# CVE-2026-8683: Overly long URLs crash the Mattermost Desktop App

> A malicious server owner can crash the Mattermost Desktop App by sending a script that opens an extremely long URL.

- **Severity:** Medium 6.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** No
- **Published:** 15 Jun 2026
- **Updated:** 15 Jun 2026
- **Also known as:** GHSA-pf78-3frq-v82v

## What is CVE-2026-8683?

CVE-2026-8683 is a medium-severity denial of service vulnerability in Mattermost, published on 15 Jun 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official description

Mattermost Desktop App versions <=6.1 5.5.13.0 fail to account for attempting to open extremely long URLs in the Mattermost Desktop App which allows a malicious server owner to crash the application via including a script to call window.open on a very large URL. Mattermost Advisory ID: MMSA-2026-00652

Weakness: CWE-770 (Allocation of Resources Without Limits or Throttling)

## Which versions are affected by CVE-2026-8683?

It affects 1 product.


Products: Mattermost Mattermost Desktop.

## How do I fix CVE-2026-8683?

No fix has been published for CVE-2026-8683 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-8683 being exploited?

There is no known exploitation of CVE-2026-8683 in the wild and no public exploit code.

## Other vulnerabilities in Mattermost

- [CVE-2016-11064](https://insights.sbomify.com/CVE-2016-11064/): Code injection in n/a (critical 9.8)
- [CVE-2019-20856](https://insights.sbomify.com/CVE-2019-20856/): Code injection in n/a (critical 9.8)
- [CVE-2019-20861](https://insights.sbomify.com/CVE-2019-20861/): Code injection in n/a (high 8.8)
- [CVE-2026-1046](https://insights.sbomify.com/CVE-2026-1046/): Arbitrary application execution via unvalidated server-controlled URLs in Help menu (high 7.6)
- [CVE-2020-14456](https://insights.sbomify.com/CVE-2020-14456/): Security feature bypass in n/a (high 7.3)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-8683 and fix it if it is affected.

What it is: CVE-2026-8683 is a medium-severity denial of service vulnerability in Mattermost, published on 15 Jun 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2026-8683 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-2026-8683/ (Markdown: https://insights.sbomify.com/CVE-2026-8683/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-8683
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-8683
- OSV: https://osv.dev/vulnerability/CVE-2026-8683

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-8683/
Summaries are generated and may contain mistakes; the official description above is the record.