# CVE-2026-9708: Incoming webhook user attribution via unvalidated webhook owner

> Attackers with webhook permissions can impersonate other users to post messages or send direct messages due to missing team/channel access validation.

- **Severity:** Medium 4.9 (`CVSS:3.1/AV:N/AC:L/PR:H/UI:N/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:** Not needed
- **Fix available:** Yes
- **Published:** 13 Jul 2026
- **Updated:** 13 Jul 2026
- **Also known as:** GHSA-8jmp-2xx6-pjg7, MINI-3hrr-w53p-c4m2, MINI-6vxq-99q8-6c8v

## What is CVE-2026-9708?

CVE-2026-9708 is a medium-severity missing authorization vulnerability in Mattermost Mattermost Server, published on 13 Jul 2026. An attacker needs administrator access to exploit it, with no action from the victim.

### Official description

Mattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, 10.11.x <= 10.11.19 fail to validate that an assigned incoming webhook user has access to the target team or channel, which allows a requester with webhook management permissions to create posts or direct messages attributed to another user via crafted incoming webhook configuration and payloads.. Mattermost Advisory ID: MMSA-2026-00683

Weakness: CWE-639 (Authorization Bypass Through User-Controlled Key)

## Which versions are affected by CVE-2026-9708?

It affects 1 product.


Products: Mattermost Mattermost Server.

## How do I fix CVE-2026-9708?

Upgrade to a fixed version.

## Is CVE-2026-9708 being exploited?

There is no known exploitation of CVE-2026-9708 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-2026-9708 and fix it if it is affected.

What it is: CVE-2026-9708 is a medium-severity missing authorization vulnerability in Mattermost Mattermost Server, published on 13 Jul 2026. An attacker needs administrator access to exploit it, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-9708 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-2026-9708/ (Markdown: https://insights.sbomify.com/CVE-2026-9708/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-9708
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-9708
- OSV: https://osv.dev/vulnerability/CVE-2026-9708

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-9708/
Summaries are generated and may contain mistakes; the official description above is the record.