---
id: CVE-2026-45122
title: MyBB is free and open source forum software
summary: >-
  MyBB is free and open source forum software. Prior to 1.8.40, the calendar
  module does not validate moderation permissions for the destination calendar
  when moving events. A user with moderation permission for the source calendar
  can mov…
severity: medium
cvss: 4.3
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N'
cwe:
  - CWE-863
published: '2026-08-18'
updated: '2026-09-08'
sourceUpdated: '2026-09-08T21:02:26.047'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-45122'
references:
  - url: >-
      https://github.com/mybb/mybb/commit/86ed2058e7f9a2c14828e731f684e997f9bb220c
    label: security-advisories@github.com
  - url: 'https://github.com/mybb/mybb/releases/tag/mybb_1840'
    label: security-advisories@github.com
  - url: 'https://github.com/mybb/mybb/security/advisories/GHSA-839m-gpw8-59j4'
    label: security-advisories@github.com
  - url: 'https://mybb.com/versions/1.8.40'
    label: security-advisories@github.com
tags:
  - nvd
epss: 0.00336
epssPercentile: 0.24225
ingestedAt: '2026-09-08T21:11:12.280Z'
---

## Overview

MyBB is free and open source forum software. Prior to 1.8.40, the calendar module does not validate moderation permissions for the destination calendar when moving events. A user with moderation permission for the source calendar can move an event to a calendar where the user has only viewing permission because the do_move action in calendar.php does not check canmoderateevents for the target calendar. The uniquely identifying implementation details include calendar event move, source calendar moderation permission, and destination calendar viewing permission. This issue is fixed in version 1.8.40.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.