CVE-2026-28735: Mattermost allows authenticated users to gain access to private repositories

May 26, 2026 (updated June 29, 2026)

Mattermost versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to validate the OAuth token scope on the callback which allows an authenticated Mattermost user to gain access to private repositories via modifying the scope parameter in the GitHub authorization URL. Mattermost Advisory ID: MMSA-2026-00628

References

github.com/advisories/GHSA-r5vf-grcx-5vqp
github.com/mattermost/mattermost-plugin-github/commit/6e6b740c4852cdfa136ee0ced160da832285c353
mattermost.com/security-updates
nvd.nist.gov/vuln/detail/CVE-2026-28735

Code Behaviors & Features

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