Mattermost 10.11.x10.11.14 & 11.x11.6.0: OAuth Scope Bypass via GitHub URL
CVE-2026-28735 Published on May 22, 2026
GitHub OAuth Scope Validation
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
Vulnerability Analysis
CVE-2026-28735 can be exploited with network access, and requires small amount of user privileges. This vulnerability is considered to have a low attack complexity. The potential impact of an exploit of this vulnerability is considered to have a small impact on confidentiality and integrity, and no impact on availability.
Weakness Type
What is an AuthZ Vulnerability?
The software performs an authorization check when an actor attempts to access a resource or perform an action, but it does not correctly perform the check. This allows attackers to bypass intended access restrictions.
CVE-2026-28735 has been classified to as an AuthZ vulnerability or weakness.
Products Associated with CVE-2026-28735
Want to know whenever a new CVE is published for MatterMost? stack.watch will email you.
Affected Versions
Mattermost:- Version 11.6.0, <= 11.6.0 is affected.
- Version 11.5.0, <= 11.5.3 is affected.
- Version 11.4.0, <= 11.4.4 is affected.
- Version 10.11.0, <= 10.11.14 is affected.
- Version 11.7.0 is unaffected.
- Version 11.6.1 is unaffected.
- Version 11.5.4 is unaffected.
- Version 11.4.5 is unaffected.
- Version 10.11.15 is unaffected.
Exploit Probability
EPSS (Exploit Prediction Scoring System) scores estimate the probability that a vulnerability will be exploited in the wild within the next 30 days. The percentile shows you how this score compares to all other vulnerabilities.