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CVE-2026-16453

Mattermost SAML RelayState forgery allows unauthorized team membership

HIGH
7.1
CVSS Score
Published: October 16, 2026 at 07:08 AM UTC
Updated: October 29, 2026 at 06:08 PM UTC
2 products affected across 1 vendorMattermost ServerMattermost

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Severity
HIGH7.1
Exploit prediction
EPSS 0.2%
SSVC
No known exploit

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 verify the integrity of the SAML RelayState POST parameter which allows an authenticated SAML user to join any team, including invite-only teams, via forging the team_id or invite_id fields in the RelayState.. Mattermost Advisory ID: MMSA-2026-00727

Affected

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

Mattermost Server· Mattermost
95%
11.7.0 ≤ v < 11.7.1011.8.0 ≤ v < 11.8.611.9.0 ≤ v < 11.9.211.10.0
Mattermost· Mattermost
90%
Vendor states this is fixed in 11.10.1 (unaffected)
Vendor states this is fixed in 11.11.0 (unaffected)
Vendor states this is fixed in 11.7.10 (unaffected)
Vendor states this is fixed in 11.8.6 (unaffected)
Vendor states this is fixed in 11.9.2 (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-472

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