CVE-2026-104183: stream-json: Prototype pollution: Assembler writes this.current[this.key] on plain objects
stream-json is a micro-library of stream components for processing JSON and JSONC with a minimal memory footprint. Prior to 3.6.0, Assembler materializes object properties with plain assignment, so an input key named proto invokes the inherited setter and causes parsed object prototype replacement instead of creating an own data property. Applications that make authorization or feature decisions from inherited values can therefore consume attacker-controlled properties, and a null prototype can disrupt code that expects Object.prototype methods. The researcher treats parsing untrusted JSON as part of the project contract, while the maintainer states that documented inputs are locally owned dumps, exports, or logs and characterizes the attack vector as local. The global Object.prototype is not polluted. This issue is fixed in version 3.6.0.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
stream-jsonto a version that resolves this vulnerability.Fixed in 3.6.0
Event History
Frequently Asked Questions
Which deployments are realistically exposed?
Deployments that use stream-json to parse attacker-controlled or otherwise untrusted JSON are exposed, particularly if application logic makes authorization or feature decisions based on inherited property values. The maintainer describes the documented input sources as locally owned dumps, exports, or logs and considers the attack vector local.
What must an attacker provide to trigger the issue?
An attacker needs to cause the application to parse JSON containing an object key named "__proto__". During assembly, that key can replace the prototype of the parsed object rather than becoming an own data property.
Does this pollute the global Object.prototype?
No. The issue affects the prototype of the parsed object; it does not pollute the global Object.prototype.
What is the remediation?
Upgrade stream-json to version 3.6.0, which fixes the issue. If an immediate upgrade is not possible, avoid parsing untrusted JSON with the affected component and ensure security-sensitive decisions do not rely on inherited properties from parsed objects.
How can I determine whether an input may have triggered the issue?
Inspect JSON inputs processed by affected versions for object properties named "__proto__". Parsed objects from such input may have attacker-controlled inherited properties or a null prototype, which can also break code expecting Object.prototype methods.