# CVE-2026-15754: Missing per-channel team-scope check in ABAC access control policy unassign allows cross-team policy removal

> Authenticated team admins can remove access control policies from channels belonging to other teams due to missing scope validation.

- **Severity:** Medium 4.2 (`CVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:U/C:N/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** An administrator
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** No
- **Published:** 17 Aug 2026
- **Updated:** 17 Aug 2026
- **Also known as:** GHSA-mg83-f953-3vq5

## What is CVE-2026-15754?

CVE-2026-15754 is a medium-severity missing authorization vulnerability in Mattermost Mattermost Server, published on 17 Aug 2026. An attacker needs administrator access to exploit it, and only if a user opens a file or follows a link.

### Official description

Mattermost versions 11.7.x <= 11.7.6, 11.8.x <= 11.8.3 The access control policy unassign endpoint fails to re-validate that each target channel still belongs to the requesting admin's team, which allows an authenticated team administrator to remove ABAC (attribute-based access control) policy assignments from channels outside their team via the policy unassign API after a channel has been moved to another team.. Mattermost Advisory ID: MMSA-2026-00718

Weakness: CWE-863 (Incorrect Authorization)

## Which versions are affected by CVE-2026-15754?

It affects 1 product.


Products: Mattermost Mattermost Server.

## How do I fix CVE-2026-15754?

No fix has been published for CVE-2026-15754 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-15754 being exploited?

There is no known exploitation of CVE-2026-15754 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-2026-15754 and fix it if it is affected.

What it is: CVE-2026-15754 is a medium-severity missing authorization vulnerability in Mattermost Mattermost Server, published on 17 Aug 2026. An attacker needs administrator access to exploit it, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2026-15754 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-2026-15754/ (Markdown: https://insights.sbomify.com/CVE-2026-15754/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-15754
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-15754
- OSV: https://osv.dev/vulnerability/CVE-2026-15754

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-15754/
Summaries are generated and may contain mistakes; the official description above is the record.