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EUVD-2026-380920

fkie_cve-2026-104183

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.

GHSA-mjw6-4jj6-33hc — stream-json has a prototype pollution issue: Assembler writes this.current[this.key] on plain objects

Affected: npm: stream-json

### Summary A prototype-pollution vulnerability in stream-json lets attacker-controlled JSON replace a parsed object's prototype.

### Details The streaming JSON parser (StreamValues/StreamObject/jsonc variants) writes keys via plain assignment, so a __proto__ key replaces the parsed object's prototype with attacker-controlled content (verified 3.5.0 npm latest; native JSON.parse twin control stays clean). Parsing untrusted JSON is the contract. Assembler in Assembler.js.

### PoC

Run: ```bash node poc-stream-json-proto-injection.mjs ```

Full proof-of-concept source (poc-stream-json-proto-injection.mjs): ```js // stream-json v3.5.0 (npm latest) — local [[Prototype]] replacement via `__proto__` keys // Class: prototype injection in a JSON deserializer (jsonparse/bser/jsonc-parser/plist family) // Root cause: Assembler builds every object as a plain `Object` and writes // `this.current[this.key] = value` (src/core/assembler.js:160,180 — plain assignment, // ToPropertyKey → inherited Object.prototype __proto__ setter fires) // Spec baseline: JSON.parse (CreateDataProperty) → own data property, no prototype change // Run: node poc-stream-json-proto-injection.mjs (from C:/Users/rncb0/AppData/Local/Temp/0day/sj-lab) import { parser } from 'stream-json'; import jsoncParser from 'stream-json/jsonc/Parser.js'; import { streamValues } from 'stream-json/streamers/stream-values.js'; import { streamObject } from 'stream-json/streamers/stream-object.js'; import { Readable } from…