Private repository access via unchecked OAuth scope
Published May 22, 2026 · Updated May 22, 2026
Incorrect authorization in the GitHub plugin for affected Mattermost releases allows remote authenticated users to access private repositories. The OAuth completion handler stored the returned token without checking GitHub's X-OAuth-Scopes header, so a modified authorization URL could add repo scope when only public access was intended. Exploitation requires a Mattermost account, an enabled and configured GitHub plugin, and completion of its OAuth flow; it exposes the connected user's private repositories.
Summary
What happened
Incorrect authorization in the GitHub plugin for affected Mattermost releases allows remote authenticated users to access private repositories. The OAuth completion handler stored the returned token without checking GitHub's X-OAuth-Scopes header, so a modified authorization URL could add repo scope when only public access was intended. Exploitation requires a Mattermost account, an enabled and configured GitHub plugin, and completion of its OAuth flow; it exposes the connected user's private repositories.
The record
- CVE
- CVE-2026-28735
- Published
- May 22, 2026
- Updated
- May 22, 2026
- Vendor
- Mattermost
- Product
- Mattermost
- Classifications
- CWE-863
- Attack vector
- network
- Privileges
- authenticated
Timeline
How it unfolded
- May 22, 2026CVE publishedPublication date reported by the CVE source.
- May 22, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=10.11.0 <=10.11.14
- Affected versionversion=11.4.0 <=11.4.4
- Affected versionversion=11.5.0 <=11.5.3
- Affected versionversion=11.6.0 <=11.6.0
What conditions does exploitation require?
What is affected?
Published CVSS scores
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Attacks
What attackers are doing with it
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Weakness, pattern, technique
Public exploit references
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Technologies
Your stack
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Your stack
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