Medium severity · published May 22, 2026
CVE-2026-28735
GitHub OAuth Scope Validation
5.4
CVSS
0.1%
EPSS
No
On KEV
Description
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
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N
Attack vector
Network
Complexity
Low
Privileges
Low
User interaction
None
Confidentiality
Low
Integrity
Low
Availability
None
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)0.1% · higher than 3% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
MA
Mattermost · Mattermost
11.6.0 through 11.6.0; 11.5.0 through 11.5.3; 11.4.0 through 11.4.4; 10.11.0 through 10.11.14 · source: vendor
MS
Mattermost Server · Mattermost
10.11.0 before 10.11.15; 11.4.0 before 11.4.5; 11.5.0 before 11.5.4; 11.6.0 before 11.6.1 · source: NVD
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-28735
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-28735
Vendor Advisory
https://mattermost.com/security-updates
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