Collaboration tools like Microsoft Teams have become indispensable in today’s hybrid work environments. But a newly highlighted security issue shows that convenience may come with hidden risks that traditional defenses simply cannot address.
According to a recent article in The Hacker News, security researchers have uncovered a cross-tenant blind spot in Microsoft Teams that can effectively remove Microsoft Defender for Office 365 protections when a user accepts a guest invite and joins an external tenant. The Hacker News
Microsoft recently introduced a feature that allows Teams users to chat with anyone using email, even if the recipient isn’t on Teams yet. This feature is designed to simplify external collaboration and is expected to be fully available globally by January 2026.
Here’s the problem: when a user accepts an invitation to join another organization’s Teams tenant as a guest, the security context governing that session switches from the user’s home organization to the hosting tenant. In plain terms:
The protections your organization pays for––like Safe Links, Safe Attachments, and other Microsoft Defender safeguards––do not follow the user into the external tenant.
Instead, the security posture of the hosting tenant applies. If that tenant has weak or no security policies, or if it is attacker-controlled, your user may be exposed to phishing links, malware, and other threats without any scanning or alerts.
What makes this attack chain even more insidious is that the initial email invitation comes from Microsoft infrastructure, allowing it to bypass common email security checks like SPF, DKIM, and DMARC.
Security researchers highlight that your organization’s security controls never trigger in these scenarios because the entire interaction takes place outside your tenant’s boundary.
Many enterprises rely on a “Detect and Respond” strategy. Modern security stacks focus on threat detection, endpoint analytics, and incident response workflows. While these tools are valuable, this Teams guest access issue exposes a critical limitation:
Detection only triggers after a threat has entered your environment, if it’s detected at all.
Response workflows assume your security controls govern the session where the risky behavior occurs.
When your users are operating inside another tenant, your security stack has no visibility, no scanning, and no control.
This means attackers can exploit legitimate collaboration functionality to evade detection and deliver malicious content right into your users’ Teams chats.
While tighter configuration controls and conditional access policies help reduce this risk, they do not eliminate the fundamental architectural gap. Teams administrators can:
Educate users not to accept unsolicited external invites.
But even with strong policies, the root issue remains: once a user is communicating in another tenant, your organization’s defenses are dormant.
This kind of blind spot underscores a broader shift in cybersecurity strategy. Rather than relying solely on detect and respond mechanisms, businesses need solutions that prevent threats from executing in the first place.
That’s where AppGuard comes in.
AppGuard is a proven endpoint protection solution with a 10-year track record of stopping advanced attacks by isolating and containing threats before they can execute. Instead of waiting for malware to be detected and then trying to respond, AppGuard:
Prevents unauthorized actions by enforcing zero-trust controls at the endpoint
Contains suspicious behavior before it impacts your environment
Stops attacks that bypass traditional defenses
Because AppGuard doesn’t rely on signatures or pattern-based detection, it can stop threats that slip past conventional tools, even those delivered through collaboration platforms like Teams.
The Teams guest access issue is a reminder that security perimeters can extend beyond traditional boundaries, and attackers will exploit every gap they can find.
The industry standard approach of “Detect and Respond” simply isn’t enough anymore. Organizations must embrace Isolation and Containment strategies that proactively neutralize threats before they execute.
Call to Action for Business Owners
If your organization relies on Microsoft Teams and Microsoft 365 for collaboration, now is the time to reconsider your endpoint protection strategy.
Talk with us at CHIPS about how AppGuard can prevent incidents like this before they happen. Let’s help you shift from a reactive detect and respond mindset to a proactive isolation and containment approach that keeps your business safe in an evolving threat landscape.
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