CVE-2026-93851

Mattermost Agents forged mention reminder can replace a plugin

Mattermost · Mattermost Agents plugin mention reminder

First documented disclosure · Review checkpoint

Summary

A crafted post can make an administrator send an unintended API request.

Severity and exploitation evidence

High · 8.0/10 · CVSS 3.1. Severity source.

Exploitation not established: Mattermost CNA describes the flaw; no exploitation campaign or victim is established.

Exploitation evidence source

Technical severity is not a measure of observed attacks or of risk to your deployment. Attempts, demonstrations, and successful compromise retain the qualifications in the source.

How the weakness works

The Agents mention reminder fails to validate a target post ID, enabling path traversal into a plugin-replacement request.

Conditions required

An authenticated user can post where a system administrator can see it; the administrator must click the forged reminder.

Potential impact

Replacement of an installed plugin and server-side code execution with administrator browser participation.

Affected versions and configurations

11.9.0–11.9.1, 11.8.0–11.8.5 and 11.7.0–11.7.10.

Fix and mitigation status

11.10.0, 11.9.2, 11.8.6 or 11.7.11.

Practical next steps

Install a supported release containing the fix; restrict plugin administration and avoid acting on untrusted reminders.

Operating lesson

Treat agent-generated UI references as untrusted identifiers and authorize the final API action.

Evidence and limits

Mattermost CNA published October 28. The bulletin delays details after fixes; its client-rendered table was unavailable in the direct HTML. Do not describe this as unauthenticated or zero-click.

Sources