GHSA-CX4M-2P55-RW7J
Vulnerability from github – Published: 2026-05-04 18:30 – Updated: 2026-05-08 17:53Arbitrary 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.
{
"affected": [
{
"package": {
"ecosystem": "Maven",
"name": "org.apache.opennlp:opennlp-tools"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "2.5.9"
}
],
"type": "ECOSYSTEM"
}
]
},
{
"package": {
"ecosystem": "Maven",
"name": "org.apache.opennlp:opennlp-tools"
},
"ranges": [
{
"events": [
{
"introduced": "3.0.0-M1"
},
{
"fixed": "3.0.0-M3"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2026-42027"
],
"database_specific": {
"cwe_ids": [
"CWE-470"
],
"github_reviewed": true,
"github_reviewed_at": "2026-05-08T17:53:14Z",
"nvd_published_at": "2026-05-04T17:16:24Z",
"severity": "CRITICAL"
},
"details": "Arbitrary Class Instantiation via Model Manifest in Apache OpenNLP ExtensionLoader\n\n\n\n\n\nVersions Affected: before 2.5.9, before 3.0.0-M3\n\n\n\n\n\nDescription:\u00a0\n\nThe ExtensionLoader.instantiateExtension(Class, String)\u00a0method loads a class by its fully-qualified name via Class.forName()\u00a0and invokes its no-arg constructor, with the class name sourced from the manifest.properties\u00a0entry of a model archive. The existing isAssignableFrom\u00a0check correctly rejects classes that are not subtypes of the expected extension interface (BaseToolFactory\u00a0for factory=, ArtifactSerializer\u00a0for serializer-class-*), but the check runs after\u00a0Class.forName()\u00a0has already loaded and initialized the named class. \n\nClass.forName()\u00a0with default initialization semantics executes the target class\u0027s 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. \n\nExploitation 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\u00a0or ArtifactSerializer\u00a0subclasses with side-effecting no-arg constructors: a malicious manifest can name such a class and force its constructor to run during model load.\n\n\n\n\n\nMitigation:\u00a0\n\n\n\n * 2.x users should upgrade to 2.5.9. \n * 3.x users should upgrade to 3.0.0-M3. \n\n\n\n\nNote: The fix introduces a package-prefix allowlist that is consulted before Class.forName()\u00a0is invoked, so the static initializer of a disallowed class is never executed. Classes under the opennlp.\u00a0prefix remain permitted by default. Deployments that load models referencing factories or serializers outside opennlp.*\u00a0must opt those packages in, either programmatically via ExtensionLoader.registerAllowedPackage(String)\u00a0before the first model load, or by setting the OPENNLP_EXT_ALLOWED_PACKAGES\u00a0system property to a comma-separated list of allowed package prefixes. \n\nUsers who cannot upgrade immediately should ensure that all model files are sourced from trusted origins\u00a0and 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.",
"id": "GHSA-cx4m-2p55-rw7j",
"modified": "2026-05-08T17:53:14Z",
"published": "2026-05-04T18:30:31Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-42027"
},
{
"type": "PACKAGE",
"url": "https://github.com/apache/opennlp"
},
{
"type": "WEB",
"url": "https://lists.apache.org/thread/ltlo4powjfc0w2w2yyl1o5tc7q1gcb2y"
},
{
"type": "WEB",
"url": "http://www.openwall.com/lists/oss-security/2026/05/01/20"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H",
"type": "CVSS_V3"
}
],
"summary": "Apache OpenNLP ExtensionLoader Vulnerable to Arbitrary Class Instantiation via Model Manifest"
}
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or observed by the user.
- Confirmed: The vulnerability has been validated from an analyst's perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.