Piscina Worker Pool: Prototype-Pollution Gadget Can Lead to Code Execution (CVE-2026-102992)

What happened

Piscina stores the ThreadPool.options object as a plain JavaScript object in src/index.ts, meaning it inherits properties from Object.prototype rather than using a null-prototype or otherwise isolated object. If an application using piscina has a separate prototype-pollution primitive (a different vulnerability that lets an attacker set arbitrary properties on Object.prototype), that primitive can supply inherited values for piscina options that do not have their own explicit defaults.

Technical cause

Because ThreadPool.options is a plain object, three security-sensitive options are reachable via prototype inheritance when no own value is set: execArgv, which is passed directly to the Node.js Worker constructor and can be used to preload attacker-controlled code into worker threads; loadBalancer, a function that executes during task scheduling and can therefore run attacker-supplied logic; and env, which can alter the environment variables available to worker threads. This is classified as CWE-1321, Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution').

Why it matters

On its own, this issue in piscina is a gadget rather than a standalone exploit: it requires a separate prototype-pollution vulnerability elsewhere in the application to be present and reachable by an attacker. However, when that precondition is met, the impact is severe, since influencing execArgv or loadBalancer can lead to execution of attacker-controlled code in worker threads. The CVSS 4.0 base score of 9.2 reflects this high potential impact on confidentiality, integrity, and availability once the gadget is triggered.

Affected versions

Piscina versions before 4.9.4 are affected. Versions from 5.0.0 up to (but not including) 5.3.2 are affected. Pre-release versions from 6.0.0-rc.1 up to (but not including) 6.0.0-rc.5 are also affected.

Fixes and mitigation

The issue is fixed in piscina versions 4.9.4, 5.3.2, and 6.0.0-rc.5. Applications should update to one of these fixed versions. Note that this gadget is only exploitable in combination with a separate prototype-pollution vulnerability elsewhere in the application; eliminating any such pollution primitives in application code is also an important mitigating step, independent of the piscina update.

Recommended action

Update piscina to version 4.9.4, 5.3.2, or 6.0.0-rc.5 depending on your current release line. Additionally, audit your application and its dependencies for other prototype-pollution vulnerabilities, since this piscina issue by itself requires such a primitive elsewhere to be exploitable.