CVE-2026-19867: Uncapped mention keys in user notification properties allow CPU exhaustion during post processing

Severity: medium (CVSS 4.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-10-16 · updated 2026-10-29
Weakness: CWE-770

Description

Mattermost versions 11.10.x <= 11.10.0, 11.9.x <= 11.9.1, 11.8.x <= 11.8.5, 11.7.x <= 11.7.10 fail to cap the number and size of mention keys stored in a user's notification properties, and to bound the quadratic multibyte-keyword matching performed for each post, which allows an authenticated user to degrade post-processing latency and consume excessive CPU on behalf of other members of any channel they share via storing an oversized notify_props.mention_keys list on their own profile.. Mattermost Advisory ID: MMSA-2026-00773

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; 11.10.0 (and their updates) — fixed in 11.7.11, 11.8.6, 11.9.2
Mattermost: from 11.10.0 through 11.10.0; from 11.9.0 through 11.9.1; from 11.8.0 through 11.8.5; from 11.7.0 through 11.7.10

How to fix

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

References

https://mattermost.com/security-updates

Get alerted when CVE-2026-19867 or the next vulnerability affects your stack