How severe is CVE-2026-0999?

CVE-2026-0999 has a CVSS score of 5.4/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2026-0999?

CVE-2026-0999 affects the following packages: github.com/mattermost/mattermost/server/v8 (Go), github.com/mattermost/mattermost-server (Go), github.com/mattermost/mattermost-server (Go), github.com/mattermost/mattermost-server (Go), github.com/mattermost/mattermost-server (Go). Ecosystems affected: Go.

How do I fix CVE-2026-0999?

Update github.com/mattermost/mattermost/server/v8 to 8.0.0-20251212052346-61651b0df7ea or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-0999 is resolved across your whole dependency graph.

How do I detect CVE-2026-0999 in my Go dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/mattermost/mattermost/server/v8, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-0999 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Is CVE-2026-0999 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-0999 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-0999?

CVE-2026-0999 has an EPSS (Exploit Prediction Scoring System) score of 0.2%, placing it in the 10th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-0999?

CVE-2026-0999 is classified as CWE-303 (CWE-303). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-0999 published, and has it been updated?

CVE-2026-0999 was published on February 16, 2026 and was last updated on November 7, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐹

🐹 Go

Not in CISA KEV

MEDIUM severity

CVE-2026-0999 — server

MEDIUMFix: mattermost/mattermost@61651b0

CVE-2026-0999 is a medium-severity (CVSS 5.4) CWE-303 vulnerability in github.com/mattermost/mattermost/server/v8. A fix is available for github.com/mattermost/mattermost/server/v8 — see the affected versions and patch details below.

Authentication bypass via userID login when email and username login are disabled

Also known asGHSA-3c9r-7f29-qp32GO-2026-4520

Published

Feb 16, 2026

Updated

Nov 7, 2026

Affected

5 pkgs

Patched

2 / 5

Exploits

None indexed

Exploitation data as of Nov 10, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-0999.

EPSS Exploitation Probability

via FIRST.org ↗

0.2%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs10th percentile — riskier than 10% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

5 pkgs affected

🐹github.com/mattermost/mattermost/server/v8🐹github.com/mattermost/mattermost-server🐹github.com/mattermost/mattermost-server🐹github.com/mattermost/mattermost-server🐹github.com/mattermost/mattermost-server

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.

Description

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

Affected Packages

5 total 2 fixed

Ecosystem	Package	Vulnerable range	Fix
🐹Go	`github.com/mattermost/mattermost/server/v8`	all versions	8.0.0-20251212052346-61651b0df7ea`go get github.com/mattermost/mattermost/server/v8@v8.0.0-20251212052346-61651b0df7ea`
🐹Go	`github.com/mattermost/mattermost-server`	≥ 11.1.0	No fix
🐹Go	`github.com/mattermost/mattermost-server`	≥ 10.11.0	No fix
🐹Go	`github.com/mattermost/mattermost-server`	≥ 11.2.0	No fix
🐹Go	`github.com/mattermost/mattermost-server`	all versions	5.3.2-0.20251212052346-61651b0df7ea`go get github.com/mattermost/mattermost-server@v5.3.2-0.20251212052346-61651b0df7ea`

Affected Products

1 product · 3 configurations

Application

mattermost servermattermost

≥ 11.2.0 && < 11.2.2

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/mattermost/mattermost/server/v8, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/mattermost/mattermost/server/v8 to 8.0.0-20251212052346-61651b0df7ea or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-0999 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Frequently Asked Questions

O3 Security · Impact-Aware SCA

Is CVE-2026-0999 in your dependencies?

Find it across Go, including transitive dependencies.

Scan my dependencies How O3 SCA works

CVSS Score

5.4/10CVSS v3 base

Vector breakdown

Attack Vector

Network

Attack Complexity

Low

Privileges Required

Low

User Interaction

None

Scope

Unchanged

Confidentiality

Low

Integrity

Low

Availability

None

Raw vector

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N

CWE Classification

CWE-303CWE-303

Weaponization

No Nuclei template, Metasploit module or public proof-of-concept found.

OpenSSF Scorecard

Package health score

mattermost

7.3/10

Code-Review10/10

Maintained10/10

Dependency-Update-Tool10/10

Branch-Protection4/10

Vulnerabilities0/10

Assessed Nov 10, 2026

Timeline

Disclosed

Feb 16, 2026

Last updated

Nov 7, 2026

Identifiers

CVE-2026-0999

GHSA-3c9r-7f29-qp32

GO-2026-4520

References

Advisorygithub.com · CVE 2026 0999.json Advisorymattermost.com · security updates Advisorynvd.nist.gov · CVE 2026 0999 Webgithub.com · 61651b0df7ea5db55d1e54f8d6fb5fce4149309c Packagegithub.com · mattermost

Related vulnerabilities

This CVE is medium severity; 7 of 8 other CVEs affecting v8 are rated as high or higher (top: high).

CVE-2026-8823CVE-2026-6046CVE-2026-7387CVE-2026-6961CVE-2026-7184CVE-2026-6689CVE-2026-6739CVE-2026-3433

Provenance

Sources

OSV.devNVDFIRST.org (EPSS)

Last verifiedNov 7, 2026