Log4JShell Bytecode Detector

This repository contains a tool to detect if a jar file is affected by the critical CVE-2021-44228. The tool scans the jar file and compares the classses against a set of vulnerable hashes for classes within the jar file. The hashes have been pre-computed for artifacts on Maven Central.

IMPORTANT UPDATE: We extended the list of affected artifacts on Maven Central. The list contains 18.816 artifacts. The artifacts either use log4j-core as declared dependency, or we identified .class files within the artifacts that match to the affected versions of log4j-core.

How to run this tool

1. Download the jar file under releases.
2. Run java -cp <PATH_TO_DOWNLOADED_JAR> de.codeshield.log4jshell.Log4JDetector <ABSOLUTE_PATH_TO_JAR_TO_CHECK>

If the jar is affected, the tool outputs information to the command-line:

CVE-2021-44228 found declared as dependency in META-INF/maven/org.apache.logging.log4j/log4j-core/pom.xml
CVE-2021-44228 found in class file org/apache/logging/log4j/core/net/JndiManager$1.class

How to run this tool on a live server (no need to stop your running Java instances)

1. Download the jar file under releases.
2. Run java -cp <PATH_TO_DOWNLOADED_JAR> de.codeshield.log4jshell.Log4JProcessDetector
3. The jar searches the classpath of all running java processes for vulnerable log4j instances

Background on CVE-2021-44228

A serious Remote Code Execution vulnerability has been discovered within log4j and version 2.0-beta9 to 2.14 are affected. The vulnerability has been classified as critical, as it affected log4j one of the most used logging libraries for Java. There are many references and article out there.

• Hackernews
• BSI (german only)
• Proof of Concept for exploiting the vulnerability
• Log4JShell Cheat Sheet

Why is it so important?

Log4j is and has been used in mostly any Java project for logging purporse. Now we need to understand which projects and libraries are actually affected. As of Java's dependency mechanism, an application can also be affected if it transitively includes the vulnerable library version. A project A includes a library lib transitively, if one of the direct dependecy B of A has a dependecy to lib. A simple test if one is affected can be done using using the maven dependecy tree:

Example: Execute command mvn dependency:tree on a maven project.

[INFO] exampleProject:mainProject:jar:0.0.1-SNAPSHOT
[INFO] +- exampleProject:lib-using-log4j:jar:0.0.1-SNAPSHOT:compile
[INFO] |  \- org.apache.logging.log4j:log4j-core:jar:2.14.1:compile

This check however, is only a first indication and does not suffices due to the following reasons.

Java programs are frequently:

• packaged as fatjar or uberjar: All class files (including direct and transitive dependencies) are shipped into a single jar file. (as we do for this software artifact)
• re-packaged or rebundled: the originaly package names are changed to avoid naming conflicts (some compilers and package mechanism apply code transformations)
• re-compiled: The source code is compiled with a different compiler and packaged anew, this leads to modified bytecode

Consquently, any library a Java project is using can include the vulnerability into your project. An in-depth bytecode analysis helps, which is what we publish as part of this repository.

How the script works

1. Extract pom.xml files from .jar
2. check declared dependencies against a pre-computed list of affected groupId:artifactId:version list for artifacts hosten on Maven Central
3. Extract .class files from .jar
4. Compute SHA hashes of the class file
5. Match SHAs against a pre-computed list

Build Instructions

To build this tool run

mvn compile

For assembling a runnable jar use

mvn clean compile assembly:single

Precomputed Hashes of Vulnerable Classes

The set of vulnerable hashes for classes has been pre-compute on entire Maven Central repository. The hashes of the classes contain all hashes that we identified as bytecode-similiar using the Fingerprinting technology.

The pre-computed information contains:

• all aritfacts that directly include log4j in any of the vulnerable version
• all artifacts that ship a class that contains a vulnerable hash of log4j in the affected version range
• all artifacts that rebundle a vulnerable class of log4j
• all artifacts that have a re-compiled class of a class of log4j in the affected version range

As affected version range we considered [2.0-beta9, 2.15) Reference.

Fingerprinting Technology

This tool uses a new bytecode fingerprinting technology for Java that has been invented by Andreas Dann. The basic flow is as follows.

1. Use the available fix commits Commit1, Commit2, and Commit3 to identify which classes are affected.
2. Compute bytecode "Fingerprints" of vulnerable classes.
3. Search for other classes on MavenCentral with same "Fingerprint".

Note: For a simpler roll out, this repository does ship SHA hashes of the vulnerable classes, the information has been computed using Fingerprinting

Disclaimers

The list of hashes and maven artifacts is not complete. We'll regularly update the lists.

Authors

• Andreas Dann [email protected]
• Johannes Spaeth [email protected]

If you want to learn more about the technology, reach out to us!

https://codeshield.io/