Critical [CVE-2026-42027] Apache OpenNLP ExtensionLoader: Arbitrary Class Instantiation

Arbitrary Class Instantiation via Model Manifest in Apache OpenNLP ExtensionLoader

Versions Affected: before 1.9.5, before 2.5.9, before 3.0.0-M3

The ExtensionLoader.instantiateExtension(Class, String) method loads a class by its fully-qualified name via Class.forName() and invokes its no-arg constructor, with the class name sourced from the manifest.properties entry of a model archive.

The existing isAssignableFrom check correctly rejects classes that are not subtypes of the expected extension interface (BaseToolFactory for factory=, ArtifactSerializer for serializer-class-*), but the check runs after Class.forName() has already loaded and initialized the named class.

Class.forName() with default initialization semantics executes the target class's static initializer before returning, so an attacker who can supply a crafted model archive can cause the static initializer of any class on the classpath to run during model loading, regardless of whether that class passes the subsequent type check.