CVE-2026-18560

Q: How do I fix CVE-2026-18560?

Upgrade mattermost server to 11.8.6 or later.

Q: Is CVE-2026-18560 being actively exploited?

Not that we know of. CVE-2026-18560 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 0.18% is the estimated probability that it will be exploited in the next 30 days. That is higher than 7% of all scored CVEs.

Q: How severe is CVE-2026-18560?

CVE-2026-18560 has a CVSS v3 base score of 4.3, 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-2026-18560 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

4.3

CVSS v3

NVD

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 validate user’s permissions before other validations which allows non-content reviewers to infer a post’s flagged status via direct API call.. Mattermost Advisory ID: MMSA-2026-00754

CWE-203

How to fix

Remediation Available

mattermost serverNVD

Affected:>= 11.8.0, < 11.8.6Fixed in:11.8.6CVE-2026-18560derived from NVD

Affected:>= 11.7.0, < 11.7.10Fixed in:11.7.10CVE-2026-18560derived from NVD

Affected:>= 11.9.0, < 11.9.2Fixed in:11.9.2CVE-2026-18560derived 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 ComplexityLow

Privileges RequiredLow

User InteractionNone

ScopeUnchanged

Impact

ConfidentialityLow

IntegrityNone

AvailabilityNone

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

Exploit Intelligence

0.18%probability of exploitation in 30 days

7thpercentile

Low risk: more likely to be exploited than 7% of all known CVEs.

References

Vendor Advisory1

mattermost.com

Related Vulnerabilities

Other CWE-203 vulnerabilities, ordered by exploit likelihood. View all

CVE	Severity	CVSS	EPSS	Exploited	Fix
CVE-2017-5753	Medium	5.6	94%	-	Fix
CVE-2003-0190	Medium	5.0	77%	-	Fix
CVE-2017-5715	Medium	5.6	73%	-	Fix
CVE-2018-3639	Medium	5.5	61%	-	Fix
CVE-2023-28770	High	7.5	58%	-	Fix
CVE-2004-1602	Medium	5.0	31%	-	-

Common questions

How do I fix CVE-2026-18560?

Upgrade mattermost server to 11.8.6 or later.

Is CVE-2026-18560 being actively exploited?

Not that we know of. CVE-2026-18560 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 0.18% is the estimated probability that it will be exploited in the next 30 days. That is higher than 7% of all scored CVEs.

How severe is CVE-2026-18560?

CVE-2026-18560 has a CVSS v3 base score of 4.3, 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-2026-18560 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 2026-10-29.