CVE-2018-21258 Mattermost Server is vulnerable to a Denial of Service attack through `invite_people` command

What is CVE-2018-21258?

CVE-2018-21258 is a high-severity denial of service vulnerability in Mattermost Mattermost Server, published on 19 Jun 2020. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

An issue was discovered in Mattermost Server before 5.1. It allows attackers to cause a denial of service via the invite_people slash command.

Weakness: CWE-74

Which versions are affected by CVE-2018-21258?

4 open source packages ship an affected version, including github.com/mattermost/mattermost-server and github.com/mattermost/mattermost-server/v5. It affects 1 product.

Products include Mattermost Mattermost Server.

How do I fix CVE-2018-21258?

Upgrade github.com/mattermost/mattermost-server to 5.1.0 or later.

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-2018-21258 being exploited?

There is no known exploitation of CVE-2018-21258 in the wild and no public exploit code.

Other vulnerabilities in Mattermost Mattermost Server

• CVE-2025-12419 Account takeover on OAuth/OpenID-enabled servers Critical 9.9
• CVE-2025-12421 Account Takeover via Code Exchange Endpoint Critical 9.9
• CVE-2025-4981 Path Traversal Leading to RCE by Any Authenticated Mattermost User Critical 9.9
• CVE-2025-20051 Arbitrary file read via block duplication in Mattermost Boards Critical 9.9
• CVE-2025-25279 Arbitrary file read in Mattermost Boards via import & export board archive Critical 9.9

All Mattermost Mattermost Server vulnerabilities

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as GHSA-5mh6-p63g-3mv5, GO-2025-4146.