Imagine this: You’re collaborating seamlessly with external partners on Microsoft Teams, but unbeknownst to you, your organization’s security defenses are silently being stripped away. This is the alarming reality cybersecurity researchers have uncovered, revealing a critical blind spot in MS Teams’ guest access feature. But here’s where it gets controversial: while Microsoft’s new ‘Chat with Anyone’ feature promises unparalleled collaboration, it inadvertently opens the door for attackers to bypass Microsoft Defender for Office 365 protections.
Here’s how it works: When users join external tenants as guests, their security is no longer governed by their home organization’s policies. Instead, they’re at the mercy of the hosting tenant’s security measures—or lack thereof. As Ontinue security researcher Rhys Downing explains, ‘These advancements increase collaboration opportunities, but they also widen the responsibility for ensuring those external environments are trustworthy and properly secured.’ And this is the part most people miss: the ‘fundamental architectural gap’ means Microsoft Defender’s protections may not apply once a user steps into another tenant’s security boundary.
Microsoft’s recent update allows users to chat with anyone via email, even those outside the Teams ecosystem. While this simplifies external engagement, it introduces significant risks. For instance, an attacker could create a malicious Microsoft 365 tenant using a low-cost license like Teams Essentials, which lacks Microsoft Defender for Office 365 by default. By inviting users to this unprotected environment, attackers can exploit the absence of Safe Links and Safe Attachments scans to deliver phishing links or malware-laced files.
What’s truly unsettling? The invitation email appears legitimate, originating from Microsoft’s infrastructure, effortlessly bypassing SPF, DKIM, and DMARC checks. Email security solutions are unlikely to flag it, leaving victims unsuspecting. Once the invitation is accepted, all communication occurs within the attacker’s tenant, completely outside the victim’s organization’s security perimeter. ‘Their security controls never triggered because the attack occurred outside their security boundary,’ Downing notes.
To mitigate this risk, organizations should restrict B2B collaboration to trusted domains, implement cross-tenant access controls, and train users to scrutinize unsolicited Teams invites. But here’s a thought-provoking question: Is Microsoft doing enough to address this vulnerability, or are organizations left to fend for themselves in an increasingly collaborative—yet risky—digital landscape?
What’s your take? Do you think Microsoft should reevaluate its security measures for external collaboration, or is this a necessary trade-off for flexibility? Let us know in the comments below!
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