CVE-2026-42027 Vulnerability Analysis & Exploit Details

CVE-2026-42027 Vulnerability Summary

CVE-2026-42027: Arbitrary Class Instantiation via Model Manifest in Apache OpenNLP ExtensionLoader Versions Affected: before 2.5.9, before 3.0.0-M3 Description:  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. Exploitation requires a class with attacker-useful side effects in its static initializer (for example, JNDI lookup, outbound network I/O, or filesystem access) to be present on the classpath, so this is not a drop-in remote code execution; however, the attack surface grows as third-party model distribution becomes more common (community model repositories, Hugging Face-style sharing), where users routinely load model files from origins they do not control. A secondary, narrower vector affects deployments that ship legitimate BaseToolFactory or ArtifactSerializer subclasses with side-effecting no-arg constructors: a malicious manifest can name such a class and force its constructor to run during model load. Mitigation:  * 2.x users should upgrade to 2.5.9. * 3.x users should upgrade to 3.0.0-M3. Note: The fix introduces a package-prefix allowlist that is consulted before Class.forName() is invoked, so the static initializer of a disallowed class is never executed. Classes under the opennlp. prefix remain permitted by default. Deployments that load models referencing factories or serializers outside opennlp.* must opt those packages in, either programmatically via ExtensionLoader.registerAllowedPackage(String) before the first model load, or by setting the OPENNLP_EXT_ALLOWED_PACKAGES system property to a comma-separated list of allowed package prefixes. Users who cannot upgrade immediately should ensure that all model files are sourced from trusted origins and should audit their classpath for classes with side-effecting static initializers or constructors, particularly any that perform JNDI lookups, network requests, or filesystem operations during class initialization.

CVE-2026-42027 References

External References

• https://lists.apache.org/thread/ltlo4powjfc0w2w2yyl1o5tc7q1gcb2y
• http://www.openwall.com/lists/oss-security/2026/05/01/20

CWE Common Weakness Enumeration

CAPEC Common Attack Pattern Enumeration and Classification

• Reflection Injection CAPEC-138 An adversary supplies a value to the target application which is then used by reflection methods to identify a class, method, or field. For example, in the Java programming language the reflection libraries permit an application to inspect, load, and invoke classes and their components by name. If an adversary can control the input into these methods including the name of the class/method/field or the parameters passed to methods, they can cause the targeted application to invoke incorrect methods, read random fields, or even to load and utilize malicious classes that the adversary created. This can lead to the application revealing sensitive information, returning incorrect results, or even having the adversary take control of the targeted application.

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