CVE-2018-21256 Missing authorization in n/a
Attackers can bypass access restrictions to create group-message channels via the slash command in Mattermost Server.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2018-21256?
CVE-2018-21256 is a medium-severity missing authorization vulnerability in Mattermost Mattermost Server, published on 19 Jun 2020. Any signed-in user can exploit it over the network, with no action from the victim.
Official description
An issue was discovered in Mattermost Server before 5.1. It allows attackers to bypass intended access restrictions (for group-message channel creation) via the Group message slash command.
Weakness: CWE-732
Which versions are affected by CVE-2018-21256?
It affects 1 product.
Products include Mattermost Mattermost Server.
How do I fix CVE-2018-21256?
No fix has been published for CVE-2018-21256 yet. Limit exposure of the affected software until one is available.
Upgrade, and check permissions on every request on the server, not only in the interface. More about Missing authorization.
Is CVE-2018-21256 being exploited?
There is no known exploitation of CVE-2018-21256 in the wild and no public exploit code.
Other vulnerabilities in Mattermost Mattermost Server
- CVE-2025-12419 Account takeover on OAuth/OpenID-enabled servers
- CVE-2025-12421 Account Takeover via Code Exchange Endpoint
- CVE-2025-4981 Path Traversal Leading to RCE by Any Authenticated Mattermost User
- CVE-2025-20051 Arbitrary file read via block duplication in Mattermost Boards
- CVE-2025-25279 Arbitrary file read in Mattermost Boards via import & export board archive
Sources
Also known as GHSA-7fjj-c6hx-jhxv.