Private channel enumeration via /mute slash command

published 2026-03-16 7 mo ago modified 2026-06-17 4 mo ago also GHSA-5mr9-crcg-8wh2 , GO-2026-4744 , EUVD-2026-12437

cvss: 4.3; version 3.1, CNA
cisa kev: Not listed; not in CISA's catalog
epss: 0.2%; EPSS 0.2% chance of exploitation in 30 days, percentile 8%
public exploits: 0; none known

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

Patch / fix

fixed in 3 distros 2 links 1

go github.com/mattermost/mattermost/server/v8 8.0.0-20260130144323-5bb5261c72fa
go github.com/mattermost/mattermost-server 5.3.2-0.20260130144323-5bb5261c72fa, 10.11.11, 10.11.11+incompatible, 11.2.3, 11.2.3+incompatible, 11.3.1, 11.3.1+incompatible
Mattermost Mattermost 10.11.11, 11.2.3, 11.3.1, 11.4.0

suse opensuse

govulncheck-vulndb openSUSE Leap 15.6 0.0.20260326T203309-150000.1.155.2

suse advisories SUSE-SU-2026:1135-1

govulncheck-vulndb openSUSE Leap 16.0 0.0.20260723T184607-160000.1.1

opensuse advisories openSUSE-SU-2026:21483-1

github.com/mattermost/mattermost@5bb5261 commit

CISA SSVC

Exploitation: none
Automatable: no
Technical impact: partial
Assessed: 7 mo ago (2026-03-16)

Exploit prediction

0.19% chance of exploitation in the next 30 days, higher than 8% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by	version	score	vector
CNA	3.1	4.3	CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N
GitHub	3.1	4.3	CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N

Affected packages

ecosystem	package	affected	fixed in
go	github.com/mattermost/mattermost/server/v8	<8.0.0-20260130144323-5bb5261c72fa	8.0.0-20260130144323-5bb5261c72fa
go	github.com/mattermost/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 >=10.11.0-rc1+incompatible, <10.11.11+incompatible >=11.2.0-rc1+incompatible, <11.2.3+incompatible >=11.3.0-rc1+incompatible, <11.3.1+incompatible	5.3.2-0.20260130144323-5bb5261c72fa 10.11.11 11.2.3 11.3.1 10.11.11+incompatible 11.2.3+incompatible 11.3.1+incompatible
go	github.com/mattermost/mattermost-server/v5	all versions	no fix listed
go	github.com/mattermost/mattermost-server/v6	all versions	no fix listed

Affected products

Mattermost Mattermost

Classification

Weakness

CWE-203 Observable Discrepancy

References

github.com /mattermost/mattermost/commit/5bb5261c72faa476558a694c23581d24b734da41
web
mattermost.com /security-updates
vendor-advisory, web
nvd.nist.gov /vuln/detail/CVE-2026-21386
advisory
github.com /mattermost/mattermost
package
github.com /advisories/GHSA-5mr9-crcg-8wh2
advisory

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

Summary