CVE-2026-18799 PUBLISHED

Mattermost Server fails to restrict Slack import account binding, allowing team admins to forge posts as existing users

Assigner: Mattermost
Reserved: 04.08.2026 Published: 16.10.2026 Updated: 16.10.2026

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 restrict Slack import account binding to system administrator-initiated imports which allows an authenticated attacker with team admin privileges to force-add existing Mattermost users to their team and forge posts attributed to those users' identities via a crafted Slack export containing target users' email addresses.. Mattermost Advisory ID: MMSA-2026-00757

Metrics

CVSS 3.1

CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N
CVSS Score: 4.3

Attack Vector	Network	Scope	Unchanged
Attack Complexity	Low	Confidentiality Impact	None
Privileges Required	Low	Integrity Impact	Low
User Interaction	None	Availability Impact	None

CVSS 3.1

Product Status

Vendor	Mattermost
Product	Mattermost
Versions	Default: unaffected affected from 11.10.0 to 11.10.0 (incl.) affected from 11.7.0 to 11.7.9 (incl.) affected from 11.9.0 to 11.9.1 (incl.) affected from 11.8.0 to 11.8.5 (incl.) Version 11.11.0 is unaffected Version 11.10.1 is unaffected Version 11.7.10 is unaffected Version 11.9.2 is unaffected Version 11.8.6 is unaffected

Solutions

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

Credits

Claude and Ada Logics finder

References

MMSA-2026-00757

Problem Types

CWE-862: Missing Authorization CWE