On February 2026, a critical issue was discovered in the popular Java logging library, logback-core, up to and including version 1.5.24, maintained by QOS.CH. Tracked as CVE-2026-1225, this vulnerability exposes Java applications to Arbitrary Code Execution (ACE) if an attacker can modify the application’s logback configuration file. The core of this issue lies in how logback parses and processes its configuration files, enabling a malicious actor to instantiate any Java class present on the classpath.

What Is Logback?

Logback is a widely-used Java logging framework, serving as the successor to log4j and *the* default backend for the popular SLF4J API.

How Does the Vulnerability Happen?

Most Java applications using logback define their logging setup in an XML configuration file, such as logback.xml. The problem? Logback allows certain XML elements (like <component> or <appender>) to refer to _arbitrary_ classes as long as they exist on the classpath.

If an attacker can modify this file and knows or supplies a malicious Java class on the classpath, they can trick logback into loading it — and possibly trigger its static block or constructor, executing any setup code embedded there.

Suppose you have a malicious class named EvilClass on your classpath

package com.example;

public class EvilClass {
    static {
        // This code runs when the class is loaded!
        Runtime.getRuntime().exec("touch /tmp/gotcha");
    }
    public EvilClass() {
        // This is also executed upon instantiation
        System.out.println("EvilClass instantiated!");
    }
}

If an attacker modifies the configuration file (logback.xml) to include

<configuration>
  <!-- Normal configuration ... -->
  <include resource="some-legit-config"/>
  
  <!-- Malicious Component Introduction -->
  <component class="com.example.EvilClass"/>
</configuration>

When the application starts (or the config file is reloaded), logback will try to instantiate com.example.EvilClass, triggering any static blocks or constructor code. The touch /tmp/gotcha command will be executed on the server.

Attack vector: Write access to logback.xml (or similar config)

- Payload: Attacker needs to inject a <component class="..."/> element or replace the class attribute of an existing appender or filter.
- Impact: If the class runs malicious code (OS commands, reverse shell, etc.), it is executed with the Java process's privileges.

Note: Instantiated objects are commonly discarded after evaluation — meaning no references remain — but the damage already happens if constructor or static blocks house malicious logic.

How Serious Is This?

- Not a Remote Code Execution (RCE) bug “by default” — attacker must have access to the configuration file *and* the necessary Java class must be present.
- However, in real-world deployments (like insecure Docker images or CI/CD pipelines), gaining write access to config file isn’t out of the question.
- If you leave config files world-writable or mount shared volumes with broad permissions, you're at risk.

Upgrade logback-core to version > 1.5.24 (latest stable preferred).

- See logback changelog for release notes and security patches.

Check for unnecessary or world-writable permissions on configuration files.

3. Use restrictive security policies to prevent untrusted classes from being placed on the classpath.

References

- CVE-2026-1225 at NVD (when published)
- Upstream logback-core project
- logback documentation on configuration

Conclusion

CVE-2026-1225 is a powerful reminder that “configuration as code” cuts both ways. If you’re using logback-core ≤1.5.24, audit your configs, patch fast, and don’t give low-privilege users access to logging configuration files. Remember — any class on the classpath is a potential weapon if your config files aren’t protected.

Feel free to share or adapt this guide for your team's security awareness!

Timeline

Published on: 01/22/2026 09:24:14 UTC
Last modified on: 01/26/2026 15:04:59 UTC