CVE-2026-86348

MS Calendar plugin: unrecovered handler panics from malformed post-action requests could crash the plugin process

Published Sep 14, 2026 CVSS 3.1 · 4.3 Medium EPSS < 1% KEV · Not listed

Description

Mattermost versions <=11.9 11.0.9 11.4.8 11.7.7 10.22.11.0 fail to recover from handler panics, which allows an authenticated user to crash the plugin via a post-action request with an unexpected field type.. Mattermost Advisory ID: MMSA-2026-00701

Analysis

Technical impact

Mattermost versions up to 11.9, 11.0.9, 11.4.8, 11.7.7, and 10.22.11.0 do not recover from handler panics caused by malformed post‑action requests, allowing an authenticated user to crash the MS Calendar plugin process.

Affected Systems

The affected product is Mattermost, specifically the MS Calendar plugin. Vulnerable versions include any release 10.22.11.0 or earlier, 11.0.9 or earlier, 11.4.8 or earlier, 11.7.7 or earlier, and 11.9 or earlier.

Risk and Exploitability

The CVSS score is 4.3 and the EPSS score is not available. The vulnerability is not listed in CISA KEV. Attack requires an authenticated user who can send a post‑action request with an unexpected field type to trigger the non‑recoverable panic and cause a denial‑of‑service of the plugin. No remote code execution or data exfiltration is possible.

Affected Products

Default status is the baseline for the product, each version can override it (e.g. patched versions marked unaffected).

Vendor Product Default status Versions

Mattermost

unaffected

Version	Status	Constraints
`<=11.9 11.0.9 11.4.8 11.7.7 10.22.11.0`	affected	≤ <=11.9 11.0.9 11.4.8 11.7.7 10.22.11.0
`11.10.0`	unaffected	—
`11.9.1`	unaffected	—
`11.8.5`	unaffected	—
`11.7.8`	unaffected	—
`10.11.23`	unaffected	—

Configuration 1 [-]

OR	cpe:2.3:a:mattermost:mattermost_server::::::::
	cpe:2.3:a:mattermost:mattermost_server::::::::
	cpe:2.3:a:mattermost:mattermost_server::::::::
	cpe:2.3:a:mattermost:mattermost_server::::::::

No data.

Vendor Product Versions

Mattermost

Version	Status	Scheme	Platform
`<=11.9 11.0.9 11.4.8 11.7.7 10.22.11.0`	affected	generic	—
`11.10.0`	unaffected	semver	—
`11.9.1`	unaffected	semver	—
`11.8.5`	unaffected	semver	—
`11.7.8`	unaffected	semver	—
`10.11.23`	unaffected	semver	—

Remediation

Vendor Solution

Update Mattermost to versions 11.10.0, 11.9.1, 11.8.5, 11.7.8, 10.11.23 or higher.

OpenCVE Recommended Actions

Update Mattermost to version 11.10.0, 11.9.1, 11.8.5, 11.7.8, 10.11.23 or newer.
If the MS Calendar plugin is not essential, disable or uninstall it to eliminate the vulnerability.
Configure the plugin or system to validate post‑action request fields and restrict permissions for authenticated users, and monitor logs for plugin panics to detect repeated exploitation attempts.

Generated by OpenCVE AI on September 15, 2026 at 14:19 UTC.

Tracking

Advisories

No advisories available

Metrics

No CVSS v4.0

Attack Vector Network

Attack Complexity Low

Privileges Required Low

Scope Unchanged

Confidentiality Impact None

Integrity Impact None

Availability Impact Low

User Interaction None

No CVSS v3.0

No CVSS v2

This CVE is not in the KEV list.

Exploit prediction score: < 1% (0.367% probability within 30 days)

Exploitation none

Automatable no

Technical Impact partial

References

Link	Providers
https://mattermost.com/security-updates

History

Oct 14, 2026 7:45 PM First appearance in the feed. Affected versions updated. Affected vendors updated View changes

Type	Values Removed	Values Added
First Time appeared		Mattermost mattermost Server
CPEs		cpe:2.3:a:mattermost:mattermost_server::::::::
Vendors & Products		Mattermost mattermost Server

Sep 14, 2026 9:00 PM First appearance in the feed. Affected vendors updated. Scoring metrics updated View changes

Type	Values Removed	Values Added
First Time appeared		Mattermost Mattermost mattermost
Vendors & Products		Mattermost Mattermost mattermost
Metrics		ssvc `{'options': {'Automatable': 'no', 'Exploitation': 'none', 'Technical Impact': 'partial'}, 'version': '2.0.3'}`

Sep 14, 2026 2:00 PM Description updated. Title updated. Weaknesses updated. Added references. Scoring metrics updated View changes

Type	Values Removed	Values Added
Description		Mattermost versions <=11.9 11.0.9 11.4.8 11.7.7 10.22.11.0 fail to recover from handler panics, which allows an authenticated user to crash the plugin via a post-action request with an unexpected field type.. Mattermost Advisory ID: MMSA-2026-00701
Title		MS Calendar plugin: unrecovered handler panics from malformed post-action requests could crash the plugin process
Weaknesses		CWE-704
References		https://mattermost.com/security-updates
Metrics		cvssV3_1 `{'score': 4.3, 'vector': 'CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L'}`