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.

Source published

1 Oct 2026 20:17:21 UTC

Source modified

1 Oct 2026 20:30:54 UTC

Observed here

23 Oct 2026 15:48:27 UTC

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CVSS 3.1 · 5.1 CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:L