CVE-2026-1225 Overview
CVE-2026-1225 is an Arbitrary Class Execution (ACE) vulnerability in the QOS.CH logback-core library, affecting versions up to and including 1.5.24. The flaw resides in how Logback processes configuration files in Java applications. An attacker with write access to an existing Logback configuration file can force the library to instantiate arbitrary classes already present on the application's classpath. The instantiated object is typically discarded immediately after construction, which substantially limits practical impact. The vulnerability is categorized under CWE-20: Improper Input Validation and requires local access with high privileges to exploit.
Critical Impact
An authenticated local attacker with write access to a Logback configuration file can instantiate arbitrary classpath-resident Java classes, with limited follow-on impact since the instance is generally discarded.
Affected Products
- QOS.CH logback-core versions up to and including 1.5.24
- Java applications that load Logback configuration from attacker-writable locations
- Fixed in logback-core1.5.25
Discovery Timeline
- 2026-01-22 - CVE-2026-1225 published to NVD
- 2026-04-15 - Last updated in NVD database
Technical Details for CVE-2026-1225
Vulnerability Analysis
Logback uses an XML-based configuration file (commonly logback.xml or logback-test.xml) to define appenders, encoders, filters, and other components. During configuration parsing, the Joran configurator instantiates Java classes referenced by class attributes in the configuration. The library does not constrain which classes may be instantiated beyond what is reachable on the classpath.
When an attacker can modify the Logback configuration file, they can specify any class on the classpath as a configuration component. Logback will then call the default constructor of that class during application initialization or configuration reload. This yields a primitive arbitrary class instantiation, classified as Arbitrary Class Execution (ACE).
Practical impact is constrained by two factors. First, the target class must already be present on the classpath. Second, the instantiated object is typically discarded shortly after creation because it does not satisfy the expected component interface. Side effects are therefore limited to whatever a class default constructor performs.
Root Cause
The root cause is improper input validation [CWE-20] in the configuration parser. Logback trusts the configuration file as a privileged input and does not enforce an allowlist of acceptable component types before invoking class instantiation through reflection.
Attack Vector
Exploitation requires local access and high privileges. The attacker must first obtain write access to the application's Logback configuration file, which is normally protected by filesystem permissions. The attacker then modifies the configuration to reference a target class on the application classpath. When the application loads or reloads its logging configuration, Logback instantiates the specified class. No remote or unauthenticated path exists for triggering the vulnerability through Logback itself.
No public proof-of-concept exploit code has been released for CVE-2026-1225. Refer to the Logback Release Announcement 1.5.25 for vendor technical details.
Detection Methods for CVE-2026-1225
Indicators of Compromise
- Unexpected modifications to logback.xml, logback-test.xml, or other Logback configuration files outside of normal deployment activity
- Application logs showing Joran configurator instantiation errors for classes that do not implement Logback component interfaces
- Process behavior showing Java applications loading unfamiliar classes immediately after a configuration reload
Detection Strategies
- Monitor filesystem integrity on directories containing Logback configuration files using file integrity monitoring (FIM)
- Compare deployed Logback configuration hashes against known-good baselines during application startup
- Inspect application startup logs for Joran warnings related to unknown or unexpected class names
Monitoring Recommendations
- Audit and alert on write events to logging configuration paths under application installation directories
- Track the version of logback-core in software bills of materials (SBOM) and flag instances at or below 1.5.24
- Correlate configuration file changes with the identity of the user or service account performing the modification
How to Mitigate CVE-2026-1225
Immediate Actions Required
- Upgrade logback-core to version 1.5.25 or later across all Java applications and build pipelines
- Restrict write permissions on Logback configuration files to administrative accounts only
- Verify that production application accounts do not have write access to their own logging configuration
Patch Information
QOS.CH addressed CVE-2026-1225 in logback-core1.5.25. Update Maven, Gradle, or other dependency manifests to require 1.5.25 or later for both logback-core and logback-classic. See the Logback Release Announcement 1.5.25 for release notes.
Workarounds
- Set strict filesystem permissions so that only privileged users can modify Logback configuration files
- Deploy Logback configuration files from immutable container images or read-only mounts where feasible
- Disable automatic configuration reload by removing scan="true" from the root <configuration> element if dynamic reload is not required
# Configuration example: enforce read-only Logback configuration in Linux
chown root:appgroup /opt/app/config/logback.xml
chmod 0644 /opt/app/config/logback.xml
# Maven dependency update to patched version
# <dependency>
# <groupId>ch.qos.logback</groupId>
# <artifactId>logback-core</artifactId>
# <version>1.5.25</version>
# </dependency>
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
