6.5

CVE-2026-18551: mattermost Incorrect Authorization

CVSS: 6.5v3.1MediumUndergoing analysis
Published 16-10-2026Last modified 20-10-2026
Open JSON

Vulnerability summary

CVE-2026-18551 is a medium (CVSS 6.5) Incorrect Authorization vulnerability in mattermost. EPSS estimates a 0.22% chance of exploitation in the next 30 days, higher than 12% of scored vulnerabilities.

Overall risk
26/100
0100
Medium · default modelHow is this calculated?
CVSS score
6.5/10
010
Medium · v3.1 (CVE.org)
EPSS score
0.22%
0%100%
Low · Change unavailable
KEVNot listedIn no KEV catalog
RansomwareNone recordedNo known use recorded
Public exploits0No public exploits recorded

Timeline

  1. 16-10-2026
    PublishedVulnerability first disclosed.
  2. 20-10-2026
    Last modifiedVulnerability information updated.

Description

Mattermost versions 11.10.x <= 11.10.0, 11.7.x <= 11.7.9, 11.9.x <= 11.9.1, 11.8.x <= 11.8.5 fail to validate the combination of guest and admin scheme roles which allows an authenticated attacker to escalate a guest account to team or channel admin via a crafted PUT request to the member roles API endpoint.. Mattermost Advisory ID: MMSA-2026-00752

CVSS metrics

2 scores · 2 sources · 1 version

EPSS trend

Probability

0.22%

Chance of exploitation in the next 30 days

Percentile

12%

Versus all other CVEs

30-day change

Change unavailable

Change unavailable

16-10-202620-10-2026
EPSS score
0.22%

Known exploited

Compare KEV catalogs

Not in any of the 3 tracked catalogs.

Public exploits

0 found

No public exploits indexed.

Weaknesses

CWE · root cause

Affected systems

Mattermost1 product
  • mattermost11.10.0 | ≥ 11.7.0, ≤ 11.7.9 | ≥ 11.9.0, ≤ 11.9.1 | ≥ 11.8.0, ≤ 11.8.5

1 vendor · 1 product

References

1 links