# yaml-language-server: $schema=../../../../schema/control.schema.json
id: CB-M365-COLLAB-024
title: Prevent anonymous users and dial-in callers from starting Teams meetings
finding:
  title: Anonymous users and dial-in callers can start Teams meetings without an organizer
  description: Anyone holding a meeting link or dial-in number can use the organization's meeting at any time, with nobody from the organization present.
status: active
profiles: [baseline]
risk:
  severity: medium
assessment: manual
tags: [teams, meetings, anonymous-access, lobby]

applicability:
  requires:
    - Microsoft Teams

description: |
  Anonymous participants and people dialing in by phone wait in the lobby until an authenticated
  participant (someone from the organization, a guest or a trusted organization user) starts the
  meeting (`AllowAnonymousUsersToStartMeeting` = `False` in every meeting policy).
rationale: |
  Leaderless meetings let unverified people meet on the organization's bridge at unscheduled times,
  reuse old meeting links for their own purposes, and run up audio conferencing costs. Keeping them in
  the lobby until a verified participant arrives keeps control of the meeting with its organizer.
impact: |
  Recurring meetings where external dial-in participants used to start without the organizer must wait
  for an internal participant. None known otherwise; Off is the default.

setting:
  location: Teams admin center > Meetings > Meeting policies > Meeting join & lobby > Anonymous users and dial-in callers can start a meeting
  default_value: "Off"
  recommended_value: Off (AllowAnonymousUsersToStartMeeting = False)

audit:
  manual: |
    1. Sign in to the Microsoft Teams admin center (https://admin.teams.microsoft.com) as at least a
       Global Reader.
    2. Browse to **Meetings** > **Meeting policies** and open the **Global (Org-wide default)** policy,
       then every custom policy assigned to users.
    3. In the **Meeting join & lobby** section, check that **Anonymous users and dial-in callers can
       start a meeting** is **Off**.
    4. With Microsoft Teams PowerShell (Global Reader is enough):
       `Get-CsTeamsMeetingPolicy | Select-Object Identity, AllowAnonymousUsersToStartMeeting`.
    5. Fail if any policy returns `True`.

remediation:
  manual: |
    1. In the Teams admin center, browse to **Meetings** > **Meeting policies** and open the **Global
       (Org-wide default)** policy.
    2. In the **Meeting join & lobby** section, set **Anonymous users and dial-in callers can start a
       meeting** to **Off** and **Save**. Role: Teams Administrator.
    3. Repeat for each custom policy, unless a documented exception applies to its users.
    4. PowerShell equivalent:
       `Set-CsTeamsMeetingPolicy -Identity Global -AllowAnonymousUsersToStartMeeting $false` (repeat
       with `-Identity <policy>` for custom policies). Changes can take up to 24 hours to apply.

references:
  - title: Manage lobby options in Microsoft Teams
    url: https://learn.microsoft.com/microsoftteams/who-can-bypass-meeting-lobby
    publisher: Microsoft
    kind: vendor-doc
    accessed: 2026-10-24
  - title: Set-CsTeamsMeetingPolicy
    url: https://learn.microsoft.com/powershell/module/microsoftteams/set-csteamsmeetingpolicy
    publisher: Microsoft
    kind: vendor-doc
    accessed: 2026-10-24
  - title: CISA SCuBA Microsoft Teams baseline
    url: https://github.com/cisagov/ScubaGear/blob/main/PowerShell/ScubaGear/baselines/teams.md
    publisher: CISA
    kind: standard
    accessed: 2026-10-24
mappings:
  - framework: cis-m365
    framework_version: "7.0.0"
    id: "8.5.2"
    relation: equivalent
  - framework: cisa-scuba-m365
    framework_version: "2.0.0"
    id: MS.TEAMS.1.2v2
    relation: equivalent
  - framework: nist-800-53r5
    framework_version: "5.1.1"
    id: AC-3
    relation: related
  - framework: nist-800-53r5
    framework_version: "5.1.1"
    id: AC-14
    relation: related
related_controls:
  - id: CB-M365-COLLAB-023
    type: related
  - id: CB-M365-COLLAB-025
    type: related

authors:
  - name: Alice
    github: Sn0wAlice
introduced_in: 0.1.0
created: 2026-10-24
updated: 2026-10-24