CVE-2026-1225: QOS.CH Logback-Core RCE Vulnerability

CVE-2026-1225 Overview

CVE-2026-1225 is an Arbitrary Code Execution (ACE) vulnerability affecting configuration file processing in QOS.CH logback-core up to and including version 1.5.24 in Java applications. This vulnerability allows an attacker to instantiate classes already present on the classpath by compromising an existing logback configuration file.

The exploitation of this vulnerability requires that the malicious Java class is already present on the user's classpath. Additionally, the attacker must have write access to a logback configuration file. However, even after successful instantiation, the instance is very likely to be discarded with no further action, limiting the overall impact.

Critical Impact

Attackers with write access to logback configuration files can instantiate arbitrary classes from the classpath, potentially leading to code execution depending on available classes.

Affected Products

• QOS.CH logback-core versions up to and including 1.5.24
• Java applications using vulnerable logback-core versions
• Applications with logback configuration files accessible to potential attackers

Discovery Timeline

• 2026-01-22 - CVE CVE-2026-1225 published to NVD
• 2026-01-22 - Last updated in NVD database

Technical Details for CVE-2026-1225

Vulnerability Analysis

This vulnerability stems from improper input validation (CWE-20) in logback-core's configuration file processing mechanism. When logback parses its XML or Groovy-based configuration files, it processes class instantiation directives without sufficient validation, allowing arbitrary class instantiation from the application's classpath.

The attack requires local access and has multiple prerequisites that significantly limit its exploitability. The attacker must first gain write access to a logback configuration file, which typically requires some form of prior access to the target system. Additionally, for meaningful exploitation, a suitable "gadget" class must already exist on the application's classpath that can perform dangerous operations upon instantiation.

The vulnerability's impact is further mitigated by the fact that instantiated objects are typically discarded immediately after creation, limiting the window for exploitation to constructor-based attacks or static initializer side effects.

Root Cause

The root cause is improper input validation in logback-core's configuration parsing mechanism. The configuration processor allows class instantiation directives to reference arbitrary classes on the classpath without validating whether those classes are safe to instantiate. This design decision, while providing flexibility for legitimate configuration needs, creates an attack surface when configuration files can be modified by malicious actors.

Attack Vector

The attack requires local access and involves compromising an existing logback configuration file. An attacker with write access to the configuration file can modify it to include directives that instantiate malicious or dangerous classes already present on the application's classpath.

The exploitation flow typically involves:

1. Gaining write access to a logback configuration file (e.g., logback.xml or logback-test.xml)
2. Identifying exploitable classes on the target application's classpath
3. Modifying the configuration file to instantiate those classes
4. Waiting for the application to reload the configuration or restart

Since no verified code examples are available, the vulnerability mechanism involves manipulating logback's XML configuration to reference arbitrary classes. Attackers would modify configuration elements that accept class names to point to dangerous classes on the classpath. For technical details, refer to the Logback Release Notes 1.5.25.

Detection Methods for CVE-2026-1225

Indicators of Compromise

• Unexpected modifications to logback configuration files (logback.xml, logback-test.xml, logback.groovy)
• Configuration file timestamps that don't align with normal deployment schedules
• Unusual class instantiation patterns in application logs

Detection Strategies

• Monitor file integrity of logback configuration files using file integrity monitoring (FIM) tools
• Implement configuration file change alerting in CI/CD pipelines and production environments
• Review application logs for unexpected class loading or instantiation errors
• Audit access controls on directories containing logback configuration files

Monitoring Recommendations

• Enable detailed logging for configuration file access and modifications
• Implement real-time alerting for changes to logback configuration files in production
• Use SentinelOne Singularity to monitor for suspicious file modifications and unauthorized configuration changes
• Regularly audit classpath contents for potentially dangerous gadget classes

How to Mitigate CVE-2026-1225

Immediate Actions Required

• Upgrade logback-core to version 1.5.25 or later which addresses this vulnerability
• Restrict file system permissions on logback configuration files to prevent unauthorized modifications
• Review and audit existing configuration files for suspicious or unexpected class references
• Implement file integrity monitoring on critical configuration files

Patch Information

QOS.CH has released logback-core version 1.5.25 which addresses this vulnerability. Organizations using affected versions should upgrade to 1.5.25 or later as soon as possible. Detailed release information is available in the Logback Release Notes 1.5.25.

Workarounds

• Implement strict file system permissions ensuring only authorized users can modify logback configuration files
• Deploy configuration files as read-only in production environments
• Use application containerization to isolate configuration files from potential attackers
• Consider implementing configuration file signing or validation before loading

# Restrict permissions on logback configuration files
chmod 640 /path/to/logback.xml
chown root:appgroup /path/to/logback.xml

# Verify current logback-core version in Maven projects
mvn dependency:tree | grep logback-core

# Update logback-core in pom.xml to patched version
# Change version from 1.5.24 or earlier to 1.5.25