NVD / CVE Program· CNA Mattermost

CVE-2026-9859: Mattermost Boards plugin didn’t enforce role-based authorization on board channel link allowing board editors to expose boards to arbitrary channels

Noroxi assessment

Priority: Monitor (action score 26/100)

The sources we track (CISA KEV, Exploit-DB, Metasploit, Nuclei, GitHub) show no public exploit code or exploitation in the wild.
Probability of exploitation in the next 30 days: 0.34% (FIRST EPSS).
Anyone who can reach it over the internet can trigger it. A low-privileged account is enough. No user action is required. No special conditions are required; it is repeatable.
According to the record, no fixed version is available yet.

Low priority for now. Update when a fix is released; follow the vendor's advisory.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · FIRST EPSS

Mattermost versions 11.7.x <= 11.7.6, 10.11.x <= 10.11.21, 11.8.x <= 11.8.3 fail to enforce PermissionManageBoardRoles on the channelId field of the batch endpoint, which allows an authenticated board editor to relink any board they can edit to an arbitrary channel via a crafted PATCH request. Mattermost Advisory ID: MMSA-2026-00686

MediumCVSS 6.5 · v3.1—No exploit No fix recorded

Broken access control (IDOR)

Published: Aug 17, 2026
Updated: Aug 19, 2026
EPSS: 0.3% · 27th percentile
CWE: CWE-863

Report tools

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Action score

Monitor

Low priority for now.

CVSS: 26 / 40 · 6.5 / 10
CISA KEV: 0 / 30 · Not listed
EPSS: 0 / 30 · 0.3%

CISA SSVC decision

Exploitation: none
Automatable: no
Technical impact: partial

Vulnrichment: CISA's decision-tree inputs.

CNA vs NVD score

NVD: —
CNA · Mattermost: 6.5; NVD has not scored this yet; the score shown is the CNA’s.

The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.

Get told when this record changes

Not in KEV: no record of exploitation in the wild
No public exploit code on record
No fixed version yet

If any of these changes, we'll tell you in the daily email digest.

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Noroxi analysis

What does it mean in your environment?

The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.

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Affected systems

Is your version affected?

Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.

Vendor	Product	CPE
mattermost	mattermost server	cpe:2.3:a:mattermost:mattermost_server

Affected versions

NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.

mattermost mattermost server10.11.0 and later · before 10.11.22
mattermost mattermost server11.7.0 and later · before 11.7.7
mattermost mattermost server11.8.0 and later · before 11.8.4

Versions reported by the vendor

Affected version ranges reported by the assigning authority (Mattermost). Independent of NVD's CPE analysis and usually ahead of it.

Mattermost Mattermost
- 11.9.0, 11.7.7, 10.11.22, 11.8.4not affected
- 11.7.0 and later · up to and including 11.7.6affected · semver
- 10.11.0 and later · up to and including 10.11.21affected · semver
- 11.8.0 and later · up to and including 11.8.3affected · semver

Same product

mattermost: all records

Other highest-scoring records for the same primary product.

CVE-2026-4858Path traversal in integration action URL leading to arbitrary API execution via system admin’s auth token.
39Monitor
CVE-2025-12421Account Takeover via Code Exchange Endpoint
39Monitor
CVE-2025-12419Account takeover on OAuth/OpenID-enabled servers
39Monitor

Published before and after this one in the same product

← EarlierCVE-2026-10080Aug 17, 2026 · Boards plugin panics on WebSocket command with non-string field types
26Monitor
← EarlierCVE-2026-9816Aug 17, 2026 · Insufficient server-side validation of board member role fields permits privilege escalation
33Monitor
Later →CVE-2026-9693Aug 17, 2026 · Mattermost thread memberships persist after team removal, exposing private channel thread metadata on re-invite
14Monitor

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

No fix version is recorded for this entry. Check the references for vendor advisories.

Exploit status

No known public exploit

No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

No dated events beyond publication.

EPSS, last 120 days

FIRST EPSS daily score; only changes of 0.01 or more are recorded (step chart).

Patch and commit links

Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.

No commit or PR link among the references.

Credits

All researchers

Finders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.

kauasilvab14fb3finder

Variant candidates

Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.

CVE-2026-738766 days apartMattermost group syncable endpoints allow privilege escalation via scheme_admin
35Monitor
CVE-2025-25274514 days apartUnauthorized Command Execution in Archived Channels
35Monitor
CVE-2026-98160 days apartInsufficient server-side validation of board member role fields permits privilege escalation
33Monitor
CVE-2026-673966 days apartMattermost: Delegated admins could patch protected default system roles
28Monitor
CVE-2026-9118128 days apartData Retention Teams Endpoint Leaks Private Team Invite ID
26Monitor
CVE-2026-1010635 days apartUnauthorized users can trigger interactive post actions in private channels via action cookie channel mismatch in Mattermost
26Monitor

Chain candidates

An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.

—

Bug bounty scope

Mattermost Public Bug Bounty Engagement Bugcrowd · paid · up to $2,000

The product’s vendor appears in a public program. Matched by name; verify the scope text in the program.

Technical details

Attack conditions

Anyone who can reach it over the internet can trigger it.
A low-privileged account is enough.
No user action is required.
No special conditions are required; it is repeatable.

If successful

Confidentiality: high · data can be read
Integrity: none
Availability: none

Attack vector: Network
Attack complexity: Low
Privileges required: Low
User interaction: None
Scope: Unchanged
Confidentiality impact: High
Integrity impact: None
Availability impact: None

Weakness class (CWE)

CWE-863 · Incorrect Authorization

CVSS vector

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

nvd-secondary

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

MITRE has no CAPEC/ATT&CK mapping for this CWE.

Change log

No changes recorded on tracked fields yet. Score, KEV, exploit maturity and fix status changes appear here.

References

mattermost.com/security-updates

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

All records