Which versions are affected?

Affected product and version evidence is listed in the product table above. Current public advisories should be checked for exact version scope before patch targeting is finalized.

Is there active exploitation?

Exploitation is reported, but no independent confirmation is available in the sources we have. If exploitation is not confirmed from current sources, teams should document that status and continue monitoring KEV, vendor, and telemetry updates.

No. CVE-2026-9859 is not listed in CISA KEV in the supplied enrichment data. KEV status should be monitored because it can change after initial disclosure or after exploitation becomes publicly confirmed.

Is there a public exploit?

6 public PoC repositories reported; 1 marked weaponized in current dataset. Public exploit or PoC availability should raise triage priority, especially for exposed or business-critical systems.

How can defenders detect it?

Start with web/proxy/WAF logs, application logs, network IDS/IPS, EDR process telemetry, host service logs where available. Look for vulnerability-class behavior, product errors, suspicious authentication events, exploit probes, and abnormal process activity based only on evidence that applies to the affected product.

How should teams prioritize it?

Prioritize CVE-2026-9859 using CVSS 6.5, EPSS 0%, KEV status, public PoC status, and asset exposure. Internet-facing, privileged, or business-critical deployments should be handled first.

Are workarounds available?

Workarounds are not confirmed from current sources. Fixed-version evidence is listed in the mitigation section above.

Which threat actors are linked publicly?

No public threat actor linkage is confirmed from current sources. Do not attribute exploitation to a threat actor unless the source evidence explicitly supports that linkage.

What should be done in the next 24 hours?

Identify exposed assets running affected vendor/product/version combinations. Prioritize based on EPSS, PoC availability, and external exposure. Search available logs for exploit probes, errors, authentication anomalies, or suspicious child processes matching the vulnerability class. Open remediation tickets with affected assets, owners, evidence status, and detection coverage so decisions are traceable.

mediumRemote Code Execpublic exploit

CVE-2026-9859

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

Q: What is CVE-2026-9859?

CVE-2026-9859 is Mattermost Boards plugin didn’t enforce role-based authorization on board channel link allowing board editors to expose boards to arbitrary channels. The supplied enrichment data classifies it as Remote Code Exec mapped to CWE-863 CWE-863.

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

Published Aug 17, 2026Updated Aug 17, 2026Sources: NVD, Mattermost (CNA), GitHub, GitHub Security Advisory, SOCRadar CTI

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the scoring sources call it.

Exploitation

Reported

not confirmed by CISA

EPSS

ahead of 0% of scored CVEs

Affects

Mattermost

7 products listed

CVSS base

6.5

medium

Remediation

The vendor's own words where we have them.

Update Mattermost to versions 11.9.0, 11.7.7, 10.11.22, 11.8.4 or higher.

First 24 hours

Ordered from the record's own fields — exposure first, because you cannot patch what you have not found.

Identify exposed assets running affected vendor/product/version combinations.
Prioritize based on EPSS, PoC availability, and external exposure.
Search available logs for exploit probes, errors, authentication anomalies, or suspicious child processes matching the vulnerability class.

Affected scope

Vendor, product and version as the advisories word them.

Vendor	Product	Versions	Status
Mattermost	Mattermost	11.7.0 to <= 11.7.6	Vulnerable
Mattermost	Mattermost	10.11.0 to <= 10.11.21	Vulnerable
Mattermost	Mattermost	11.8.0 to <= 11.8.3	Vulnerable
Mattermost	Mattermost	11.9.0	Patched
Mattermost	Mattermost	11.7.7	Patched
Mattermost	Mattermost	10.11.22	Patched
Mattermost	Mattermost	11.8.4	Patched

Attack characteristics

The CVSS vector, decoded. It describes the attack, not your exposure to it.

Availability

None

Confidentiality

High

Integrity

None

Scope

Unchanged

Attack Complexity

Low

Attack Vector

Network

Privileges Req

Low

User Interaction

None

Every base score collected

Sources score independently and disagree; each row says who scored it and under which version.

Score	Version	Severity	Expl.	Impact	Source
6.5	CVSS 3.1	medium	2.8	3.6	mattermost.com (CNA), Mattermost (CNA), GHSA

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

What this weakness leads to

MITRE's own consequences and mitigations for the weakness class — the authority's wording, not guidance derived from the CVSS vector.

MITRE

CWE-863 · Incorrect Authorization

Read Application Data
Read Files or Directories
Modify Application Data
Modify Files or Directories

Mitigation: Divide the software into anonymous, normal, privileged, and administrative areas. Reduce the attack surface by carefully mapping roles with data and functionality. Use role-based access control (RBAC) [REF-229] to enforce the roles at the appropriate boundaries. Note that this approach may not protect against horizontal authorization, i.e., it will not protect a user from attacking others with the same role.

Weakness & attack patterns

CWE-863

Inferred from the weakness class — not observed against this CVE.

CAPEC-242 · Code InjectionCAPEC-35 · Leverage Executable Code in Non-Executable Files

Public exploit

Capability, not use: code existing is a different claim from anyone running it.

Repositories

1 of the 5 sampled are weaponized

Social mentions

Posts naming this CVE. Attention, not evidence of exploitation — it spikes on disclosure and decays whether or not anything is exploited.

CVE-2026-9859 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, whi… https://t.co/KV7GGlbJKG
@CVEnew· Aug 17, 2026

Detection

Read off the CVSS vector and the weakness class. Starting points, not rules we have tested.

Prioritize edge telemetry for network-reachable Mattermost.
Monitor for scanner or exploit-pattern traffic after 6 public PoC repositories were reported.

web/proxy/WAF logsapplication logsnetwork IDS/IPSEDR process telemetryhost service logs

Timeline

What happened to this CVE, newest first — with the readings a source repeats on a schedule counted underneath rather than listed.

2026
Added · GHSA-27x7-qjjm-4f83 published (medium)
Aug 18, 2026 · GitHub Security Advisory
Added · CVSS 3.1 6.5 (AV:N)
Aug 17, 2026 · NVD
Initial · CVE published
Aug 17, 2026 · NVD

Source activity

Readings a source repeats on a schedule, counted rather than listed.

1×CVE modified by NIST, Modified, LinkAug 18, 2026 · SOCRadar CTI

References

3 on the record

mattermost.com/security-updates
Vendor Advisory, Mitre, NIST
nvd.nist.gov/vuln/detail/CVE-2026-9859
NVD
github.com/advisories/GHSA-27x7-qjjm-4f83
Exploit, Third Party Advisory

Elsewhere on this site

Mattermostevery CVE for this vendor
Remote Code Execsame class
CWE-863other pages naming this weakness

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
No exploit packet captures, log samples, or incident case IDs supplied.

Answered from this record1

What should defenders know first?

CVE-2026-9859 is Mattermost Boards plugin didn’t enforce role-based authorization on board channel link allowing board editors to expose boards to arbitrary channels, a medium vulnerability affecting Mattermost from Mattermost. The current evidence does not list it in CISA KEV, and the exploit status is: Exploitation is reported, but no independent confirmation is available in the sources we have. Public exploit evidence is: 6 public PoC repositories reported; 1 marked weaponized in current dataset. The affected-version evidence is listed in the key facts and affected products tables. Defenders should first verify whether exposed or business-critical assets run those versions, then apply vendor patches or mitigations, restrict reachable attack surface, and preserve logs for detection review. CVSS 6.5 describes technical severity, while EPSS 0% helps estimate near-term exploit likelihood; neither replaces asset context. Unknown fields should remain explicit in tickets, and threat actor, IOC, victimology, or payload claims should not be added unless a cited source supports them. Monitor CISA KEV, vendor advisories, NVD changes, public PoC repositories, and internal telemetry for update triggers.