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Mattermost CVE-2026-21386

| EUVDEUVD-2026-12437 MEDIUM
Observable Discrepancy (CWE-203)
2026-03-16 responsibledisclosure@mattermost.com GHSA-5mr9-crcg-8wh2
Information Disclosure Mattermost Mattermost Server Suse
4.3
CVSS 3.1 · NVD
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Vulnerability Assessment

Weakness
CWE
CWE-203
OWASP Top 10:2025
A06:2021 - Vulnerable and Outdated Components
ASVS 5.0
V6 - Authentication Timing-safe comparisons
Impact

CVSS 4.3. EPSS 0.0%. No patch confirmed.
Exploit ScenarioDerived

An attacker could exploit this vulnerability in the affected application to compromise the security of affected deployments, potentially impacting confidentiality, integrity, or availability.
This Month Plan within 30 days. Why: MEDIUM severity (CVSS 4.0-6.9).
Patch available 0.0% exploitation probability (EPSS)

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AnalysisAIv1 – Mar 16, 2026

CVE-2026-21386 is a security vulnerability (CVSS 4.3) that allows an authenticated team member. Remediation should follow standard vulnerability management procedures.

Earlier versions of this analysis (1)
v1 Mar 16, 2026 - 15:30 CVSS 4.3

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 enumerat

RemediationAI

Monitor vendor channels for patch availability.

Affected VersionsENISA

Mattermost 11.3.0 ≤11.3.0
Mattermost 11.2.0 ≤11.2.2
Mattermost 10.11.0 ≤10.11.10

Affected Packages

go github.com/mattermost/mattermost-server fix: 5.3.2-0.20260130144323-5bb5261c72fa, 10.11.11+incompatible, 10.11.11, 11.2.3+incompatible, 11.2.3, 11.3.1+incompatible, 11.3.1
go github.com/mattermost/mattermost-server/v5 no fix
go github.com/mattermost/mattermost-server/v6 no fix
go github.com/mattermost/mattermost/server/v8 fix: 8.0.0-20260130144323-5bb5261c72fa
euvd mattermost no fix

Source: OSV (Open Source Vulnerabilities)

Affects your stack

Affects 5 packages in go, euvd

Checking this CVE against your own stacks and servers is part of the Basic plan and up.

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Technical ContextAI · show details

Vulnerability type not specified by vendor.

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  • Exploit and PoC references, threat intelligence from 22 sources
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Severity by source

NVD PRIMARY 4.3 MEDIUM
AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N
SUSE MEDIUM

Primary rating from NVD.

CVSS VectorNVD

Attack Vector
Network
Attack Complexity
Low
Privileges Required
Low
User Interaction
None
Scope
Unchanged
Confidentiality
Low
Integrity
None
Availability
None

Lifecycle Timeline

2
Patch released
Mar 31, 2026 - 21:13 nvd
Patch available
CVE Published
Mar 16, 2026 - 15:16 nvd
MEDIUM 4.3

DescriptionCVE.org

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

Executive ViewAI

Why It Matters

CVE-2026-21386 presents moderate security risk rated CVSS 4.3. Exploitation could compromise the security of affected deployments. Organizations should apply vendor updates when available and monitor for exploitation.

Business Impact: Low
Compensating Controls

Apply vendor patch. If patching is delayed: restrict network access and enable monitoring.

Threat Intelligence Overview

EPSS Score
0.0%
Top 93% exploitable
Data Quality
5 sources
Confidence: Medium
EUVD NVD CVE.org EPSS Vendor VulDB

How this decision was reached

The engine reads each signal below and applies a fixed list of rules from top to bottom. The first rule that matches sets the recommended action – like a statement, every line is shown.

SignalValue
Severity MEDIUM
CISA KEV No
Public POC No
EPSS 0.0%
Patch available Yes
Active threat No
Deciding rule
MEDIUM severity (CVSS 4.0-6.9)
What every action level means
Emergency Immediate action required. Criteria: CRITICAL + in CISA KEV (actively exploited) without patch, OR CRITICAL + EPSS > 30% without patch, OR CRITICAL + active threat signals without patch.
Act Now Prioritize within 24-48h. Criteria: in KEV with patch available, OR CRITICAL + public POC, OR HIGH + EPSS > 10%, OR CRITICAL + active threat, OR any CRITICAL severity.
This Week Schedule remediation this week. Criteria: HIGH severity (CVSS 7.0-8.9). Patch within normal change window.
This Month Plan within 30 days. Criteria: MEDIUM severity (CVSS 4.0-6.9), lower exploitation probability. Include in regular patch cycle.
Monitor Track and reassess. Criteria: LOW severity (CVSS 0.1-3.9), minimal exploitation risk. No immediate action needed.

SSVC AssessmentENISA

SSVC Decision· derived
Defer
Exploitation· derived
None
System Exposure· derived
Open
Automatable· derived
No
Human Impact
Low

Priority Score

22
Low Medium High Critical
KEV: 0
EPSS: 0
CVSS: +22
PoC: 0
Weaponized: 0
Threat intel: 0
Gov advisory: 0

Attack Technique

Information Disclosure MEDIUM

Information disclosure occurs when an application unintentionally exposes sensitive data that aids attackers in reconnaissance or directly compromises security.

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CVE-2026-22545 LOW
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Same weakness CWE-203 – Observable Discrepancy

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Same technique Information Disclosure

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CVE-2013-2251 CRITICAL POC
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CVE-2017-5638 CRITICAL POC
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CVE-2025-68613 HIGH POC
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CVE-2015-1701 HIGH POC
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Vendor StatusVendor

SUSE

Severity: Medium
Product Status
openSUSE Leap 15.6 Fixed
SUSE Linux Enterprise Module for Package Hub 15 SP5 Fixed
SUSE Linux Enterprise Module for Package Hub 15 SP6 Fixed
openSUSE Leap 15.5 Fixed

References

Advisories & patches
ADVISORY https://nvd.nist.gov/vuln/detail/CVE-2026-21386 ADVISORY https://vuldb.com/?id.351278 ADVISORY https://www.suse.com/support/update/SUSE-SU-2026:1135/ REF https://mattermost.com/security-updates

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