# CVE-2026-86349: Mattermost Server Algorithmic DoS via Unbounded Markdown Block Nesting

> An authenticated attacker can cause CPU exhaustion and denial of service by submitting posts with deeply nested Markdown blocks.

- **Severity:** Medium 4.3 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 14 Sep 2026
- **Updated:** 14 Sep 2026
- **Also known as:** GHSA-6mc8-c94m-mjg8, MINI-27gp-c6j5-m7mg, MINI-989r-gg9q-mvxw

## What is CVE-2026-86349?

CVE-2026-86349 is a medium-severity denial of service vulnerability in Mattermost Mattermost Server, published on 14 Sep 2026. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.8, 10.11.x <= 10.11.22 fail to limit the nesting depth in the server-side Markdown parser which allows an authenticated attacker to cause a denial of service (CPU resource exhaustion) via a crafted post containing deeply nested blockquotes or list items.. Mattermost Advisory ID: MMSA-2026-00707

Weakness: CWE-407 (Inefficient Algorithmic Complexity)

## Which versions are affected by CVE-2026-86349?

It affects 1 product.


Products: Mattermost Mattermost Server.

## How do I fix CVE-2026-86349?

Upgrade to a fixed version.

## Is CVE-2026-86349 being exploited?

There is no known exploitation of CVE-2026-86349 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-2026-86349 and fix it if it is affected.

What it is: CVE-2026-86349 is a medium-severity denial of service vulnerability in Mattermost Mattermost Server, published on 14 Sep 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-2026-86349 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-2026-86349/ (Markdown: https://insights.sbomify.com/CVE-2026-86349/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-86349
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-86349
- OSV: https://osv.dev/vulnerability/CVE-2026-86349

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-86349/
Summaries are generated and may contain mistakes; the official description above is the record.