CVE-2026-63317 Apache OpenNLP: Arbitrary Class Instantiation in GeneratorFactory via Feature Descriptor XML

Medium 5.6 Published 24 Jul 2026 Updated 24 Jul 2026

Attackers can trigger arbitrary class instantiation via crafted model archives or format names, potentially leading to code execution if vulnerable classes are on the classpath.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: Not needed
Fix: Available

What is CVE-2026-63317?

CVE-2026-63317 is a medium-severity unsafe deserialization vulnerability in Apache OpenNLP, published on 24 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

Arbitrary Class Instantiation via XML Feature Generator Descriptor and Format Name in Apache OpenNLP

Versions Affected:

- before 2.5.10 - before 3.0.0-M5

Description:

Three code paths in Apache OpenNLP load a class by its fully-qualified name via Class.forName() and invoke its no-arg constructor without any prior validation of the class name or its type.

The affected paths are:

(1) GeneratorFactory, which reads the class attribute of generator elements in an XML feature generator descriptor; such descriptors are embedded as artifacts in model archives (e.g. TokenNameFinder and POSTagger models) and are parsed during model loading, so an attacker who can supply a crafted model archive controls the class name directly.

(2) StreamFactoryRegistry.getFactory(Class, String), which falls back to interpreting an unregistered format name as the fully-qualified class name of an ObjectStreamFactory; this is exploitable in applications that pass untrusted format names (e.g. exposing the -format parameter of the command-line tooling to external input).

(3) StringInterners, which instantiates the interner implementation named by the opennlp.interner.class system property; this value is normally deployer-controlled, so it is hardened as defense in depth rather than being independently attacker-reachable.

Exploitation requires a class with attacker-useful side effects in its static initializer or no-arg constructor (JNDI lookup, outbound network I/O, filesystem access) to be present on the classpath, so this is not drop-in remote code execution. T

Mitigation:

Upgrade to a fixed release.

The fix routes all three paths through ExtensionLoader.instantiateExtension(...), which consults a package-prefix allowlist before Class.forName() is invoked, so a disallowed class is never loaded, initialized, or constructed. Classes under the opennlp. prefix remain permitted by default. Deployments that load models referencing feature generator factories, object stream factories, or string interners 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 all model files and format names are sourced from trusted origins and should audit their classpath for classes with side-effecting static initializers or constructors.

Weakness: CWE-470

Which versions are affected by CVE-2026-63317?

It affects 1 product.

Linux distribution packages (1)

opennlp openSUSE:Tumbleweed All versions 1.9.5-2.1

Products include Apache Opennlp.

How do I fix CVE-2026-63317?

Upgrade to a fixed version.

Upgrade, and only deserialise data from trusted sources or with an allow-list of types. More about Unsafe deserialization.

Is CVE-2026-63317 being exploited?

There is no known exploitation of CVE-2026-63317 in the wild and no public exploit code.

Sources

Also known as DEBIAN-CVE-2026-63317, GHSA-ccq5-x868-5p6x, RHLW-2026-2aebdb8547d9b041, UBUNTU-CVE-2026-63317, openSUSE-SU-2026:11369-1.