CVE-2026-116486 in Objenesis
Summary
by MITRE • 10/26/2026
Objenesis before 3.6 contains a type confusion vulnerability that allows code in one class loader to substitute objects into another by exploiting instantiator caching keyed on class name. When a shared ObjenesisStd instance caches instantiators by clazz.getName() in ObjenesisBase.getInstantiatorOf(), same-named classes collide, causing newInstance() to return wrong-class objects and ClassCastException failures.
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Analysis
by VulDB Data Team • 10/26/2026
The vulnerability identified in versions of the Objenesis library prior to 3.6 represents a critical type confusion flaw rooted in the internal caching mechanism used for object instantiation. Objenesis is widely utilized as a dependency injection framework, particularly within popular Java testing and mocking libraries such as Mockito, where it facilitates the creation of objects without invoking their constructors. The core issue lies in how the library manages its cache of instantiators to optimize performance by reusing previously created factory instances. Specifically, the static instance ObjenesisStd caches these instantiators using a map keyed exclusively on the fully qualified class name obtained via clazz.getName(). This design choice assumes that every unique string returned by getName() corresponds to exactly one distinct Class object within the JVM's type system.
This assumption fails in environments where multiple class loaders are present, such as application servers like Apache Tomcat or JBoss WildFly, or even within complex microservice architectures using custom class loading strategies. In these scenarios, it is possible for two different classes with identical fully qualified names to exist simultaneously but be loaded by separate ClassLoader instances. Because the cache key relies solely on the string representation of the class name rather than a combination of the class name and its defining ClassLoader or identity hash code, the library treats these distinct classes as identical. When ObjenesisBase.getInstantiatorOf() is invoked for one of these classes, it retrieves an instantiator cached under that shared key, which was originally created for the other class loaded by a different loader.
The operational impact of this type confusion is severe and manifests primarily through runtime exceptions that can destabilize applications relying on dynamic object creation. When the application attempts to instantiate the second class using the misaligned cache entry, it receives an instance of the first class instead. This results in immediate ClassCastException failures when the code expects an instance of the target class but receives a different type. Beyond simple crashes, this behavior can lead to subtle logical errors if the two classes share similar method signatures or inheritance structures, potentially allowing for unexpected object substitution that bypasses intended security controls or business logic validations. In testing environments, it causes flaky test failures and unreliable mock behaviors, undermining confidence in automated quality assurance processes.
From a vulnerability classification perspective, this flaw aligns with CWE-843, which describes the access of an incorrect type through its pointer, specifically highlighting issues arising from improper handling of object types due to caching or reference errors. It also relates closely to CWE-170, indicating Improper Null Termination that leads to buffer over-read in some contexts, though here it is more accurately categorized under logic flaws leading to type confusion. In the context of the MITRE ATT&CK framework for enterprise security, this vulnerability could be leveraged as part of a broader attack chain involving improper input validation or exploitation of dependency management weaknesses, potentially facilitating unauthorized code execution if an attacker can control which classes are loaded and trigger the collision scenario in a privileged context.
Mitigation strategies focus primarily on upgrading to Objenesis version 3.6 or later, where this caching logic has been corrected to include ClassLoader identity in the cache key, ensuring that instantiators are properly isolated per class loader environment. For applications unable to upgrade immediately due to legacy constraints, developers should ensure strict isolation of class loading contexts and avoid sharing Objenesis instances across different ClassLoaders if possible. Additionally, auditing dependencies for known vulnerabilities using tools like OWASP Dependency-Check is recommended to identify other potential risks within the software supply chain that might exploit similar assumptions about type uniqueness in dynamic languages.