CVE-2026-9571

Q: How do I fix CVE-2026-9571?

Upgrade mattermost server to 10.11.20 or later.

Q: Is CVE-2026-9571 being actively exploited?

Not that we know of. CVE-2026-9571 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 0.30% is the estimated probability that it will be exploited in the next 30 days. That is higher than 22% of all scored CVEs.

Q: How severe is CVE-2026-9571?

CVE-2026-9571 has a CVSS v3 base score of 5.9, rated medium. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.

Q: What does CVE-2026-9571 affect?

Published advisories record a fix for mattermost server (NVD). Only products with a sourced advisory are listed, so treat this as what we can cite rather than a complete inventory.

MEDIUM

5.9

CVSS v3

NVD

Description

Mattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, 10.11.x <= 10.11.19 fail to invalidate OAuth refresh tokens upon user account deactivation, which allows a deactivated user or an attacker in possession of a valid refresh token to obtain new functional access tokens via the OAuth refresh token grant endpoint.. Mattermost Advisory ID: MMSA-2026-00680

CWE-305

How to fix

Remediation Available

mattermost serverNVD

Affected:>= 10.11.0, < 10.11.20Fixed in:10.11.20CVE-2026-9571derived from NVD

Affected:>= 11.6.0, < 11.6.5Fixed in:11.6.5CVE-2026-9571derived from NVD

Affected:>= 11.7.0, < 11.7.3Fixed in:11.7.3CVE-2026-9571derived from NVD

TridentStack Control can deploy fixes like this automatically across your Windows, macOS, and Linux fleet. See how it works

Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.

CVSS v3 Vector

Exploitability

Attack VectorNetwork

Attack ComplexityHigh

Privileges RequiredHigh

User InteractionNone

ScopeUnchanged

Impact

ConfidentialityHigh

IntegrityHigh

AvailabilityNone

CVSS:3.1/AV:N/AC:H/PR:H/UI:N/S:U/C:H/I:H/A:N

Exploit Intelligence

0.30%probability of exploitation in 30 days

22ndpercentile

Low risk: more likely to be exploited than 22% of all known CVEs.

References

Vendor Advisory1

mattermost.com

Related Vulnerabilities

Other CWE-305 vulnerabilities, ordered by exploit likelihood. View all

CVE	Severity	CVSS	EPSS	Exploited	Fix
CVE-2025-31161	Critical	9.8	100%	KEV + Ransom	Fix
CVE-2020-10923	High	8.8	85%	-	-
CVE-2023-28126	Medium	5.9	67%	-	-
CVE-2023-34124	Critical	9.8	49%	-	Fix
CVE-2025-32011	Critical	9.8	27%	-	-
CVE-2024-37085	Medium	6.8	27%	KEV + Ransom	Fix

Common questions

How do I fix CVE-2026-9571?

Upgrade mattermost server to 10.11.20 or later.

Is CVE-2026-9571 being actively exploited?

Not that we know of. CVE-2026-9571 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 0.30% is the estimated probability that it will be exploited in the next 30 days. That is higher than 22% of all scored CVEs.

How severe is CVE-2026-9571?

CVE-2026-9571 has a CVSS v3 base score of 5.9, rated medium. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.

What does CVE-2026-9571 affect?

Published advisories record a fix for mattermost server (NVD). Only products with a sourced advisory are listed, so treat this as what we can cite rather than a complete inventory.

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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-07-15.