An unknown external user joined a teams chat, they have no info and I can't remove them/block them

Question

An unknown external user joined a teams chat, they have no info and I can't remove them/block them

Cristiano Olivari 0

I wanted to create a chat with a user outside of my organization, and after a while an unknown user (which we will call John Cert) joined said chat: there is no Linkedin data, no email visible, no organization info, nothing.

Is there some way to limit people joining Temas chats? So that if a user is invited to a chat, at least is my organization deciding it and nobody else.

Also it seems i cannot block or remove the person from the chat, which is kind of strange. Is there some way for me or the admin to do it?

Thank you in advance for the support!

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Answer 1

Fazal Qureshi 5

If the unknown user joined a Teams group chat, you cannot remove or block that participant from the existing chat. If the user is a Guest, an administrator can remove them from Microsoft Entra ID or the Team. If they are an external (federated) user, the recommended solution is to create a new group chat without that user and, if needed, configure Teams External Access policies to block or restrict external users in the future. Also, check the Teams Admin Center and audit logs to identify how the user joined.

Cristiano Olivari 0 Reputation points

2026-07-15T10:58:03.09+00:00

Thank you for reaching out, I will check what I can do. Do you know if having a user like so poses a security issue for the company I'm in? Should I be worried?
Fazal Qureshi 5 Reputation points

2026-07-15T11:02:02.85+00:00

Whether this is a security risk depends on how the user joined. If they were intentionally invited or joined through your organization's allowed External Access (federation) or as a Guest, it isn't necessarily a security issue. However, if you don't recognize the participant or can't determine how they were added, it's worth investigating.

I recommend checking the Teams Admin Center, Microsoft Entra ID (for guest accounts), and Microsoft Purview Audit logs to identify the user's identity and how they joined the chat. If your organization doesn't require external chats, consider reviewing and tightening your External Access and Guest Access policies to reduce the risk of unauthorized external participation. check with your M365 Admin Team.