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nccgroup / log4j-jndi-be-gone

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

Home Page:https://research.nccgroup.com/2021/12/12/log4j-jndi-be-gone-a-simple-mitigation-for-cve-2021-44228/

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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

As of version 1.1.0, log4j-jndi-be-gone defaults to attempting to handle repackaged (aka "shaded") versions of log4j that may be embedded in a JAR under an alternate package name to prevent collisions between an application's version of a depdencency and a depdendency's version of the same depdendency. However, it should be noted that log4j seems not to easily be repckaged under alternate package names/prefixes due to the use of reflection with static classnames and/or classnames from embedded configuration files.

This behavior can be disabled by placing =structureMatch=0 after the agent JAR path in the -javaagent: argument, e.g.:

-javaagent:path/to/log4j-jndi-be-gone-1.0.0-standalone.jar=structureMatch=0

which will result in the same matching behavior as 1.0.0, a simple exact string comparison against the class name.

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+.

Class Matching

The implementation starts by matching against classes with suffixes that match the innermost sub-packages and classname of org.apache.logging.log4j.core.lookup.JndiLookup, lookup.JndiLookup, as org.apache.logging.log4j.core can reasonably be expected to have been mangled by repackaging rules that did not seek to preserve package names. Additionally, instead of just performing similar checks for all other expected log4j types, it ensures that they exist under the same base package as well.

The implementation then walks through the structure of any identified potential log4j lookup.JndiLookup classes, attempting to validate against:

  • the modifiers on the class itself
  • the class' parent class and/or implemented interfaces (these differ between log4j versions)
  • the org.apache.logging.log4j.core.config.plugins.Plugin annotation expected across all 2.x versions, including annotation parameters and their values
  • the method lookup(), matching against its modifiers and type signature (and ignoring the 1-argument version from 2.0)
  • the method convertJndiName(), matching against its modifiers and type signature
  • the field CONTAINER_JNDI_RESOURCE_PATH_PREFIX, matching against its modifiers

Caveats

  • log4j-jndi-be-gone will not work if the log4j library has been obfuscated or if its class packages/names have been modified other than basic re-packaging (i.e. "shading").

    • FWIW, log4j 2.x is pretty inflexible with regards to being repackaged, so it's unclear how common such practices are.

  • 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. As of version 1.1.0, the log4j-jndi-be-gone standalone JAR bundles a repackaged Byte Buddy under its own package prefix. This should prevent any collisions.

  • If you have replaced your JndiLookup classes with implementations that attempt to do honeypotting or log lookup() calls, log4j-jndi-be-gone will potentially disable their lookup method, preventing them from working.

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.

Compatability

Java Versions Tested

log4j-jndi-be-gone has been tested on OpenJDK 6, 8, 11, and 17, and on the HotSpot and OpenJ9 JVMs.

Log4j Versions Tested

  • 2.0
  • 2.0.1
  • 2.0.2
  • 2.1
  • 2.2
  • 2.3
  • 2.4
  • 2.4.1
  • 2.5
  • 2.6
  • 2.6.1
  • 2.6.2
  • 2.7
  • 2.8
  • 2.8.1
  • 2.8.2
  • 2.9.0
  • 2.9.1
  • 2.10.0
  • 2.11.0
  • 2.11.1
  • 2.11.2
  • 2.12.0
  • 2.12.1
  • 2.12.2
  • 2.13.0
  • 2.13.1
  • 2.13.2
  • 2.13.3
  • 2.14.0
  • 2.14.1
  • 2.15.0
  • 2.16.0
  • 2.17.0

About

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

https://research.nccgroup.com/2021/12/12/log4j-jndi-be-gone-a-simple-mitigation-for-cve-2021-44228/

License:Apache License 2.0

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