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Log4j zero-day “Log4Shell” arrives just in time to ruin your weekend

Log4j zero-day “Log4Shell” arrives just in time to ruin your weekend

If you’re running a service that relies on Apache Struts or uses the popular Apache Log4j utility we hope you haven’t made plans for the weekend.

An exploit listed as CVE-2021-44228 was made public on December 9, 2021. The exploit is simple, easy to trigger, and can be used to perform remote code execution (RCE) in vulnerable systems, which could allow an attacker to gain full control of them. All an attacker has to do is get the affected app to log a special string. For that reason, researchers have dubbed the vulnerability “Log4Shell”.

The vulnerability has a CVSS score of 10.0 out of a possible 10. It impacts Apache Log4j versions 2.0-beta9 to 2.14.1. Mitigations are available for version 2.10 and higher.

Log4j is an open source logging library written in Java that was developed by the Apache Software Foundation. Millions of applications use it, and some of them are enormously popular—such as iCloud, Steam, and Minecraft—so the potential reach of this problem is enormous.

Used in the wild

After the 0-day was posted on Twitter, along with a proof-of-concept that was published on GitHub, the exploit has already been spotted being used in the wild by CERT New Zealand, CERT Austria, and CERT Germany. Along with many others, they are seeing automated systems trying to exploit the vulnerability.

The vulnerability is triggered by a simple string sent to a vulnerable server:


When the vulnerable application logs the string it triggers a lookup to an attacker-controlled remote LDAP server (example.com in our scenario). The response from the malicious server contains a path to a remote Java class file that’s injected into the server process. Attackers can execute commands with the same level of privilege as the application that uses the logging library.

Given how common this library is and how serious the consequences of a relatively easy-to-exploit vulnerability can be, this is a recipe for disaster. Many organizations will not even realize they are vulnerable.

According to researcher Marcus Hutchins, in the case of Minecraft, attackers were able to get remote code execution on Minecraft servers by simply pasting the malicious string into the chat box. Similar examples exist for a number of other popular services.


Mitigations are available for versions of log4j 2.10.0 and up. Version 2.15.0 is not vulnerable by default. Note that there may be other dependencies, such as your Java version, that need to be updated before you can upgrade. Fixing the vulnerability may not be straightforward, but it is urgent.

According to the Apache log4j project, if you are unable to upgrade, for whatever reason, you can mitigate this vulnerability in version 2.10.0 or higher by switching log4j2.formatMsgNoLookups to true. This can be done by adding ‐Dlog4j2.formatMsgNoLookups=True to the JVM command for starting the application.

Sadly, there is little, if anything, that users of affected systems can do to make themselves less vulnerable to the consequences. No doubt many systems will be affected and system administrators will want to treat anomalies with extreme caution.

So, if you’re an administrator looking forward to a quiet weekend, you know what to do!

Stay safe, everyone!

The post Log4j zero-day “Log4Shell” arrives just in time to ruin your weekend appeared first on Malwarebytes Labs.