[18.10] Mattermost Security Updates May 13, 2026

What does this MR do and why?

Update Mattermost from version 10.11.12 to 10.11.17 and backport it directly to ensure we ship up-to-date versions for the stable branches.

The full changelog for this release can be found here.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
  • Note: There is no default branch MR for this change because Mattermost was dropped from Omnibus in 19.0.
• The original MR has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).