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To determine if a host is vulnerable to log4j CVE‐2021‐44228

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check-log4j

This tool will try to determine if the host it is running on is likely vulnerable to the latest reason that the internet is on fire: the log4j RCE CVE‐2021‐44228.

This is different from other tools that attempt to verify whether a specific service is vulnerable by triggering the exploit and e.g., tracking pingbacks on a DNS canary token. That approach tells you whether a service is vulnerable, but it doesn't even tell you which specific systems: the payload may have been proxied on to another system and from there logged via log4j on yet another one. So inspection of the service does not tell you that that specific host is vulnerable.

On the other hand, host owners may not know whether they have a vulnerable version of log4j on their system: The log4j package may be pulled in as a dependency by various packages, or included inside a Java application jar.

The check-log4j tool attempts to give host owners a tool to determine likely vulnerability by looking at running java processes and inside of any common Java archive files found.

Please see the manual page for full details.

Installation

To install the command and manual page somewhere convenient, run make install; the Makefile defaults to '/usr/local' but you can change the PREFIX:

$ make PREFIX=~ install

FAQ

Dude, this is a shell script. You suck. Why isn't this written in $MyFavoriteLanguage?

check-log4j is intended to run on any Unix-like system without depending on any particular language runtime. It's not pretty, but hey.

Why does it say "Possibly vulnerable"?

Actual vulnerability depends on runtime configuration. check-log4j basically checks whether the file JndiLookup.class is found in any archive files. If so, the system becomes suspect. If check-log4j can determine that this might be a log4j-2.16.x version, it will remain silent, but otherwise, it simply doesn't know whether that class might be used or just sits there as an unused dependency or what.

This doesn't work on my system, explodes in some way, or doesn't correctly detect a vulnerable host!

I'm sorry. Please let me know about this via email or a GitHub issue or, better yet, a pull request with a fix.

Documentation

NAME
     check-log4j -- try to determine if a host is vulnerable to log4j
     CVE-2021-44228

SYNOPSIS
     check-log4j [-Vhv] [-j jar] [-p path] [-s skip]

DESCRIPTION
     The check-log4j tool attempts to determine whether the host it is exe-
     cuted on is vulnerable to the log4j RCE vulnerability identified as
     CVE-2021-4428.

     Since this vulnerability is in a specific Java class that may be inside
     nested Java archive files, check-log4j may be somewhat intrusive to run
     and should be executed with care and consideration of the system's load.
     Please see DETAILS for more information.

OPTIONS
     The following options are supported by check-log4j:

     -V	      Print version number and exit.

     -h	      Print a short help message and exit.

     -j jar   Check only this archive, nothing else.  Can be specified multi-
	      ple times for multiple JAR (or other zip formatted archive)
	      files.

     -p path  Limit filesystem traversal to this directory.  Can be specified
	      multiple times.  If not specified, check-log4j will default to
	      '/'.

     -s skip  Skip the given checks.  Valid arguments are 'files', 'packages',
	      and 'processes'.

     -v	      Be verbose.  Can be specified multiple times.

DETAILS
     CVE-2021-4428 describes a possible remote code execution (RCE) vulnera-
     bility in the popular log4j framework.  Simply causing the vulnerable
     system to log a specifically crafted message can the attacker gain com-
     mand execution and information disclosure capabilities.  This vulnerabil-
     ity relies on an insecure default setting applying to the Java Naming and
     Directory Interface (JNDI).

     Specifically, a system that contains the JndiLookup.class may enable the
     attack path in question.

     To determine whether a host is vulnerable, the check-log4j tool will per-
     form the following checks:
     o	 check for the existence of likely vulnerable packages
     o	 check for the existence of java processes using the 'JndiLookup'
	 class

     The discovery process may include running find(1), lsof(1), or rpm(1);
     please use the -s flag to skip any checks that might have a negative
     impact on your host.

     The output of the command attempts to be human readable and provide suf-
     ficient information to judge whether the host requires attention.

ENVIRONMENT
     The following environment variables influence the behavior of
     check-log4j:

     CHECK_LOG4J_FIND_OPTS_PRE
		   Additional options to pass to find(1) prior to the path
		   name(s).

		   By default, check-log4j runs "find / -type f -name
		   '*.[ejw]ar'"; the contents of this variable are placed
		   immediately after the 'find' and before the path name(s).

     CHECK_LOG4J_FIND_OPTS_POST
		   Additional options to pass to find(1) immediately after the
		   path name(s).

EXAMPLES
     Sample invocation on a non-vulnerable host:

	   $ check-log4j
	   No obvious indicators of vulnerability found.
	   $

     Sample invocation only looking at processes

	   $ ./check-log4j.sh -s files -s packages -v -v
	   => Running all checks...
	   ==> Skipping package check.
	   ==> Looking for jars...
	   ==> Skipping files check.
	   ==> Checking all found jars...
	   check-log4j.sh 1.0 localhost: Possibly vulnerable jar 'BOOT-INF/lib/log4j-core-2.14.1.jar' (inside of /home/jans/log4shell-vulnerable-app-0.0.1-SNAPSHOT.jar) used by process 15569.

	   $

     Sample invocation searching only /var and /usr/local/lib and skipping
     package and process checks:

	   $ check-log4j -p /var -p /usr/local/lib -s packages -s processes
	   Possibly vulnerable jar '/usr/local/lib/jars/log4j-core-2.15.0.jar'.
	   Possibly vulnerable jar '/usr/local/lib/jars/log4j-core-2.15.jar'.
	   Possibly vulnerable jar '/usr/local/lib/jars/log4j-core-2.jar'.
	   Possibly vulnerable jar '/usr/local/lib/jars/log4j-core.jar'.

	   $

     Note version comparisons are only done for packages, which is why the
     above output incudes files ending in a seemingly non-vulnerable version.

     To avoid mountpoint traversal on a Unix system where find(1) requires the
     -x flag to precede the paths:

	   $ env CHECK_LOG4J_FIND_OPTS_PRE="-x" check-log4j
	   No obvious indicators of vulnerability found.

     To only search files newer than '/tmp/foo':

	   $ env CHECK_LOG4J_FIND_OPTS_POST="-newer /tmp/foo" check-log4j
	   No obvious indicators of vulnerability found.

EXIT STATUS
     check-log4j will return 0 if the host was found not to be vulnerable and
     not in need of any update; it will return 1 if a vulnerable jar or pack-
     age was detected.

     If no vulnerability to CVE-2021-44228 / CVE-2021-45046 was found, but
     versions below the desired minimum were found, check-log4j will return 2.

SEE ALSO
     find(1), lsof(1), rpm(1)

HISTORY
     check-log4j was originally written by Jan Schaumann <jans@yahooinc.com>
     in December 2021.

BUGS
     Please file bugs and feature requests via GitHub pull requests and issues
     or by emailing the author.

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