# CVE-2026-75588: Mattermost Desktop App plugin popout scheme validation bypass

> An attacker can bypass URL scheme validation to load plugin popout windows over insecure connections in the Mattermost Desktop App.

- **Severity:** Low 2.6 (`CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:N/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** No
- **Published:** 17 Sep 2026
- **Updated:** 17 Sep 2026
- **Also known as:** GHSA-p8rx-2r7q-5wfv

## What is CVE-2026-75588?

CVE-2026-75588 is a low-severity vulnerability in Mattermost, published on 17 Sep 2026. Any signed-in user can exploit it over the network, and only if a user opens a file or follows a link.

### Official description

Mattermost Desktop App versions <=6.2 6.2.2.0 fail to validate the URL scheme when checking whether a target URL is internal to the connected server, which allows a network-positioned attacker to load a plugin popout window over an insecure connection via a link using a downgraded URL scheme. Mattermost Advisory ID: MMSA-2026-00717

Weakness: CWE-1287 (Improper Validation of Specified Type of Input)

## Which versions are affected by CVE-2026-75588?

It affects 1 product.


Products: Mattermost Mattermost Desktop.

## How do I fix CVE-2026-75588?

No fix has been published for CVE-2026-75588 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-75588 being exploited?

There is no known exploitation of CVE-2026-75588 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-2026-75588 and fix it if it is affected.

What it is: CVE-2026-75588 is a low-severity vulnerability in Mattermost, published on 17 Sep 2026. Any signed-in user can exploit it over the network, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2026-75588 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-2026-75588/ (Markdown: https://insights.sbomify.com/CVE-2026-75588/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-75588
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-75588
- OSV: https://osv.dev/vulnerability/CVE-2026-75588

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-75588/
Summaries are generated and may contain mistakes; the official description above is the record.