{
  "id": "CB-M365-COLLAB-030",
  "standard": "m365",
  "version": "main",
  "url": "https://baselines.cirsec.org/b/m365/CB-M365-COLLAB-030",
  "json": "https://baselines.cirsec.org/b/m365/CB-M365-COLLAB-030.json",
  "yaml": "https://baselines.cirsec.org/b/m365/CB-M365-COLLAB-030.yml",
  "profiles_effective": [
    "hardened"
  ],
  "license": "Text CC BY 4.0, code Apache-2.0",
  "attribution": "CIRSEC Baselines, https://baselines.cirsec.org",
  "control": {
    "id": "CB-M365-COLLAB-030",
    "title": "Block chat in external meetings hosted by untrusted organizations",
    "finding": {
      "title": "Users can chat in meetings hosted by organizations the tenant does not trust",
      "description": "In a meeting set up by an unknown organization, users can receive links and files in the meeting chat and post internal content to it."
    },
    "status": "active",
    "profiles": [
      "hardened"
    ],
    "risk": {
      "severity": "low"
    },
    "assessment": "manual",
    "tags": [
      "teams",
      "meetings",
      "external-access",
      "chat"
    ],
    "applicability": {
      "requires": [
        "Microsoft Teams"
      ]
    },
    "description": "Users of the organization cannot read or write chat messages in Teams meetings hosted by\norganizations that are not trusted in external access (`AllowExternalNonTrustedMeetingChat` =\n`False` in every meeting policy). They can still join those meetings with audio, video and screen\nsharing.\n",
    "rationale": "A meeting hosted by an untrusted tenant is an easy pretext for phishing: the attacker invites the\nvictim, then drops links or files in the meeting chat, outside the organization's chat protections.\nThe same chat is a channel to paste internal content to an unknown party. Blocking chat in those\nmeetings narrows both.\n",
    "impact": "Users attending meetings of customers or suppliers that are not in the trusted list cannot use the\nmeeting chat. Add frequent partners to external access (CB-M365-COLLAB-018) to restore it.\n",
    "setting": {
      "location": "Teams admin center > Meetings > Meeting policies > Meeting engagement > External meeting chat",
      "default_value": "On",
      "recommended_value": "Off (AllowExternalNonTrustedMeetingChat = False)"
    },
    "audit": {
      "manual": "1. Sign in to the Microsoft Teams admin center (https://admin.teams.microsoft.com) as at least a\n   Global Reader.\n2. Browse to **Meetings** > **Meeting policies** and open the **Global (Org-wide default)** policy,\n   then every custom policy assigned to users.\n3. In the **Meeting engagement** section, check that **External meeting chat** is **Off**.\n4. With Microsoft Teams PowerShell (Global Reader is enough):\n   `Get-CsTeamsMeetingPolicy | Select-Object Identity, AllowExternalNonTrustedMeetingChat`.\n5. Fail if any policy returns `True`.\n"
    },
    "remediation": {
      "manual": "1. In the Teams admin center, browse to **Meetings** > **Meeting policies** and open the **Global\n   (Org-wide default)** policy.\n2. In the **Meeting engagement** section, set **External meeting chat** to **Off** and **Save**.\n   Role: Teams Administrator.\n3. Repeat for each custom policy, unless a documented exception applies to its users.\n4. PowerShell equivalent:\n   `Set-CsTeamsMeetingPolicy -Identity Global -AllowExternalNonTrustedMeetingChat $false` (repeat\n   with `-Identity <policy>` for custom policies). Changes can take up to 24 hours to apply.\n"
    },
    "references": [
      {
        "title": "Set-CsTeamsMeetingPolicy",
        "url": "https://learn.microsoft.com/powershell/module/microsoftteams/set-csteamsmeetingpolicy",
        "publisher": "Microsoft",
        "kind": "vendor-doc",
        "accessed": "2026-10-24"
      },
      {
        "title": "Teams settings and policies reference",
        "url": "https://learn.microsoft.com/microsoftteams/settings-policies-reference",
        "publisher": "Microsoft",
        "kind": "vendor-doc",
        "accessed": "2026-10-24"
      }
    ],
    "mappings": [
      {
        "framework": "nist-800-53r5",
        "framework_version": "5.1.1",
        "id": "SC-7",
        "relation": "related"
      },
      {
        "framework": "nist-800-53r5",
        "framework_version": "5.1.1",
        "id": "AC-4",
        "relation": "related"
      }
    ],
    "threats": [
      {
        "framework": "mitre-attack",
        "id": "T1566.002"
      }
    ],
    "related_controls": [
      {
        "id": "CB-M365-COLLAB-018",
        "type": "related"
      },
      {
        "id": "CB-M365-COLLAB-027",
        "type": "related"
      }
    ],
    "authors": [
      {
        "name": "Alice",
        "github": "Sn0wAlice"
      }
    ],
    "introduced_in": "0.1.0",
    "created": "2026-10-24",
    "updated": "2026-10-24"
  }
}