Medium severity · published Sep 14, 2026
CVE-2026-10556
Unauthenticated webhook request with null notification entry could crash the Microsoft Calendar plugin.
5.3
CVSS
0.4%
EPSS
No
On KEV
Description
Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7, 10.11.x <= 10.11.22 fail to validate null entries in Microsoft Graph webhook notification payloads, which allows an unauthenticated attacker to crash the Microsoft Calendar plugin process and deny calendar integration service to all users on the instance via a crafted {{POST}} request to the public webhook endpoint.. Mattermost Advisory ID: MMSA-2026-00693
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Confidentiality
None
Integrity
None
Availability
Low
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)0.4% · higher than 36% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
MA
Mattermost · Mattermost
11.9.0 through 11.9.0; 11.8.0 through 11.8.4; 11.7.0 through 11.7.7; 10.11.0 through 10.11.22 · source: vendor
MS
Mattermost Server · Mattermost
10.11.0 before 10.11.23; 11.7.0 before 11.7.8; 11.8.0 before 11.8.5; 11.9.0 before 11.9.1 · source: NVD
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-10556
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-10556
Vendor Advisory
https://mattermost.com/security-updates
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