What is CVE-2018-21257?

CVE-2018-21257 is a medium-severity vulnerability in Mattermost Server, classified under Missing Authorization. CVSS score: 5.3/10. Published 2020-06-19.

How severe is CVE-2018-21257?

Medium severity. CVSS v3 base score is 5.3 out of 10.

Auth bypass in Mattermost Server

CVE-2018-21257

An issue was discovered in Mattermost Server before 5.1. It allows attackers to bypass intended access restrictions (for setting a channel header) via the Channel header slash command API.

Vulnerability class: Broken Access Control

Published 2020-06-19 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 5.3 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N.

EPSS exploit prediction

EPSS: 0.008 (55.3th percentile), scored 2026-09-30.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 55th percentile — 55.3% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.008 · newest: 0.008 · change: 0.000

Affected products

Mattermost Server

Weakness classification (CWE)

CWE-862 — Missing Authorization

References

mattermost.com/security-updates/ (Vendor Advisory)

Frequently asked questions

What is CVE-2018-21257?: CVE-2018-21257 is a medium-severity vulnerability in Mattermost Server, classified under Missing Authorization. CVSS score: 5.3/10. Published 2020-06-19.
How severe is CVE-2018-21257?: Medium severity. CVSS v3 base score is 5.3 out of 10.