High

CVE-2026-16453 — Mattermost SAML RelayState forgery allows unauthorized team membership

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

Published Oct 16, 2026 · Updated Oct 16, 2026 · Sources: NIST National Vulnerability Database, CVE Program (cve.org) e CISA ADP · CNA: Mattermost

Low

Who exploits it and how urgent it is

A 0–100 score combining confirmed exploitation, ransomware and APT use, public exploits, EPSS and severity.

No exploitation signal in the sources consulted so far.

How the score was calculated

Technical severity (high)+6

The score orders the patch queue; it does not replace your asset context (internet exposure, criticality, compensating controls).

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the sources call it.

Exploitation

Not confirmed

Not in CISA KEV — an absence of record, not proof it is unexploited.

EPSS FIRST

—

No EPSS score.

Affects

Mattermost

CVSS base

7.1

High · 2 source(s)

Remediation

Fixes and mitigations documented by the sources — linked to the original advisory.

No fix documented by the sources we poll. That does not mean none exists: check the vendor advisory.

First 24 hours

Ordered from the record’s own fields — exposure first, because you cannot patch what you have not found. A rule-generated starting point.

Identify exposed assets running the affected vendor/product/version combinations (Mattermost Mattermost).
No fix documented in the sources: reduce exposure and monitor the vendor advisory.
Search available logs for exploit probes, errors and authentication anomalies.

Affected scope

Vendor, product and versions as the sources word them.

Vendor	Product	Versions	Status	Source
Mattermost	Mattermost	≥ 11.10.0 and ≤ 11.10.0≥ 11.7.0 and ≤ 11.7.9≥ 11.9.0 and ≤ 11.9.1≥ 11.8.0 and ≤ 11.8.5	Vulnerable	CVE Program (cve.org) e CISA ADP

Attack characteristics

The CVSS vector, decoded. It describes the attack, not your exposure to it.

Attack vector

Network

Attack complexity

Low

Privileges required

Low

User interaction

None

Scope

Unchanged

Confidentiality

High

Integrity

Low

Availability

None

Every base score collected

Sources score independently and may disagree; each row says who scored it and under which version.

Score	Version	Severity	Expl.	Impact	Scored by
7.1	CVSS 3.1	High	2.8	4.2	responsibledisclosure@mattermost.com via NIST National Vulnerability Database
7.1	CVSS 3.1	High	—	—	Mattermost via CVE Program (cve.org) e CISA ADP

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:N (score shown at the top: responsibledisclosure@mattermost.com)

Weakness MITRE

Weakness class according to MITRE’s CWE catalog.

CWE-472

Other CVEs with this weakness →

Detection

Read off the CVSS vector and the weakness class. Starting points, not tested rules.

Prioritize edge telemetry for the network-reachable component (proxy, WAF, IDS/IPS).

web/proxy/WAF logsnetwork IDS/IPS

Timeline

What happened to this CVE, newest first.

2026
Affected products added · mattermost/mattermost
Oct 16, 2026 · CVE Program (cve.org) e CISA ADP
Record published
Oct 16, 2026
CVSS changed · 7.1
Oct 16, 2026 · NIST National Vulnerability Database
Source status changed · → Received
Oct 16, 2026 · NIST National Vulnerability Database
Record created in this database
Oct 16, 2026 · Alertas do Telegram
CVE published
Oct 16, 2026 · Mattermost

Source activity

Repeated readings are counted rather than listed.

1× References added · Oct 16, 2026 – Oct 16, 2026
1× Official description updated · Oct 16, 2026 – Oct 16, 2026

References

1 on the record

mattermost.com/security-updates
vendor-advisory, NIST National Vulnerability Database

Elsewhere on this site

Mattermost
every CVE for this vendor

CWE-472
other pages naming this weakness

Related CVEs (same products)

Medium CVE-2026-19621 Mattermost leaks interactive-message integration secrets to non-admin users via post-deletion events and thread reads 3h
Medium CVE-2026-18799 Mattermost Server fails to restrict Slack import account binding, allowing team admins to forge posts as existing users 3h
Medium CVE-2026-19076 Mattermost fails to reject OAuth application sessions in account type switch, allowing account takeover via bearer token 3h
Medium CVE-2026-19600 Mattermost logs plaintext authentication tokens in debug-level server logs on failed auth attempts 3h
Medium CVE-2026-16251 Mattermost local image proxy allows server memory exhaustion via unbounded image response buffering 3h
Medium CVE-2026-18223 Mattermost invitation tokens not bound to invited email address allowing unauthorized team and private channel access 3h

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

No FIRST EPSS score.
No fixed version, patch or mitigation documented.
No CISA SSVC assessment.
No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
No exploit packet captures, log samples, or incident case IDs supplied.

Answered from this record

Text assembled only from the fields above — no additional claims.

What should defenders know first?

CVE-2026-16453 is "Mattermost SAML RelayState forgery allows unauthorized team membership", rated High. It affects Mattermost (Mattermost); versions are in the scope table. It is not in CISA KEV, which does not rule out exploitation. CVSS 7.1 describes technical severity. First verify whether exposed or business-critical assets run the affected versions, apply vendor fixes or mitigations, and preserve logs for review.

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