stream-json has a prototype pollution issue: Assembler writes this.current[this.key] on plain objects

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

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

EPSS

• CVE-2026-104183: 0.19% estimated exploitation activity in the next 30 days; percentile 9.32 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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: Full proof-of-concept source (poc-stream-json-proto-injection.mjs): Observed output (verbatim, Node 24.15.0, Windows): 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…

Published by the source

5 Oct 2026, 22:49 UTC

Last modified by the source

5 Oct 2026, 23:00 UTC

Source last checked

26 Oct 2026, 13:06 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-mjw6-4jj6-33hc

2. Revision 2 · amended · 24 Oct 2026

   Amended from osv record GHSA-mjw6-4jj6-33hc

3. Revision 1 · created · 23 Oct 2026

   Published from ghsa record GHSA-mjw6-4jj6-33hc

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# Patch Next advisory brief: CVE-2026-104183

Advisory: stream-json has a prototype pollution issue: Assembler writes this.current[this.key] on plain objects
IDs: CVE-2026-104183, GHSA-mjw6-4jj6-33hc
Record: https://patchnext.com/advisories/cve-2026-104183 (revision 3, source checked 26 Oct 2026, 13:06 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.1)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: stream-json 3.6.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm stream-json: affected < 3.6.0; fixed in 3.6.0

## Suggested inspection (read-only)
- `npm ls stream-json --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on stream-json.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-mjw6-4jj6-33hc (GitHub-reviewed advisory GHSA-mjw6-4jj6-33hc lists affected ranges: npm stream-json >=0 <3.6.0)
- Verified: https://osv.dev/vulnerability/GHSA-mjw6-4jj6-33hc (OSV record GHSA-mjw6-4jj6-33hc lists affected ranges: npm stream-json >=0 <3.6.0 (fixed))

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