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Vulnerabilities

Vulnerability · CVE-2026-5163

Mattermost versions 11.5.x <= 11.5.1 fail to verify channel membership when processing AI-assisted message rewrites which allows an authenticated attacker to read the…

Published May 18, 2026 · catalog updated Oct 14, 2026 · mattermost server

Check if you run it

No confirmed exploitation is on record for CVE-2026-5163. EPSS is 0.34%. Act only if a listed version of mattermost server is installed.

Gaps on this record

  • No vendor fix or required action is stored. Compare the linked advisory before you change production.
Severity
Medium6.5
Known exploitation
Not in CISA/VulnCheck KEV
EPSS
0.34%
Remediation
Version ranges on file
Affected products
cpe mattermost/mattermost server · stated mattermost server

Mattermost versions 11.5.x <= 11.5.1 fail to verify channel membership when processing AI-assisted message rewrites which allows an authenticated attacker to read the content of threads in private channels and direct messages they do not have access to via a crafted request to the post rewrite endpoint.. Mattermost Advisory ID: MMSA-2026-00645

Are you affected?

Enter the version you run. The result uses only the ranges stored on this record.

The check returns affected, not affected, or incomplete. It does not guess a version that is missing from the record.

  • cpe mattermost/mattermost serverversion unspecified
  • stated mattermost server<= 11.5.1

How to fix CVE-2026-5163

What to do

Three questions, each answered from this record only. Confirm the vendor advisory before you patch.

Should you patch?

No confirmed exploitation is on record for CVE-2026-5163. EPSS is 0.34%. Act only if a listed version of mattermost server is installed.

Which versions are affected?

Compare what you have installed with mattermost server <= 11.5.1. Compare the installed version with the linked vendor advisory before you change production.

What should an analyst verify?

CWE-862: Missing Authorization. EPSS 0.34%. There is no CISA KEV listing. No Exploit-DB or Metasploit reference is linked. Use the source links for the original advisory.

Code

The project this record applies to. Exploit listings stay in References.

No public source repository is identified for this product.

Timeline

Detection, updates, and resolution for this record.

  1. May 18, 2026 · NIST NVD

    Detected

    Mattermost versions 11.5.x <= 11.5.1 fail to verify channel membership when processing AI-assisted message rewrites which allows an authenticated attacker to read the content of threads in private channels and direct messages they do not have access to via a crafted request to the post rewrite endpoint.. Mattermost Advisory ID: MMSA-2026-00645

  2. Oct 14, 2026 · NIST NVD

    Updated

    Mattermost versions 11.5.x <= 11.5.1 fail to verify channel membership when processing AI-assisted message rewrites which allows an authenticated attacker to read the content of threads in private channels and direct messages they do not have access to via a crafted request to the post rewrite endpoint.. Mattermost Advisory ID: MMSA-2026-00645

Sources

  • CVE.orgVulnerability recordCVE-2026-5163

    Official CVE identifier

    cve.org
  • FIRST EPSSExploitation probabilityCVE-2026-5163

    30-day exploitation probability 0.3%

    first.org
  • NIST NVDVulnerability recordCVE-2026-5163

    Mattermost versions 11.5.x <= 11.5.1 fail to verify channel membership when processing AI-assisted message rewrites which allows an authenticated attacker to read the content of threads in private channels and direct messages they do not have access to via a crafted request to th

    nvd.nist.gov

References

Freshness

Last verified in this catalog Oct 18, 2026. Significant claims (KEV, EPSS, CVSS, affected ranges) come from the source rows above — open the original publisher for the authoritative advisory.