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If your business relies on Teams for conversations, file sharing, and getting things done, you need to know about this Microsoft Teams bug recently discovered by cybersecurity researchers. A new feature designed to make collaboration easier has accidentally opened the door to attackers. If you don’t act now, this cross-tenant vulnerability could put your company data at risk.
A “Convenience” Feature That Creates a Security Gap
Microsoft recently rolled out a feature that lets any Teams user start a new chat with anyone simply by entering their email address. Even if the other person doesn’t use Teams, they’ll get an invite in their inbox and can join the chat as a guest.
On paper, this sounds helpful. In practice, researchers at cybersecurity firm Ontinue say it introduced a fundamental architectural gap: a security flaw that allows cybercriminals to slip past standard protections. This feature is enabled by default on many SMB licenses (including Teams Essentials, Business Basic, and Business Standard), exposing millions of users.
What’s the risk? A hacker (or someone pretending to be a client or vendor) can use this Microsoft Teams exploit to message your employees from outside your organization, drop malicious files, or send phishing links without Microsoft’s usual safety nets catching them.
Why Normal Security Tools Are Blind to This Microsoft Teams Exploit
Many companies rely on Microsoft Defender, email filters, or third-party tools to block bad stuff, which work fine most of the time.
But when the attack comes from another legitimate Teams tenant, it looks “trusted.” It won’t throw up red flags or block attachments. Your people just get a friendly-looking message that can install ransomware or steal credentials the moment they click on it.
Because the chat invite comes through email and provides guest access, it bypasses many of the security controls that organizations rely on, allowing attackers to come in through the side door.
The Damage From This Bug Isn’t Hypothetical
This Microsoft Teams bug creates a substantial data breach risk. Because conversations take place in Teams, employees feel safe. That trust makes it much easier for threat actors to trick users into clicking a link or downloading a malicious file.
You can reduce this cybersecurity threat with a few simple steps:
- Review your Teams settings: Check whether external chat and guest access options are enabled. Disable them if your team doesn’t need the feature.
- Create a policy for contacting outside users: Make sure your employees know how to verify unexpected messages or invitations.
- Tighten MFA and identity controls: Strong two-factor authentication helps block attackers that slip through.
- Provide ongoing user awareness training: Your employees are your first (and often best) defense. Remind them not to click unexpected links or open files, even if they show up in Teams.
Don’t Ignore This Microsoft Teams Bug
The new Microsoft Teams bug isn’t some obscure zero-day exploit. It’s a fundamental architectural gap that any scammer with a Teams account can exploit.
As collaboration tools continue to evolve, stay alert and review your default settings regularly. A few small adjustments today can prevent a major headache tomorrow.
