CVE-2026-6850 Crafted message attachment causes client-side denial of service via markdown parser regex backtracking in Mattermost

Q: Is CVE-2026-6850 being exploited?

There is no known exploitation of CVE-2026-6850 in the wild and no public exploit code.

Medium 6.5 Published 13 Jul 2026 Updated 13 Jul 2026

An authenticated attacker can cause client-side denial of service for all channel users by sending a crafted message attachment that triggers regex backtracking.

Exploited in the wild: Not known
Login needed: Any signed-in user
Attackable: Over the internet
Victim action needed: Not needed
Fix: Available

What is CVE-2026-6850?

CVE-2026-6850 is a medium-severity denial of service vulnerability in Mattermost Mattermost Server, published on 13 Jul 2026. Any signed-in user can exploit it over the network, with no action from the victim.

Official description

Mattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, 10.11.x <= 10.11.19 fail to validate the length and content of message attachment field values, which allows an authenticated attacker to cause a denial of service for all users in a channel via a post containing a specially crafted payload that triggers catastrophic backtracking in the client-side markdown parser.. Mattermost Advisory ID: MMSA-2026-00658

Weakness: CWE-1333

Which versions are affected by CVE-2026-6850?

It affects 1 product.

Products include Mattermost Mattermost Server.

How do I fix CVE-2026-6850?

Upgrade to a fixed version.

Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.

Is CVE-2026-6850 being exploited?

There is no known exploitation of CVE-2026-6850 in the wild and no public exploit code.

All Mattermost Mattermost Server vulnerabilities

Sources

Also known as GHSA-c8hv-77r5-h5f7, MINI-4hm5-6787-jc6f, MINI-q4rj-3pxh-4wfx.