CVE-2026-28735
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedMattermost 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
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · moderate confidenceMattermost fails to validate OAuth token scopes on the callback endpoint during GitHub OAuth flow. An authenticated attacker can manipulate the scope parameter in the initial GitHub authorization URL to request broader permissions (e.g., repo scope for private repositories), then use the resulting token on callback to access resources they shouldn't have authorization for.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data>= 10.11.0, < 10.11.15>= 11.4.0, < 11.4.5>= 11.5.0, < 11.5.4>= 11.6.0, < 11.6.1CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- Low
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- Low
- Integrity
- Low
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Identify Mattermost Server versionAccess the Mattermost System Console > About > Mattermost, or use the /about slash command in the system console channel. Note the full version number (e.g., 10.11.14, 11.5.3, 11.6.0).Affected if The installed version falls within any of these ranges: >=10.11.0 and <10.11.15, >=11.4.0 and <11.4.5, >=11.5.0 and <11.5.4, or >=11.6.0 and <11.6.1
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Determine if GitHub OAuth is enabledNavigate to System Console > Authentication > GitHub, or check if GitHub OAuth has been configured in the OAuth 2.0 settings. Verify whether GitHub as an OAuth provider is active and users can authenticate via GitHub.Affected if GitHub OAuth integration is enabled and configured in Mattermost
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Verify OAuth scope handling in callbackInspect the OAuth authorization URL generated during GitHub login. The vulnerability allows manipulation of the scope parameter in the authorization request. In a vulnerable version, the server does not validate that the scopes in the token response match what was requested.Affected if The server accepts and uses OAuth tokens without validating that the granted scopes match the scopes requested in the authorization URL, potentially allowing access to additional GitHub resources beyond what was originally authorized
You are affected if your Mattermost Server version is within the affected ranges AND GitHub OAuth is enabled, as the scope validation failure only applies when OAuth authentication is configured.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
dbcve · scoped10.11.1511.4.511.5.4
Upgrade to the latest Mattermost version that includes proper OAuth scope validation on the callback. Until patched, disable GitHub OAuth integration or implement additional validation on the callback handler.
Upgrade to 10.11.15 (for 10.11.x), 11.4.5 (for 11.4.x), 11.5.4 (for 11.5.x), or 11.6.1 (for 11.6.x) depending on your current major version branch
- 1. Identify the currently running Mattermost Server version using the system console or mmctl command
- 2. Based on your current major version line, plan upgrade to the fixed release: For 10.11.x, upgrade to 10.11.15; For 11.4.x, upgrade to 11.4.5; For 11.5.x, upgrade to 11.5.4; For 11.6.x, upgrade to 11.6.1
- 3. Review Mattermost upgrade documentation and ensure backup of database and configuration files
- 4. Execute the upgrade following standard Mattermost upgrade procedures for your deployment method
- 5. After upgrade, verify OAuth integration with GitHub continues to function correctly
- 6. Confirm the fix by testing that scope parameter modification in authorization URL no longer grants unauthorized access
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation3.0 h
- Implementation6.0 h
- Testing3.0 h
- Review / QA2.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $3,984.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-28735 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2026-28735 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Version or environment caveats, and links to real fixes
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