What’s Vulnerable › Vulnerabilities

CVE-2026-95666: Unbounded post ID array in the bulk reactions endpoint allows denial of service

• Severity: medium (CVSS 4.3 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.4% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-09-22 · updated 2026-10-07
• Weakness: CWE-770

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

Affected products

• Mattermost: from 11.7.0 before 11.7.11; from 11.8.0 before 11.8.6; from 11.9.0 before 11.9.2; from 11.10.0 before 11.10.2 — fixed in 11.10.2, 11.7.11, 11.8.6, 11.9.2

How to fix

Upgrade to a fixed version: Mattermost 11.10.2, 11.7.11, 11.8.6, 11.9.2. Check the vendor advisory for mitigations.

References

• https://mattermost.com/security-updates

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