CVE-2026-42027: Apache OpenNLP: Arbitrary Class Instantiation via Model Manifest in ExtensionLoader

[Richard Zowalla <rzo1 () apache org>]

Severity: moderate 

Affected versions:

- Apache OpenNLP (org.apache.opennlp:opennlp-tools) before 2.5.9
- Apache OpenNLP (org.apache.opennlp:opennlp-tools) 3.0 before 3.0.0-M3

Description:

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.

This issue is being tracked as OPENNLP-1820 

Credit:

Subramanian S (finder)

References:

https://opennlp.apache.org/
https://www.cve.org/CVERecord?id=CVE-2026-42027
https://issues.apache.org/jira/browse/OPENNLP-1820