# CVE-2026-6342: Group prefix matching bypass for subscriptions

> Attackers can bypass subscription restrictions by creating groups with matching prefixes, allowing unauthorized access to restricted group subscriptions.

- **Severity:** Medium 4.3 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 18 May 2026
- **Updated:** 18 May 2026
- **Also known as:** GHSA-x3f7-3cx7-2cw7

## What is CVE-2026-6342?

CVE-2026-6342 is a medium-severity missing authorization vulnerability in Mattermost Mattermost Server, published on 18 May 2026. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

Mattermost Plugins versions <=11.5 11.1.5 10.13.11 11.3.4.0 fail to appropriately check for valid namespaces which allows plugin users to create subscriptions to groups that were not whitelisted via creating groups that share the same prefix as a whitelisted group. Mattermost Advisory ID: MMSA-2026-00601

Weakness: CWE-863 (Incorrect Authorization)

## Which versions are affected by CVE-2026-6342?

It affects 1 product.


Products: Mattermost Mattermost Server.

## How do I fix CVE-2026-6342?

No fix has been published for CVE-2026-6342 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-6342 being exploited?

There is no known exploitation of CVE-2026-6342 in the wild and no public exploit code.

## Other vulnerabilities in Mattermost Mattermost Server

- [CVE-2025-12419](https://insights.sbomify.com/CVE-2025-12419/): Account takeover on OAuth/OpenID-enabled servers (critical 9.9)
- [CVE-2025-12421](https://insights.sbomify.com/CVE-2025-12421/): Account Takeover via Code Exchange Endpoint (critical 9.9)
- [CVE-2025-4981](https://insights.sbomify.com/CVE-2025-4981/): Path Traversal Leading to RCE by Any Authenticated Mattermost User (critical 9.9)
- [CVE-2025-20051](https://insights.sbomify.com/CVE-2025-20051/): Arbitrary file read via block duplication in Mattermost Boards (critical 9.9)
- [CVE-2025-25279](https://insights.sbomify.com/CVE-2025-25279/): Arbitrary file read in Mattermost Boards via import & export board archive (critical 9.9)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-6342 and fix it if it is affected.

What it is: CVE-2026-6342 is a medium-severity missing authorization vulnerability in Mattermost Mattermost Server, published on 18 May 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-6342 in the wild and no public exploit code.

Affected product: Mattermost Mattermost Server. No package-level version data is available, so check the vendor advisory linked from the reference page.

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-2026-6342/ (Markdown: https://insights.sbomify.com/CVE-2026-6342/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-6342
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-6342
- OSV: https://osv.dev/vulnerability/CVE-2026-6342

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-6342/
Summaries are generated and may contain mistakes; the official description above is the record.