What is the CVSS score of CVE-2026-0999?

CVE-2026-0999 has a CVSS 3.1 base score of 5.4 (Medium). CVSS vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N. EPSS exploitation probability: 0.0%.

How to fix or mitigate CVE-2026-0999?

Within 30 days: Identify affected systems and apply vendor patches as part of regular patch cycle. Monitor vendor channels for patch availability.

Mattermost CVE-2026-0999

Q: Which versions of Mattermost are affected by CVE-2026-0999?

CVE-2026-0999 presents moderate security risk rated CVSS 5.4. Exploitation could compromise the security of affected deployments.. A patch is available.

MEDIUM

Incorrect Implementation of Authentication Algorithm (CWE-303)

2026-02-16 responsibledisclosure@mattermost.com

Authentication Bypass Mattermost Mattermost Server Suse

5.4

CVSS 3.1 · NVD

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Vulnerability Assessment

Weakness	CWE CWE-303 OWASP Top 10:2025 A07:2025 - Authentication Failures ASVS 5.0 V6 - Authentication Authentication algorithm review
Impact	CVSS 5.4 (MEDIUM). Attack vector: Network. Complexity: Low. Privileges required: Low. User interaction: None. Exploitation requires authentication.
Exploit ScenarioDerived	An attacker (requires authentication) could exploit this vulnerability to compromise the affected system.

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 12, 2026

Mattermost Server versions 11.1.x through 11.1.2, 10.11.x through 10.11.9, and 11.2.x through 11.2.1 inadequately enforce login method restrictions, permitting authenticated users to circumvent SSO-only requirements by authenticating with a userID instead. This allows an attacker with valid credentials to gain unauthorized access to accounts restricted to single sign-on authentication. No patch is currently available for this vulnerability.

Earlier versions of this analysis (1)

v1 Mar 12, 2026 - 21:54 CVSS 5.4

Mattermost versions 11.1.x <= 11.1.2, 10.11.x <= 10.11.9, 11.2.x <= 11.2.1 fail to properly validate login method restrictions which allows an authenticated user to bypass SSO-only login requirements via userID-based authentication. Mattermost Advisory ID: MMSA-2025-00548 [CVSS 5.4 MEDIUM]

RemediationAI

Monitor vendor advisories for a patch. Restrict network access to the affected service where possible.

Affected Packages

go github.com/mattermost/mattermost-server fix: 5.3.2-0.20251212052346-61651b0df7ea

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: 8.0.0-20251212052346-61651b0df7ea

Source: OSV (Open Source Vulnerabilities)

Affects your stack

Affects 4 packages in go

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

Technical ContextAI · show details

Affects Mattermost Server. Mattermost versions 11.1.x <= 11.1.2, 10.11.x <= 10.11.9, 11.2.x <= 11.2.1 fail to properly validate login method restrictions which allows an authenticated user to bypass SSO-only login requirements via userID-based authentication. Mattermost Advisory ID: MMSA-2025-00548

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Exploit and PoC references, threat intelligence from 22 sources

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Severity by source

NVD PRIMARY 5.4 MEDIUM

AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/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

Low

Availability

None

Lifecycle Timeline

Patch released

Mar 31, 2026 - 21:13 nvd

Patch available

CVE Published

Feb 16, 2026 - 10:16 nvd

MEDIUM 5.4

DescriptionCVE.org

Executive ViewAI

Why It Matters

CVE-2026-0999 presents moderate security risk rated CVSS 5.4. 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 88% exploitable

Data Quality

3 sources

Confidence: Low

NVD

CVE.org

EPSS Vendor

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

Human Impact

Low

Priority Score

Low Medium High Critical

KEV: 0

EPSS: 0

CVSS: +27

PoC: 0

Weaponized: 0

Threat intel: 0

Gov advisory: 0

Attack Technique

Authentication Bypass CRITICAL

Authentication bypass attacks exploit flaws in the verification mechanisms that control access to systems and applications.

Affected Products (CPE)CVE.org

cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:*

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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://mattermost.com/security-updates ADVISORY https://www.suse.com/support/update/SUSE-SU-2026:0757/

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