Medium severity · published Sep 22, 2026
CVE-2026-95666
Unbounded post ID array in the bulk reactions endpoint allows denial of service
4.3
CVSS
0.4%
EPSS
No
On KEV
Description
Mattermost versions 11.9.x <= 11.9.1, 11.8.x <= 11.8.5, 11.7.x <= 11.7.10, 11.10.x <= 11.10.1 fail to limit the length of the post ID array accepted by the bulk reactions endpoint which allows an authenticated user to cause excessive database load via a crafted request to {{POST /api/v4/posts/ids/reactions}}.. Mattermost Advisory ID: MMSA-2026-00771
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L
Attack vector
Network
Complexity
Low
Privileges
Low
User interaction
None
Confidentiality
None
Integrity
None
Availability
Low
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)0.4% · higher than 30% 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 11.9.1; 11.8.0 through 11.8.5; 11.7.0 through 11.7.10; 11.10.0 through 11.10.1 · source: vendor
MS
Mattermost Server · Mattermost
11.7.0 before 11.7.11; 11.8.0 before 11.8.6; 11.9.0 before 11.9.2; 11.10.0 before 11.10.2 · source: NVD
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-95666
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-95666
Vendor Advisory
https://mattermost.com/security-updates
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