CVE-2026-18223

Mattermost invitation tokens not bound to invited email address allowing unauthorized team and private channel access

Published Oct 16, 2026 CVSS 3.1 · 5 Medium EPSS < 1% KEV · Not listed

Description

Mattermost versions 11.10.x <= 11.10.0, 11.7.x <= 11.7.9, 11.9.x <= 11.9.1, 11.8.x <= 11.8.5 fail to validate the invited recipient's email address when redeeming team and guest invitation tokens, which allows an authenticated attacker to join a team or gain unauthorized access to private channels via a forwarded or leaked invitation link.. Mattermost Advisory ID: MMSA-2026-00747

Affected Products

Default status is the baseline for the product, each version can override it (e.g. patched versions marked unaffected).

Vendor Product Default status Versions

Mattermost

unaffected

Version	Status	Constraints
`11.10.0`	affected	≤ 11.10.0
`11.7.0`	affected	≤ 11.7.9
`11.9.0`	affected	≤ 11.9.1
`11.8.0`	affected	≤ 11.8.5
`11.11.0`	unaffected	—
`11.10.1`	unaffected	—
`11.7.10`	unaffected	—
`11.9.2`	unaffected	—
`11.8.6`	unaffected	—

Configuration 1 [-]

OR	cpe:2.3:a:mattermost:mattermost_server::::::::
	cpe:2.3:a:mattermost:mattermost_server::::::::
	cpe:2.3:a:mattermost:mattermost_server::::::::
	cpe:2.3:a:mattermost:mattermost_server:11.10.0:::::::*

No data.

Vendor Product Versions

Mattermost

Version	Status	Scheme	Platform
`[11.10.0,11.10.0]`	affected	semver	—
`[11.7.0,11.7.9]`	affected	semver	—
`[11.9.0,11.9.1]`	affected	semver	—
`[11.8.0,11.8.5]`	affected	semver	—
`[11.11.0,11.11.0]`	unaffected	semver	—
`[11.10.1,11.10.1]`	unaffected	semver	—
`[11.7.10,11.7.10]`	unaffected	semver	—
`[11.9.2,11.9.2]`	unaffected	semver	—
`[11.8.6,11.8.6]`	unaffected	semver	—

Remediation

Vendor Solution

Update Mattermost to versions 11.11.0, 11.10.1, 11.7.10, 11.9.2, 11.8.6 or higher.

Tracking

Advisories

No advisories available

Metrics

No CVSS v4.0

Attack Vector Network

Attack Complexity High

Privileges Required High

Scope Unchanged

Confidentiality Impact High

Integrity Impact Low

Availability Impact None

User Interaction None

No CVSS v3.0

No CVSS v2

This CVE is not in the KEV list.

Exploit prediction score: < 1% (0.161% probability within 30 days)

Exploitation none

Automatable no

Technical Impact partial

References

Link	Providers
https://mattermost.com/security-updates

History

Oct 29, 2026 6:15 PM First appearance in the feed. Affected versions updated. Affected vendors updated View changes

Type	Values Removed	Values Added
First Time appeared		Mattermost mattermost Server
CPEs		cpe:2.3:a:mattermost:mattermost_server:::::::: cpe:2.3:a:mattermost:mattermost_server:11.10.0:::::::*
Vendors & Products		Mattermost mattermost Server

Oct 23, 2026 11:30 AM First appearance in the feed. Affected vendors updated View changes

Type	Values Removed	Values Added
First Time appeared		Mattermost Mattermost mattermost
Vendors & Products		Mattermost Mattermost mattermost

Oct 16, 2026 3:30 PM Scoring metrics updated View changes

Type	Values Removed	Values Added
Metrics		ssvc `{'options': {'Automatable': 'no', 'Exploitation': 'none', 'Technical Impact': 'partial'}, 'version': '2.0.3'}`

Oct 16, 2026 7:30 AM Description updated. Title updated. Weaknesses updated. Added references. Scoring metrics updated View changes

Type	Values Removed	Values Added
Description		Mattermost versions 11.10.x <= 11.10.0, 11.7.x <= 11.7.9, 11.9.x <= 11.9.1, 11.8.x <= 11.8.5 fail to validate the invited recipient's email address when redeeming team and guest invitation tokens, which allows an authenticated attacker to join a team or gain unauthorized access to private channels via a forwarded or leaked invitation link.. Mattermost Advisory ID: MMSA-2026-00747
Title		Mattermost invitation tokens not bound to invited email address allowing unauthorized team and private channel access
Weaknesses		CWE-863
References		https://mattermost.com/security-updates
Metrics		cvssV3_1 `{'score': 5, 'vector': 'CVSS:3.1/AV:N/AC:H/PR:H/UI:N/S:U/C:H/I:L/A:N'}`