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CVE-2026-8821

Playbooks run owner channel membership permission bypass

HIGH
7.1
CVSS Score
Published: September 14, 2026 at 10:42 AM UTC
Updated: October 7, 2026 at 03:43 PM UTC
2 products affected across 1 vendorMattermost ServerMattermost

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Severity
HIGH7.1
Exploit prediction
EPSS 0.3%
SSVC
No known exploit

Description

Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7, 10.11.x <= 10.11.22 fail to validate channel member-management permission during playbook run creation, allowing an authenticated channel member to add an arbitrary user to a restricted channel via the run owner field.. Mattermost Advisory ID: MMSA-2026-00677

Affected

Our own answer: ranges merged, affected separated from platform-only, with confidence and review flags.

Mattermost Server· Mattermost
95%
10.11.0 ≤ v < 10.11.2311.7.0 ≤ v < 11.7.811.8.0 ≤ v < 11.8.511.9.0 ≤ v < 11.9.1
Affected range stops below 10.11.23, so it looks safe — our inference, not a vendor statement
Affected range stops below 11.7.8, so it looks safe — our inference, not a vendor statement
Affected range stops below 11.8.5, so it looks safe — our inference, not a vendor statement
Affected range stops below 11.9.1, so it looks safe — our inference, not a vendor statement
Mattermost· Mattermost
60%
10.11.0 ≤ v ≤ 10.11.2211.7.0 ≤ v ≤ 11.7.711.8.0 ≤ v ≤ 11.8.410.11.011.7.011.8.011.9.0
Vendor states this is fixed in 10.11.23 (unaffected)
Vendor states this is fixed in 11.10.0 (unaffected)
Vendor states this is fixed in 11.7.8 (unaffected)
Vendor states this is fixed in 11.8.5 (unaffected)
Vendor states this is fixed in 11.9.1 (unaffected)

Remediation

No guide yet

No remediation guidance has been written for this CVE yet. What the record does show:

Patch and advisory references

References (1)

Vendor Advisory
Source: cverecord
https://mattermost.com/security-updates

SSVC — Action Prioritization

SSVCNo known exploitNot automatablePartial impact

SSVC signals action urgency (what to patch first), separate from CVSS which measures severity.

CVSS

7.1HIGHMattermost
Attack Vector
NETWORK
Complexity
LOW
Privileges
LOW
User Interaction
NONE
Scope
UNCHANGED
Confidentiality
HIGH
Integrity
LOW
Availability
NONE
Vector?

Quick Info

StatusANALYZED
CWE
CWE-862

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