done with log data. To put it simply, it is used to write logs to files. Here are a few different types of appenders.

    • Logs are sent to standard output by ConsoleAppender.
    • A file appender that prints logs.
    • A rolling file appender to a file with the maximum size.
  • Layouts: It is responsible for formatting and logging data in various formats. In an application, the Layout class is defined as abstract, and we never use it directly; instead, we deal with its subclasses.

Now let us move forward with our blog and discuss Log4j vulnerability.

What is Log4J Vulnerability?

What is Log4J Vulnerability?

Understanding JNDI (Java Naming and Directory Interface) is important for a deeper understanding of this issue. A Java program can use JNDI, a directory service, to find data in a directory. LDAP (Lightweight Directory Access Protocol), RMI (Remote Method Interface) RegistryCORBA COS (Common Object Service), and other directory services can all be used with JNDI thanks to the SPIs it has access to.

A Java program can use JNDI and LDAP to find the Java object containing the data it needs. So the LDAP server can run on another computer. In other words, it can be located anywhere on the Internet. This flexibility means an attacker can control the LDAP URL by trying to write a string like ${jndi:ldap://} to their Log4j. 

In this case, Log4j connects to their LDAP server at and retrieves the object. Imagine a Java-based system that logs when a customer's name is not found. A malicious user can create an order with a first name containing an exploit and make several jumps before executing the script on the server.

This vulnerability does not affect Log4j 1. x versions because the JNDILookup plugin was only added in version 2.0-beta-9 and beyond.

This issue affects all versions of Log4j 2. x from 2.0-beta-9 and 2.14.1.

How the Log4j Vulnerability Works

Alright! As we have already discussed, the Log4j vulnerability. Now let us discuss how this vulnerability actually works. 

An attacker can send messages to Log4j instructing it to download malware from a server, which can either give the attacker access to the system or directly work on sabotaging it, thanks to a vulnerability that takes advantage of the logging library's access to the internal system functions like directory services.

How the Log4j Vulnerability Works

If the server that gets this string uses the Log4j library, the string is logged into the console, and depending on how the console logs are handled, the string can launch the malware download directly. The attacker can use forms to insert the string that will download the malware; even a tiny name field in a log-in page can be exploited to enter the string.

How to Mitigate Log4j Vulnerability

There are some measures you can follow to mitigate the vulnerability.

  • You should update to the latest version, 2.15.0, as soon as possible for all organizations using Log4j. 
  • Keeping credentials in a secret store rather than as env Variables is another, more general action.
  • If you're using Log4j version 2.10 or later and you're unable to upgrade for whatever reason, set the property:
  • The JndiLookup class should be removed from the classpath. To achieve it, enter the following command:
    zip -q -d log4j-core-*.jar 

What is Logging?

Logging is the process of recording messages, events, or information that occur during the execution of a software application. These events are saved as log messages in specific locations, such as log files or centralized logging systems. Logging is essential in software development and provides valuable insights into the application's performance and behavior.

Pros and Cons of Logging 

The benefits and drawbacks of logging are as follows:



A crucial part of software development is logging. It could cause an application to lag.
Quick debugging. If overly wordy, it may result in scrolling blindness.
Simple upkeep of data. Logging is rarely an application's primary priority.
Structured storage of an application's runtime data. Complex to implement.

Alright! Ninjas, we have discussed everything about the Log4j vulnerability. Now let us see some frequently asked questions.

Frequently Asked Questions

What is Log4j's primary goal?

A simple method for Selenium logging is provided by the Java logging package Log4j. In a word, the framework details everything that occurs when the software is being executed. Additionally, Log4j offers the perception of everything that might have gone wrong while using software or automation.

What is the impact of Log4j?

Log4j's impact is significant as it is widely used for logging in Java apps. However, recent security vulnerabilities like CVE-2021-44228 have affected multiple systems.

Why is Log4j so popular?

Log4j is popular due to its flexibility, ease of use, and robust logging features, making it a standard choice for Java applications.

Is it safe to use log4j?

Log4j can be safe to use, but it is essential to keep regular updates crucial to mitigate security risks, such as the vulnerability CVE-2021-44228.

When was Log4j discovered?

Log4j's security vulnerability, CVE-2021-44228, was discovered in December 2021, leading to security measures and updates.

How risky is Log4j?

The Log4j issue enables an attacker quickly and silently gain complete control of susceptible systems without having to go through any security checks. Threat actors can remotely access networks, take control of systems, and steal sensitive data like files and passwords.

Where is log4j stored?

It is stored in the file app_data /conf/server/log4j. properties, where app_data is the application data folder. You can open this file by selecting Settings > Logging or edit it directly on the server.


As we have reached the end of this blog, let us see what we have discussed so far. In this blog, we have discussed what vulnerability is, the Log4j library, its need, features, components, Log4j vulnerability, it's working, and various ways to mitigate it. In the end, we discussed the pros and cons of logging. 

If you like to learn more, you can check out our articles: 

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