CVE-2026-6333 PUBLISHED

SSRF via Host Header Spoofing in Custom Slash Commands

Assigner: Mattermost
Reserved: 15.04.2026 Published: 18.05.2026 Updated: 18.05.2026

Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13 fail to validate the Host header when constructing response URLs for custom slash commands which allows an authenticated attacker to redirect slash command responses to an attacker-controlled server via a spoofed Host header.. Mattermost Advisory ID: MMSA-2026-00582

Metrics

CVSS 3.1

CVSS Vector: CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:N/I:L/A:N
CVSS Score: 3.5

Attack Vector	Network	Scope	Changed
Attack Complexity	High	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 11.5.0 to 11.5.1 (incl.) affected from 10.11.0 to 10.11.13 (incl.) Version 11.6.0 is unaffected Version 11.5.2 is unaffected Version 10.11.14 is unaffected

Solutions

Update Mattermost to versions 11.6.0, 11.5.2, 10.11.14 or higher.

Credits

Juho Forsén finder

References

MMSA-2026-00582

Problem Types

CWE-918: Server-Side Request Forgery (SSRF) CWE