log4j-finder

A Python3 script to scan the filesystem to find Log4j2 that is vulnerable to Log4Shell (CVE-2021-44228 & CVE-2021-45046 & CVE-2021-45105). It scans recursively both on disk and inside (nested) Java Archive files (JARs).

How it works

log4j-finder identifies log4j2 libraries on your filesystem using a list of known bad and known good MD5 hashes of specific files (currently only JndiManager.class) present in log4j2-core-* packages; the main package that is affected by log4shell. It searches for these files inside Java Archive files and on the filesystem. The log4j2 version is then identified based on the MD5 hash of this file.

To optimize scanning speed, it searches the filesystem and processes ONLY the following filenames:

• All files with Java ARchive file extensions in the filename (also nested in these archives):
  • *.jar, *.war, *.ear
• Filenames that we have known bad and good hashes for (also inside above archives, and nested):
  • JndiManager.class

If the file matches one of the extensions mentioned above, it will check inside these archives (all in memory, nothing is unpacked) to search for the filenames that the script has known hashes for. It also looks inside nested archives, for example, a JAR file in a WAR file.

The script does NOT scan other archive file extensions such as 7z, RAR, TAR, BZ2, etc. So, for example, if a JAR file is inside a 7z file, the script will not find it. The rationale is that Java can only load Java ARchive formats so we only scan those.

Unknown MD5 hashes are shown as UNKNOWN; this could happen if a non log4j2 Java package uses the same filename that this script searches for. It's most likely not log4j2 if the identified file path does not contain references to org/apache/logging/log4j. However, manual verification is still recommended.

Downloading and running

You can install log4j-finder using one of the following methods:

Using the release binary

You can download the correct binary for your Operating System:

• Windows latest: log4j-finder.exe
  • This can trigger your AntiVirus due to it being a PyInstaller executable. You can also generate the executable yourself, see "Generating log4j-finder executables" on how to do this.
• Linux x86_64 latest: log4j-finder

If you are on Linux you can also download the latest release and run using one of the following ways:

curl -L https://github.com/fox-it/log4j-finder/releases/latest/download/log4j-finder -o log4j-finder
chmod +x log4j-finder
sudo ./log4j-finder

wget https://github.com/fox-it/log4j-finder/releases/latest/download/log4j-finder -O log4j-finder
chmod +x log4j-finder
sudo ./log4j-finder

Using Python 3

For distribution with Python 3.6+ installed, one following methods also work:

curl -L https://github.com/fox-it/log4j-finder/raw/main/log4j-finder.py -o log4j-finder.py
sudo python3 log4j-finder.py

wget https://github.com/fox-it/log4j-finder/raw/main/log4j-finder.py
sudo python3 log4j-finder.py

git clone https://github.com/fox-it/log4j-finder
cd log4j-finder
sudo python3 log4j-finder.py

Generating log4j-finder executables

Auto generated executables

There is a GitHub Action in the repository that automatically generates a Windows and Linux binary of the log4j-finder.py script using PyInstaller on every commit. The build artifacts of these workflow runs are used to attach to the Releases page.

We are aware that some Anti Virus vendors don't like the Windows binaries, in that case we recommend using generating the executable yourself using the following steps.

Generating the Windows executable

1. If you don't have Python 3.6 or higher installed, download it first from https://www.python.org/downloads/

   • Choose Python 3.8.10 if you want your binary to work on Windows 7:
     • Download Python 3.8.10 Windows installer (32 bit)
     • Download Python 3.8.10 Windows installer (64-bit)
   • Ensure that during install you choose: Add Python 3.x to PATH, this makes the following steps much easier.

2. Open a command prompt and use pip to install the pyinstaller package:

   pip install pyinstaller
   
   # In the output you will see where pyinstaller is installed, for example:
   # C:\Users\User\AppData\Roaming\Python\Python310\Scripts
   #
   # Verify using --version
   C:\Users\User\AppData\Roaming\Python\Python310\Scripts\pyinstaller.exe --version
   4.7

3. Optionally install the colorama package to add support for colors:

   pip install colorama

4. Download the latest version of the log4j-finder.py script and then run PyInstaller:

pyinstaller --onefile --hidden-import colorama log4j-finder.py

The Windows executable is then in the dist directory: dist\log4j-finder.exe

Generating the Linux executable

Example given for Debian 11:

# Install PyInstaller using pip3
sudo apt update
sudo apt install python3-pip git
pip3 install --user pyinstaller

# Git clone and build using PyInstaller
git clone https://github.com/fox-it/log4j-finder
cd log4j-finder
~/.local/bin/pyinstaller --onefile log4j-finder.spec

# Verify that the binary works
./dist/log4j-finder --help

Usage

Example usage to scan a path (defaults to /):

$ python3 log4j-finder.py /path/to/scan

Or directly a JAR file:

$ python3 log4j-finder.py /path/to/jarfile.jar

Or multiple directories and or files:

$ python3 log4j-finder.py /path/to/dir1 /path/to/dir2 /path/to/jarfile.jar

Exclude files or directories:

$ python3 log4j-finder.py / --exclude "/*/.dontgohere" --exclude "/home/user/*.war"

Note that on Windows it only scans the root c:\ drive if you don't give any extra arguments. We recommend specifying the drives you need to scan on the commandline such as (drives that don't exist are skipped):

log4j-finder.exe c:\ d:\ e:\ f:\

Files or directories that cannot be accessed (Permission denied errors) are not printed. If you want to see more output, you can give the -v flag for verbose, or -vv for debug mode (only recommended for debugging purposes).