How severe is CVE-2026-6347?

CVE-2026-6347 has a CVSS score of 7.6/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2026-6347?

CVE-2026-6347 affects the following packages: github.com/mattermost/mattermost-server (Go), github.com/mattermost/mattermost-server (Go), github.com/mattermost/mattermost-server (Go), github.com/mattermost/mattermost-plugin-calls (Go). Ecosystems affected: Go.

How do I fix CVE-2026-6347?

Update github.com/mattermost/mattermost-server to 11.5.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-6347 is resolved across your whole dependency graph.

How do I detect CVE-2026-6347 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, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-6347 if there is no patch (or I can't update yet)?

Assume what was exposed is already known: rotate any credential, token or key that the affected component could return, restrict the endpoint to callers that genuinely need it, and strip sensitive fields from responses and error output at the boundary rather than relying on the client not to read them.

Is CVE-2026-6347 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-6347 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-2026-6347?

CVE-2026-6347 has an EPSS (Exploit Prediction Scoring System) score of 0.4%, placing it in the 34th 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-2026-6347?

CVE-2026-6347 is classified as Information Exposure (CWE-200). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-6347 published, and has it been updated?

CVE-2026-6347 was published on May 18, 2026 and was last updated on August 12, 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

HIGH severity

CVE-2026-6347 — mattermost-server

HIGHFix: mattermost/mattermost-plugin-calls@d48893c

CVE-2026-6347 is a high-severity (CVSS 7.6) Information Exposure vulnerability in github.com/mattermost/mattermost-server. A fix is available for github.com/mattermost/mattermost-server — see the affected versions and patch details below.

Mattermost Calls plugin exposes TURN server credentials in plaintext in support packets

Also known asGHSA-82j6-4fq7-fx62GO-2026-5238

Published

May 18, 2026

Updated

Aug 12, 2026

Affected

4 pkgs

Patched

4 / 4

Exploits

None indexed

Exploitation data as of Sep 27, 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-2026-6347.

EPSS Exploitation Probability

via FIRST.org ↗

0.4%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs34th percentile — riskier than 34% of all scored CVEsHighest risk

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

How urgent is this, really

CVE-2026-6347 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 380,526 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

4 pkgs affected

🐹github.com/mattermost/mattermost-server🐹github.com/mattermost/mattermost-server🐹github.com/mattermost/mattermost-server🐹github.com/mattermost/mattermost-plugin-calls

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 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, 11.4.x <= 11.4.3 fail to sanitize sensitive configuration fields in the Mattermost Calls plugin which allows an attacker with access to a support packet to obtain TURN server credentials via the plaintext values present in the exported plugin configuration.. Mattermost Advisory ID: MMSA-2026-00605

Affected Packages

4 total 4 fixed

Ecosystem	Package	Vulnerable range	Fix
🐹Go	`github.com/mattermost/mattermost-server`	≥ 11.5.0&&< 11.5.2	11.5.2`go get github.com/mattermost/mattermost-server@v11.5.2`
🐹Go	`github.com/mattermost/mattermost-server`	≥ 10.11.0&&< 10.11.14	10.11.14`go get github.com/mattermost/mattermost-server@v10.11.14`
🐹Go	`github.com/mattermost/mattermost-server`	≥ 11.4.0&&< 11.4.4	11.4.4`go get github.com/mattermost/mattermost-server@v11.4.4`
🐹Go	`github.com/mattermost/mattermost-plugin-calls`	all versions	1.12.0-rc2`go get github.com/mattermost/mattermost-plugin-calls@v1.12.0-rc2`

Affected Products

1 product · 3 configurations

Application

mattermost servermattermost

≥ 11.5.0 && < 11.5.2

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, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/mattermost/mattermost-server to 11.5.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-6347 is resolved across your whole dependency graph.
Workarounds
Assume what was exposed is already known: rotate any credential, token or key that the affected component could return, restrict the endpoint to callers that genuinely need it, and strip sensitive fields from responses and error output at the boundary rather than relying on the client not to read them.

Frequently Asked Questions

O3 Security · Impact-Aware SCA

Is CVE-2026-6347 in your dependencies?

Find it across Go, including transitive dependencies.

Scan my dependencies How O3 SCA works

CVSS Score

7.6/10CVSS v3 base

Vector breakdown

Attack Vector

Network

Attack Complexity

Low

Privileges Required

High

User Interaction

None

Scope

Changed

Confidentiality

High

Integrity

None

Availability

Low

Raw vector

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

CWE Classification

CWE-200Information Exposure

Weaponization

No known public exploit tooling found.

Timeline

Disclosed

May 18, 2026

Last updated

Aug 12, 2026

Identifiers

CVE-2026-6347

GHSA-82j6-4fq7-fx62

GO-2026-5238

References

Advisorygithub.com Advisorymattermost.com Advisorynvd.nist.gov Webgithub.com Packagegithub.com

Related vulnerabilities

This CVE is high severity; 2 of 8 other CVEs affecting mattermost-server are rated as high or higher (top: high).

CVE-2026-5139CVE-2026-8074CVE-2026-9162CVE-2026-6673CVE-2026-6062CVE-2026-6046CVE-2026-7387CVE-2026-6961

Provenance

Sources

OSV.devNVDFIRST.org (EPSS)

Last verifiedAug 12, 2026