Role Escalation Vulnerability in Mattermost by Mattermost Inc.
CVE-2026-18551

6.5MEDIUM

Key Information:

Vendor

Mattermost
Status
Mattermost
Vendor
CVE Published:
16 October 2026

What is CVE-2026-18551?

Mattermost suffers from a vulnerability that allows authenticated attackers to escalate guest accounts to team or channel admin privileges. This exploitation occurs through a manipulated PUT request directed at the member roles API endpoint. Versions vulnerable to this issue include 11.10.x up to 11.10.0, 11.7.x up to 11.7.9, 11.9.x up to 11.9.1, and 11.8.x up to 11.8.5. Organizations using affected versions should apply security updates promptly to mitigate risks.

Affected Version(s)

Mattermost 11.10.0

Mattermost 11.7.0 <= 11.7.9

Mattermost 11.9.0 <= 11.9.1

References

https://mattermost.com/security-updates
vendor-advisory

CVSS V3.1

Score:
6.5
Severity:
MEDIUM
Confidentiality:
High
Integrity:
High
Availability:
High
Attack Vector:
Network
Attack Complexity:
Low
Privileges Required:
High
User Interaction:
None
Scope:
Unchanged

Timeline

  • Vulnerability published

  • Vulnerability Reserved

Credit

0x7oda7123
.