How severe is CVE-2026-4274?

CVE-2026-4274 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-2026-4274?

CVE-2026-4274 affects the following packages: github.com/mattermost/mattermost/server/v8 (Go), github.com/mattermost/mattermost/server/v8 (Go), github.com/mattermost/mattermost/server/v8 (Go), github.com/mattermost/mattermost/server/v8 (Go), github.com/mattermost/mattermost/server/v8 (Go). Ecosystems affected: Go.

How do I fix CVE-2026-4274?

Update github.com/mattermost/mattermost/server/v8 to 11.4.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-4274 is resolved across your whole dependency graph.

How do I detect CVE-2026-4274 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-2026-4274 if there is no patch (or I can't update yet)?

Put an independent control in front of the weakness: restrict the affected endpoint or interface to trusted networks, require an additional authentication factor or proxy-level check, and invalidate existing sessions and credentials in case the flaw has already been used.

Is CVE-2026-4274 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-4274 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-2026-4274?

CVE-2026-4274 has an EPSS (Exploit Prediction Scoring System) score of 0.1%, placing it in the 4th 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-2026-4274?

CVE-2026-4274 is classified as CWE-863 (CWE-863). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-4274 published, and has it been updated?

CVE-2026-4274 was published on March 26, 2026 and was last updated on October 28, 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-2026-4274 — server

MEDIUM

CVE-2026-4274 is a medium-severity (CVSS 5.4) CWE-863 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.

Insufficient authorization in shared channel membership sync grants team-level access instead of channel-level access

Also known asGHSA-g7fp-cqj5-x8hfGO-2026-5393

Published

Mar 26, 2026

Updated

Oct 28, 2026

Affected

5 pkgs

Patched

5 / 5

Exploits

None indexed

Exploitation data as of Nov 3, 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-2026-4274.

EPSS Exploitation Probability

via FIRST.org ↗

0.1%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs4th percentile — riskier than 4% 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/v8🐹github.com/mattermost/mattermost/server/v8🐹github.com/mattermost/mattermost/server/v8🐹github.com/mattermost/mattermost/server/v8

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 11.2.x <= 11.2.2, 10.11.x <= 10.11.10, 11.4.x <= 11.4.0, 11.3.x <= 11.3.1 fail to restrict team-level access when processing membership sync from a remote cluster, which allows a malicious remote cluster to grant a user access to an entire private team instead of only the shared channel via sending crafted membership sync messages that trigger team membership assignment. Mattermost Advisory ID: MMSA-2026-00574

Affected Packages

5 total 5 fixed

Ecosystem	Package	Vulnerable range	Fix
🐹Go	`github.com/mattermost/mattermost/server/v8`	≥ 11.4.0-rc1&&< 11.4.1	11.4.1`go get github.com/mattermost/mattermost/server/v8@v11.4.1`
🐹Go	`github.com/mattermost/mattermost/server/v8`	≥ 11.3.0-rc1&&< 11.3.2	11.3.2`go get github.com/mattermost/mattermost/server/v8@v11.3.2`
🐹Go	`github.com/mattermost/mattermost/server/v8`	≥ 11.2.0-rc1&&< 11.2.3	11.2.3`go get github.com/mattermost/mattermost/server/v8@v11.2.3`
🐹Go	`github.com/mattermost/mattermost/server/v8`	≥ 10.11.0-rc1&&< 10.11.11	10.11.11`go get github.com/mattermost/mattermost/server/v8@v10.11.11`
🐹Go	`github.com/mattermost/mattermost/server/v8`	≥ 8.0.0-20260105080200-d27a2195068d&&< 8.0.0-20260217110922-b7d4a1f1f59b	8.0.0-20260217110922-b7d4a1f1f59b`go get github.com/mattermost/mattermost/server/v8@v8.0.0-20260217110922-b7d4a1f1f59b`

Affected Products

1 product · 4 configurations

Application

mattermost servermattermost

≥ 11.4.0 && < 11.4.1

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 11.4.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-4274 is resolved across your whole dependency graph.
Workarounds
Put an independent control in front of the weakness: restrict the affected endpoint or interface to trusted networks, require an additional authentication factor or proxy-level check, and invalidate existing sessions and credentials in case the flaw has already been used.

Frequently Asked Questions

O3 Security · Impact-Aware SCA

Is CVE-2026-4274 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-863CWE-863

Weaponization

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

Timeline

Disclosed

Mar 26, 2026

Last updated

Oct 28, 2026

Identifiers

CVE-2026-4274

GHSA-g7fp-cqj5-x8hf

GO-2026-5393

References

Advisorygithub.com · CVE 2026 4274.json Advisorymattermost.com · security updates Advisorynvd.nist.gov · CVE 2026 4274 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 verifiedOct 28, 2026