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


- **Severity:** Medium 4.8 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Fix available:** Yes
- **Published:** 8 Jul 2026
- **Updated:** 9 Jul 2026
- **Also known as:** CLEANSTART-2026-FF01212, GHSA-jfj6-75fj-8934, MINI-8g7c-2p9h-899g

## What is CVE-2026-59876?

CVE-2026-59876 is a medium-severity vulnerability in protobufjs protobuf.js, published on 8 Jul 2026. 

### Official 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.

Weakness: CWE-1321 (Improperly Controlled Modification of Object Prototype Attributes)

## Which versions are affected by CVE-2026-59876?

2 open source packages ship an affected version, including protobufjs. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `protobufjs` | npm | From 8.2.0 | 8.6.5 |
| `protobufjs` | npm | From 8.2.0 | 8.6.5 |

Products: Protobufjs Project Protobufjs.

## How do I fix CVE-2026-59876?

Upgrade protobufjs to 8.6.5 or later.

## Is CVE-2026-59876 being exploited?

There is no known exploitation of CVE-2026-59876 in the wild and no public exploit code.

## Other vulnerabilities in protobufjs protobuf.js

- [CVE-2023-36665](https://insights.sbomify.com/CVE-2023-36665/): protobufjs Prototype Pollution vulnerability (critical 9.8)
- [CVE-2026-41242](https://insights.sbomify.com/CVE-2026-41242/): protobufjs has an arbitrary code execution issue (critical 9.4)
- [CVE-2022-25878](https://insights.sbomify.com/CVE-2022-25878/): Prototype Pollution (high 8.2)
- [CVE-2026-44291](https://insights.sbomify.com/CVE-2026-44291/): protobufjs: Code generation gadget after prototype pollution (high 8.1)
- [CVE-2026-44293](https://insights.sbomify.com/CVE-2026-44293/): protobufjs: Code injection through bytes field defaults in generated toObject code (high 7.7)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-59876 and fix it if it is affected.

What it is: CVE-2026-59876 is a medium-severity vulnerability in protobufjs protobuf.js, published on 8 Jul 2026. 
Exploitation: There is no known exploitation of CVE-2026-59876 in the wild and no public exploit code.

Affected packages and fixed versions:
- protobufjs (npm): affected from 8.2.0; fixed in 8.6.5
- protobufjs (npm): affected from 8.2.0; fixed in 8.6.5

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-59876/ (Markdown: https://insights.sbomify.com/CVE-2026-59876/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-59876
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-59876
- OSV: https://osv.dev/vulnerability/CVE-2026-59876

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-59876/
Summaries are generated and may contain mistakes; the official description above is the record.