# CVE-2026-6517: Mattermost Desktop App fails to restrict the allow list of domains which NTLM credentials are passed

> Attackers can intercept NTLM credentials by embedding images routing to external servers, affecting users on servers without image proxy enabled.

- **Severity:** Medium 6.3 (`CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/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:** 15 Jun 2026
- **Updated:** 15 Jun 2026
- **Also known as:** GHSA-2wmq-3m94-g4jr

## What is CVE-2026-6517?

CVE-2026-6517 is a medium-severity vulnerability in Mattermost, published on 15 Jun 2026. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

Mattermost Desktop App versions <=6.1 5.5.13.0 fail to restrict the allow list of domains to which NTLM credentials were forwarded to in the Mattermost Desktop App which allows any user on a server without the image proxy enabled to intercept other users credentials via embedding an image that routes to an external web server. Mattermost Advisory ID: MMSA-2026-00651

Weakness: CWE-522 (Insufficiently Protected Credentials)

## Which versions are affected by CVE-2026-6517?

It affects 1 product.


Products: Mattermost Mattermost Desktop.

## How do I fix CVE-2026-6517?

No fix has been published for CVE-2026-6517 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-6517 being exploited?

There is no known exploitation of CVE-2026-6517 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-2026-6517 and fix it if it is affected.

What it is: CVE-2026-6517 is a medium-severity vulnerability in Mattermost, published on 15 Jun 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-2026-6517 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-2026-6517/ (Markdown: https://insights.sbomify.com/CVE-2026-6517/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-6517
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-6517
- OSV: https://osv.dev/vulnerability/CVE-2026-6517

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-6517/
Summaries are generated and may contain mistakes; the official description above is the record.