Locutus has Prototype Pollution via __proto__ Key Injection in unserialize()

Exploitation

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

CVSS 4.0

6.9 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N · score source

EPSS

• CVE-2026-33993: 0.59% estimated exploitation activity in the next 30 days; percentile 48.35 (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 The unserialize() function in locutus/php/var/unserialize assigns deserialized keys to plain objects via bracket notation without filtering the proto key. When a PHP serialized payload contains proto as an array or object key, JavaScript's proto setter is invoked, replacing the deserialized object's prototype with attacker-controlled content. This enables property injection, for...in propagation of injected properties, and denial of service via built-in method override. This is distinct from the previously reported prototype pollution in parsestr (GHSA-f98m-q3hr-p5wq, GHSA-rxrv-835q-v5mh) — unserialize is a different function with no mitigation applied. Details The vulnerable code is in two functions within src/php/var/unserialize.ts: expectArrayItems() at line 358: expectObject() at line 278: Both functions create a plain object ({}) and assign user-controlled keys via bracket notation. When the key is proto, JavaScript's proto setter replaces the object's prototype rather…

Published by the source

27 Mar 2026, 17:57 UTC

Last modified by the source

30 Mar 2026, 20:34 UTC

Source last checked

26 Oct 2026, 12:47 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-4mph-v827-f877

2. Revision 2 · amended · 24 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-4mph-v827-f877

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# Patch Next advisory brief: CVE-2026-33993

Advisory: Locutus has Prototype Pollution via __proto__ Key Injection in unserialize()
IDs: CVE-2026-33993, GHSA-4mph-v827-f877
Record: https://patchnext.com/advisories/cve-2026-33993 (revision 3, source checked 26 Oct 2026, 12:47 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.9)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: locutus 3.0.25. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm locutus: affected < 3.0.25; fixed in 3.0.25

## Suggested inspection (read-only)
- `npm ls locutus --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 locutus.
- 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://osv.dev/vulnerability/GHSA-4mph-v827-f877 (OSV record GHSA-4mph-v827-f877 lists affected ranges: npm locutus >=0 <3.0.25 (fixed))
- Verified: https://github.com/advisories/GHSA-4mph-v827-f877 (GitHub-reviewed advisory GHSA-4mph-v827-f877 lists affected ranges: npm locutus >=0 <3.0.25)

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