log4j-jndi-be-gone

A Byte Buddy Java agent-based fix for CVE-2021-44228, the log4j 2.x "JNDI LDAP" vulnerability.

It does three things:

Disables the internal method handler for jndi: format strings ("lookups").
Logs a message to System.err (i.e stderr) indicating that a log4j JNDI attempt has been made (including the format string attempted, with any ${} characters sanitized to prevent transitive injections).
Resolves the format string to "(log4j jndi disabled)" in the log message (to prevent transitive injections).

Usage

Add -javaagent:path/to/log4j-jndi-be-gone-1.0.0-standalone.jar to your java commands.

Note: If you already have Byte Buddy in the classpath, try using log4j-jndi-be-gone-1.0.0.jar.

$ java -javaagent:path/to/log4j-jndi-be-gone-1.0.0-standalone.jar -jar path/to/some.jar

Obtaining log4j-jndi-be-gone

You can build the JAR with ./gradlew (build/libs/log4j-jndi-be-gone-1.0.0(-standalone).jar) or get it from the releases page.

Compatibility

The log4j-jndi-be-gone agent JAR supports Java 6-17+.

Caveats

log4j-jndi-be-gone will not work if the log4j library has been obfuscated or if its class packages/names have been modified.
log4j-jndi-be-gone-1.0.0-standalone.jar bundles in Byte Buddy. If you already use Byte Buddy, you may run into issues with it. Try log4j-jndi-be-gone-1.0.0.jar instead, though note that log4j-jndi-be-gone expects Byte Buddy 1.12.x.

Example

The tests/jnditest directory has a simple test case where a log4j logging call passes in a JNDI LDAP format string. It also sets up its own port listener to determine if a connection attempt was made by log4j and fails the test if a connection was received.

$ ./tests/jnditest/test-uninstrumented.sh

BUILD SUCCESSFUL in 1s
6 actionable tasks: 5 executed, 1 up-to-date

BUILD SUCCESSFUL in 1s
3 actionable tasks: 3 up-to-date
JUnit version 4.12
.16:08:49.547 [main] ERROR trust.nccgroup.jnditest.test.JndiTest - Hello, _${jndi:ldap://127.0.0.1:8899/evil}_!
E
Time: 0.929
There was 1 failure:
1) logging(trust.nccgroup.jnditest.test.JndiTest)
java.lang.AssertionError: jndi ldap connection received
	at org.junit.Assert.fail(Assert.java:88)
	at trust.nccgroup.jnditest.test.JndiTest.logging(JndiTest.java:55)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:568)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
	at org.junit.runners.Suite.runChild(Suite.java:128)
	at org.junit.runners.Suite.runChild(Suite.java:27)
	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
	at org.junit.runners.Suite.runChild(Suite.java:128)
	at org.junit.runners.Suite.runChild(Suite.java:27)
	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
	at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
	at org.junit.runner.JUnitCore.run(JUnitCore.java:115)
	at org.junit.runner.JUnitCore.runMain(JUnitCore.java:77)
	at org.junit.runner.JUnitCore.main(JUnitCore.java:36)
	at trust.nccgroup.jnditest.Main.main(Main.java:24)

FAILURES!!!
Tests run: 1,  Failures: 1

$ ./tests/jnditest/test-instrumented.sh

BUILD SUCCESSFUL in 1s
6 actionable tasks: 5 executed, 1 up-to-date

BUILD SUCCESSFUL in 1s
3 actionable tasks: 3 up-to-date
JUnit version 4.12
.log4j jndi lookup attempted: (sanitized) ldap://127.0.0.1:8899/evil
16:09:06.064 [main] ERROR trust.nccgroup.jnditest.test.JndiTest - Hello, _(log4j jndi disabled)_!

Time: 1.362

OK (1 test)

License

Licensed under the Apache 2 license.

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