What is the CVSS score of CVE-2026-28735?

CVE-2026-28735 has a CVSS 3.1 base score of 5.4 (Medium). CVSS vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N. EPSS exploitation probability: 0.0%.

Is there a patch available for CVE-2026-28735?

Yes, a patch is available for CVE-2026-28735. Update the affected software to the latest version immediately.

Mattermost CVE-2026-28735

Q: Which versions of Mattermost are affected by CVE-2026-28735?

OAuth scope validation bypass in Mattermost's GitHub integration allows authenticated users to escalate repository access beyond what was originally authorized.. A patch is available.

EUVD-2026-31465 MEDIUM

Incorrect Authorization (CWE-863)

2026-05-22 Mattermost

GHSA-r5vf-grcx-5vqp

Authentication Bypass Mattermost

5.4

CVSS 3.1 · NVD

Severity by source

NVD PRIMARY

5.4 MEDIUM

AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N

Primary rating from NVD · only source for this CVE.

CVSS VectorNVD

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N

Attack Vector

Network

Attack Complexity

Low

Privileges Required

Low

User Interaction

None

Scope

Unchanged

Confidentiality

Low

Integrity

Low

Availability

None

Lifecycle Timeline

Analysis Generated

May 22, 2026 - 16:45 vuln.today

DescriptionCVE.org

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

AnalysisAI

OAuth scope validation bypass in Mattermost's GitHub integration allows authenticated users to escalate repository access beyond what was originally authorized. By manipulating the scope parameter in the GitHub OAuth authorization URL before the callback is processed, a low-privileged Mattermost user can obtain a GitHub token with broader permissions - including access to private repositories - than the application intended to grant. Affecting versions across four active release branches (10.11.x through 11.6.x), this is no public exploit identified at time of analysis and is not listed in CISA KEV, but the low complexity and authentication-only barrier make it a realistic insider or compromised-account risk.

Technical ContextAI

Mattermost integrates with GitHub via OAuth 2.0, allowing users to link their GitHub accounts for repository browsing or plugin functionality. During the OAuth authorization flow, a client initiates a request to GitHub with a defined scope parameter that specifies what permissions are being requested (e.g., read:user, public_repo). After the user authorizes the app on GitHub's side, GitHub redirects back to Mattermost's callback endpoint with an authorization code. CWE-863 (Incorrect Authorization) identifies the root cause: Mattermost's callback handler fails to verify that the scope embedded in the returned token matches the scope originally requested. This means an attacker can modify the scope in the outbound authorization URL to request elevated permissions (e.g., repo, which grants access to private repositories), and Mattermost will silently accept and store the resulting over-privileged token. The affected CPE is cpe:2.3:a:mattermost:mattermost:*:*:*:*:*:*:*:* across branches 10.11.x, 11.4.x, 11.5.x, and 11.6.x.

RemediationAI

Upgrade to a patched release per Mattermost's security advisory MMSA-2026-00628, available at https://mattermost.com/security-updates. The exact fixed version numbers are not independently confirmed in the available reference data beyond the vendor advisory page - administrators should consult that page for the specific target versions in their branch (10.11.x, 11.4.x, 11.5.x, or 11.6.x). As a compensating control for organizations that cannot immediately patch, disabling the GitHub OAuth integration in Mattermost's system console (System Console > Integrations > OAuth 2.0) will eliminate the attack surface entirely, though this removes GitHub-linked functionality for all users. Restricting which users can initiate OAuth flows via role-based access controls may limit exposure but does not eliminate the underlying flaw. Auditing existing GitHub OAuth tokens granted through Mattermost for unexpected scope elevation is advisable regardless of patch status.