# CVE-2026-16049: Missing authorization in Mattermost

> Authenticated attackers can inject bot messages with arbitrary URLs into unauthorized channels via missing permission checks in the GitLab plugin.

- **Severity:** Low 3.9 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/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:** 17 Aug 2026
- **Updated:** 17 Aug 2026
- **Also known as:** GHSA-vr3j-2q9w-h26r

## What is CVE-2026-16049?

CVE-2026-16049 is a low-severity missing authorization vulnerability in Mattermost, published on 17 Aug 2026. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

Mattermost Plugins versions <=11.8 10.20.11 11.5.7.0 _The Mattermost GitLab plugin fails to verify channel permissions when processing API requests with a caller-supplied_ {{post_id}}_, and fails to validate the_ {{web_url}} _parameter against the configured GitLab instance, which allows an authenticated attacker to inject bot-authored messages containing arbitrary URLs into channels they do not have access to via the_ {{createIssue}} _and_ {{attachCommentToIssue}} _API endpoints._ Mattermost Advisory ID: MMSA-2026-00673

Weakness: CWE-862 (Missing Authorization)

## Which versions are affected by CVE-2026-16049?


## How do I fix CVE-2026-16049?

No fix has been published for CVE-2026-16049 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-16049 being exploited?

There is no known exploitation of CVE-2026-16049 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-16049 and fix it if it is affected.

What it is: CVE-2026-16049 is a low-severity missing authorization vulnerability in Mattermost, published on 17 Aug 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-2026-16049 in the wild and no public exploit code.

Affected product: Mattermost. 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-2026-16049/ (Markdown: https://insights.sbomify.com/CVE-2026-16049/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-16049
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-16049
- OSV: https://osv.dev/vulnerability/CVE-2026-16049

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-16049/
Summaries are generated and may contain mistakes; the official description above is the record.