Blog Post #2 – How Did Log4j Attack Organizations?

Table of Contents

Welcome back, classmates, instructors, future employers?! and random lurkers! Here I present to you, another installment of my Log4j blog series. In my previous blog, you learned what Log4j is. Today, you get to learn about how Log4j was used to attack organizations. Very exciting stuff!


It has been awhile since you visited my first blog post on Log4j, so let’s review what we learned.

You received a crash course on what Log4j is and a brief demo on how the Log4j code is structured. We have had a lot going on since we last discussed Log4j together, so I’ll give you the too long didn’t read version of the recap below, to quickly bring you back up to speed on Log4j. That way you can impress your new security instructor Nermine with your vast Log4j knowledge. Grab a beer my friend, and let’s get going!

PLEASE NOTE: If you’re feeling good about your Log4j smarts, you can skip straight to the new material in the table of contents at the top of the screen. If you need a refresher, then disregard this and carry on.

What is Log4j?

We previously discussed that Log4j is a popular Java-based logging utility that is a part of Apache Logging Services, which is one of many projects under the Apache Software Foundation (ASF). Log4j is widely used for logging messages on your code to a file when something goes wrong in your program. The file contains error messages and debug messages that can be referenced by a developer for troubleshooting purposes.

Additional context

We also discussed how Log4j is similar to the ever-familiar System.out.println(); statement. While System.out.println(); outputs to a console window, Log4j can output to a console as well, but it can also output to a log file, a database table or an email server to name a few places.1

Built-in log levels

Another feature of Log4j that we discussed were the built-in log levels and messages, ranked in order starting from the least severe message to the most:


Custom log levels

Developers can make use of the built-in log levels, or they can create their own as Log4j allows us to define our own custom levels. We noted that you can define a custom level in code, or you can do it in XML configuration. Either way works fine as long as you remember to define the level before using it – seems simple enough, but you know how it goes…


When we left off, I had briefly introduced the Java Naming and Directory interface (JNDI) to you. To quickly recap, we established that JNDI is an application programming interface (API) that provides naming and directory functionality to applications written using Java. JNDI isn’t Log4j, but is a feature that’s included in the Java SDK. In order to use JNDI, you need to download at least one service provider; one example of a service provider that works with JNDI is Lightweight Directory Access Protocol (LDAP). JNDI performs lookups, which is a capability that Log4j supports. In simple terms, we can add a lookup (non-malicious) to a log message. A Java application can also use JNDI and its service to retrieve an object with data it needs.


Is it all coming back now? If you said no, then I don’t blame you. Let’s keep humming along on our journey of learning *heh*, and venture into today’s topic: how did Log4j attack organizations’ systems? Even more exciting, today’s demo will show you how to run Log4j on your system. I will also show you in a very bare bones way how the Log4j hack was perform. You’ll be able to follow along with the set-up process and get it going on your own system.

Let’s get to it!

Basic Facts on the Log4j vulnerability

The Log4j vulnerability was publicly disclosed on Thursday, December 9th 2021. It was discovered by a researcher from the Alibaba Cloud Security Team. The Log4j vulnerability was assigned the CVE identifier of CVE-2021-44228 by the Mitre Corporation. For those who don’t yet know, CVE stands for Common Vulnerabilities and Exposures. If a vulnerability is a publicly known security flaw, it will be assigned a CVE identifier so it can be easily referenced, and information on it can be easily shared. It is a system that is maintained by the Mitre Corporation, which is an American not-for-profit organization. Mitre manages federally funded research and development centres which support various American government agencies; cybersecurity is one of them.2 You may see CVE identifiers in your job even if you aren’t working in security, so it’s good to have a basic knowledge of what it is.

What is Log4Shell?

Up to now, I’ve referred to the security flaw with Log4j as the Log4j vulnerability. Now, if you were to discuss this with IT or computer security professionals, you may hear it referred to as Log4Shell. This was the official name it was given, and according to one of my favourite YouTube channels Java Brains, that’s because anyone could open a shell on any server and issue commands.3

CVSS Score

As you’ve heard, Log4Shell was an incredibly serious vulnerability. It was rated a 10 in the CVSS scoring system – yet another acronym you get to learn today! CVSS stands for Common Vulnerability Scoring System. These scores are used by security teams as a way to compare vulnerabilities to one another as a triage system of sorts, so vulnerabilities that are a higher priority on the list will get fixed sooner. CVSS scores range from 0-10, where 10 is critical. Hopefully now, some of the urgency surrounding Log4Shell is starting to become more clear.

I know that Log4Shell is serious, but why?

So, the biggest issue with Log4Shell was the remote code execution (RCE). Interestingly, this was first discovered in the game Minecraft. A community forum discovered the vulnerability after a user sent an RCE exploit to a friend’s server, which successfully executed.4 This was concerning to businesses because the same code format could be used to attack servers running vulnerable applications. And even more concerning is that it’s more common for organizations to use the Log4j library in their applications rather than not use it. The organization Snyk estimated that 39.2% of Java applications directly use a Log4j library, and 60.8% indirectly use Log4j, perhaps through a library that then utilizes part of the Log4j library. Another estimate pegged the number of affected devices at three billion.5 And with the simplicity of the malicious code that was needed to execute the attack, it became imperative to patch the vulnerability especially when it was made public – it wasn’t just a distant risk from state-level hackers that could create mayhem, but lower level hackers could throw their hat in the ring and give the attack a go as well.

Log4j versions

The versions of Log4j that were affected by this RCE attack were Apache Log4j2 versions 2.0-beta7 through If you’re already running Log4j on your system, then you won’t be vulnerable to this attack as long as you upgrade to Log4j 2.3.2 (for Java 6), 2.12.4 (for Java 7), or 2.17.1 (for Java 8 and later).

More excitement

Just when you thought you could relax and move on to your neighbour’s cat drama (don’t ask), another Log4j vulnerability was discovered! This exploit was assigned the CVE identifier of CVE 2021-45046. It was discovered shortly after the first vulnerability on December 16, 2021, and was the result of an incomplete patch for the first vulnerability.7 This exploit allowed attackers to create malicious input data using a JNDI (remember this?!) lookup pattern so they could launch denial-of-service attacks. The patch for this attack removed support for message lookup patterns, and disabled JNDI functionality.8


I introduced JNDI and LDAP briefly at the beginning of the blog, and you’re probably still wondering what exactly they are. Allow me to explain them now with some more detail. So, LDP stands for Lightweight Directory Access Protocol. It is a cross platform protocol used for directory services authentication. Basically, LDAP provides the communication language that applications use to communicate with other directory services servers. User information such as passwords are stored in directory services, which then share that information with other entities on the network.9

If you’re still having a tough time understanding JNDI and LDAP, that’s OK, let me give you an analogy that may feel more familiar. I found this explanation on and I’ll note it here verbatim (you can find the article in my sources). The article quoted was written by Sameer Tyagi:10

“JNDI does for LDAP what JDBC does for Oracle — it provides a standard API for interacting with naming and directory services using a service provider interface (SPI), which is analogous to an JDBC driver. LDAP is a standard way to provide access to directory information. JNDI gives Java applications and objects a powerful and transparent interface to access directory services like LDAP.”

I hope that makes a bit more sense, and you have a better grasp of JNDI and LDAP. In short: JNDI can be used to retrieve an object with needed data, and LDAP is a communication language.

What does the code look like?

Enough with the theory, so let’s see what the attack code would look like.



This is it:


It’s literally that simple. Note: the domain is the attacking server’s domain, and hackerman is the Java class where the code will be executed from.

Let’s walk through the steps of the attack: the attacker would insert this lookup in a request that is likely to be logged. Then, it’s passed to Log4j for logging. Next, Log4j essentially accepts it and queries the malicious LDP server. And finally, the server responds with the malicious information and the malicious Java class is downloaded and executed.

Log4j makes frequent use of lookups. Here are some other examples of lookups that would have been possible to exploit. If you remember expression language (EL) from our CPRG352 web applications class, they’ll feel familiar:






If you remember the XML configuration from my first blog post, here is an example of what it would look like with an environment variable included. This example is from apache.org11:

  • <File name=”Application” fileName=”application.log”>
  • <PatternLayout>
  • <pattern>%d %p %c{1.} [%t] $${env:USER} %m%n</pattern>
  • </PatternLayout>
  • </File>

There are other pieces of code that could be used for attacks. For the purposes of this blog I’ve kept it simple, but in case you’re curious (of course you are!) here’s another example that can be manipulated to initiate an attack:12



On to the even more exciting stuff before we wrap things up; please see my video below for a demonstration on how to set up and run Log4j on a basic Java program. I will also walk you through (in a very basic way) how the Log4j hack was performed.

Resolving Security Vulnerabilities

If you find yourself working at an organization where for whatever reason it isn’t possible to update a library like Log4j, you can have a workaround patch. In the case of Log4j, if users couldn’t upgrade their library, then Apache recommended users could either add -Dlog4j.formatMsgNoLookups=true as a command-line option or add log4j.formatMsgNoLookups=true to a file on the classpath to prevent lookups in log event messages. Users could also specify %m{nolookups} in the PatternLayout configuration to prevent lookups in log event messages. Remove the JndiLookup and JndiManager classes from the log4j-core jar.13 I just wanted to note that because on the individual level we can simply download the patch for our system, but at the enterprise level, things can be a bit more complex.

Parting words

Log4Shell is nowhere near the first security vulnerability to exist, and it sure won’t be the last. When moving on to your fruitful and highly successful career in Software Development, always approach your work with a security mindset – stay curious, and always seek out new knowledge (just don’t burn yourself out). It has been wonderful learning more about Log4j and having the opportunity to teach you more about my newfound knowledge. I hope despite your crazy schedule, you’ve also learned something new. I wish you the utmost happiness and success in your future career and throughout your life, whoever you may be.


If you would like to learn more about Log4j or just want to pay a visit to some of my sources for this blog post, please see the links below:














Image 1:

Image 2:

Extra source that I didn’t use, but seems interesting:


Video code

Log4j download (binary zip for Windows laptop):

Java class:

import org.apache.logging.log4j.*;
public class Log4jDemo {
static Logger name = LogManager.getLogger(Log4jDemo.class);
public static void main(String[] args) {
name.fatal(“Fatal message”); // 100
name.error(“Error message”); // 200
name.warn(“Warn message”); // 300“Info message”); // 400
name.debug(“Debug message”); // 500
name.trace(“Trace message”); // 600

Default XML file:

<?xml version=”1.0″ encoding=”UTF-8″?>
<Configuration status=”WARN”>
<Console name=”Console” target=”SYSTEM_OUT”>
<PatternLayout pattern=”%d{HH:mm:ss.SSS} [%t] %-5level %logger{36} – %msg%n”/>
<Root level=”error”>
<AppenderRef ref=”Console”/>

XML file for log files:

<?xml version=“1.0” encoding=”UTF-8″?>
<Configuration status=“WARN”>
<Property name=“filename”>./logfilesyay</Property>
<Console name=“Console” target=”SYSTEM_OUT”>
<PatternLayout pattern=“%d{HH:mm:ss.SSS} [%t] %-5level %logger{36} – %msg%n”/>
<RollingFile name=“thisismyfile” fileName=”${filename}/loginfohere.log”
<pattern>%d %p %c{1.} [%t] %m%n</pattern>
<SizeBasedTriggeringPolicy size=“1 KB” />
<Root level=“trace”>
<AppenderRef ref=“Console”/>
<AppenderRef ref=“thisismyfile”/>

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