4.3

CVE-2026-21386: Private channel enumeration via /mute slash command

Analyzed
AliasesGHSA-5mr9-crcg-8wh2GO-2026-4744
Published 16-03-2026Last modified 16-03-2026

Vulnerability summary

CVE-2026-21386 is a medium (CVSS 4.3) Observable Discrepancy vulnerability in mattermost. EPSS estimates a 0.19% chance of exploitation in the next 30 days, higher than 8.8% of scored vulnerabilities.

Priority
Routine

Not severe (CVSS 4.3, network access); no exploitation reported by the 5 checked sources

Source freshness unavailable

How is this decided?
CVSS score
4.3/10
010
Medium · v3.1 (CVE.org)
EPSS score
0.19%
0%100%
Low · +0.01 pp in 30 days
KEVNot listedNot listed by any tracked catalog.
RansomwareNot flaggedNo ransomware use recorded.
Public exploits0No public exploits recorded

Advisory lineage

Upstream 0 · Downstream 2 · Related 2
OPENSUSE-SU-2026:21483-1SUSE-SU-2026:1135-1

Timeline

  1. 16-03-2026
    PublishedVulnerability first disclosed.
  2. 16-03-2026
    Last modifiedVulnerability information updated.

Description

Mattermost versions 11.3.x <= 11.3.0, 11.2.x <= 11.2.2, 10.11.x <= 10.11.10 fail to use consistent error responses when handling the /mute command which allows an authenticated team member to enumerate private channels they are not authorized to know about via differing error messages for nonexistent versus private channels. Mattermost Advisory ID: MMSA-2026-00588

CVSS metrics

3 scores · 3 sources · 1 version

EPSS trend

Probability

0.19%

Chance of exploitation in the next 30 days

Percentile

9%

Versus all other CVEs

30-day change

Increase +0.01 pp

From 0.18% on 04-10-2026

04-10-202603-11-2026

Known exploited

Compare KEV catalogs
CISA KEV

Not in the CISA KEV catalog.

ENISA CNW

Not in the ENISA CNW catalog.

CIRCL

Not in the CIRCL catalog.

Public exploits

0 found

No public exploits indexed.

Weaknesses

CWE · root cause

Affected systems

Version check

Known conditions: network-reachable, low-privilege account, no user interaction (CVSS vector). Other conditions: not stated in public data.

Github.com/mattermost/mattermost-server2 products
  • v5All versions (No fix recorded)
  • v6All versions (No fix recorded)
Mattermost2 products
  • mattermost11.3.0 | ≥ 11.2.0, ≤ 11.2.2 | ≥ 10.11.0, ≤ 10.11.10
  • mattermost_server≥ 10.11.0, < 10.11.11 | ≥ 11.2.0, < 11.2.3 | ≥ 11.3.0, < 11.3.1
Github.com/mattermost1 product
  • mattermost-server< 5.3.2-0.20260130144323-5bb5261c72fa | ≥ 10.11.0-rc1, < 10.11.11 | ≥ 11.2.0-rc1, < 11.2.3 | ≥ 11.3.0-rc1, < 11.3.1 | ≥ 11.3.0-rc1+incompatible, < 11.3.1+incompatible
Github.com/mattermost/mattermost/server1 product
  • v8< 8.0.0-20260130144323-5bb5261c72fa

4 vendors · 6 products

References

5 links