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CVE-2026-19867

4.3 · MEDIUM

Mattermost versions 11.10.x <= 11.10.0, 11.9.x <= 11.9.1, 11.8.x <= 11.8.5, 11.7.x <= 11.7.10 fail to cap the number and size of mention keys stored in a user's notification properties, and to boun…

Published Oct 16, 2026mattermostCWE-770EPSS 0.22%

Overview

CVE-2026-19867 is a medium-severity vulnerability affecting mattermost mattermost_server. It was published on October 16, 2026 and has a CVSS 3.1 base score of 4.3 (MEDIUM).

This vulnerability has a CVSS 3.1 base score of 4.3, rated MEDIUM. It can be exploited remotely over the network. Some level of privileges is required for exploitation.

Technical description

Mattermost versions 11.10.x <= 11.10.0, 11.9.x <= 11.9.1, 11.8.x <= 11.8.5, 11.7.x <= 11.7.10 fail to cap the number and size of mention keys stored in a user's notification properties, and to bound the quadratic multibyte-keyword matching performed for each post, which allows an authenticated user to degrade post-processing latency and consume excessive CPU on behalf of other members of any channel they share via storing an oversized notify_props.mention_keys list on their own profile.. Mattermost Advisory ID: MMSA-2026-00773

Remediation

Check the references section for vendor advisories and patches from mattermost. Update mattermost_server to the latest patched version. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

Affected products

VendorProductVersionsStatus
mattermostmattermost_server>= 11.7.0, < 11.7.11Affected

References

Refmattermost.com/security-updates

How CyberStrike helps

Frequently asked questions

What is CVE-2026-19867?

CVE-2026-19867 is a medium-severity vulnerability affecting mattermost mattermost_server. It was published on October 16, 2026 and has a CVSS 3.1 base score of 4.3 (MEDIUM).

How severe is CVE-2026-19867?

This vulnerability has a CVSS 3.1 base score of 4.3, rated MEDIUM. It can be exploited remotely over the network. Some level of privileges is required for exploitation.

How do I fix or remediate CVE-2026-19867?

Check the references section for vendor advisories and patches from mattermost. Update mattermost_server to the latest patched version. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

How can CyberStrike help with CVE-2026-19867?

CyberStrike is an open-source, AI-augmented pentesting engine. Rather than flag a version number, it tests whether an issue like CVE-2026-19867 is actually reachable in your own applications and APIs, then verifies each finding before reporting it. That tells you if the weakness is a real, exploitable risk in your environment or a false positive you can safely deprioritize.

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