protobufjs: Text Format string map parsing can mutate returned map object prototype

Exploitation

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

CVSS

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

EPSS

• CVE-2026-59876: 0.35% estimated exploitation activity in the next 30 days; percentile 28.62 (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 protobuf.js text format extension parsed string-keyed map entries using ordinary property assignment. A text-format map entry with key proto could therefore change the prototype of the returned map object instead of creating an own map entry. This affected the optional Text Format extension. Other protobufjs input paths, including binary decode, fromObject, and ProtoJSON conversion, are not affected. Impact An attacker who can provide protobuf text-format input parsed by an application using protobufjs/ext/textformat may be able to create message objects whose string-keyed map fields have attacker-controlled prototypes. This is per-object prototype mutation, not global Object.prototype pollution. Impact depends on downstream application logic treating inherited properties as meaningful, for example by using in, truthiness checks, or direct property access on parsed map objects instead of own-property checks. Applications that do not parse untrusted Text Format input, or…

Published by the source

20 Jul 2026, 22:04 UTC

Last modified by the source

10 Sep 2026, 03:50 UTC

Source last checked

26 Oct 2026, 12:57 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-jfj6-75fj-8934

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-jfj6-75fj-8934

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# Patch Next advisory brief: CVE-2026-59876

Advisory: protobufjs: Text Format string map parsing can mutate returned map object prototype
IDs: CVE-2026-59876, GHSA-jfj6-75fj-8934
Record: https://patchnext.com/advisories/cve-2026-59876 (revision 2, source checked 26 Oct 2026, 12:57 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 4.8)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: protobufjs 8.6.5. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm protobufjs: affected >= 8.2.0, <= 8.6.4; fixed in 8.6.5

## Suggested inspection (read-only)
- `npm ls protobufjs --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 protobufjs.
- 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-jfj6-75fj-8934 (OSV record GHSA-jfj6-75fj-8934 lists affected ranges: npm protobufjs >=8.2.0 <8.6.5 (fixed))
- Verified: https://github.com/advisories/GHSA-jfj6-75fj-8934 (GitHub-reviewed advisory GHSA-jfj6-75fj-8934 lists affected ranges: npm protobufjs >=8.2.0 <=8.6.4)

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