Thieves Enter Through the Guest Door in Microsoft Teams
- Javier Conejo del Cerro
- hace 5 días
- 3 Min. de lectura
Modern collaboration tools are built to make teamwork easier — but some doors open both ways. A newly surfaced blind spot in Microsoft Teams allows attackers to impersonate invited guests, bypassing Microsoft Defender for Office 365 protections and walking straight into corporate environments. Because the invitation email comes from Microsoft infrastructure itself, it appears trustworthy — yet once inside, the attackers operate in a tenant with no security controls, enabling phishing, malware distribution, and data theft outside the victim organization’s visibility.
Phase 1: The Guest Invitation — A Knock at the Door
Attackers set up malicious Microsoft 365 tenants using low-cost licenses that do not include Microsoft Defender for Office 365 protections by default. They disable all built-in safeguards — effectively designing a home with no alarm system. Then they identify users in target organizations and request guest access to chat with them in Teams by simply entering their email addresses.
Since the invitation originates from Microsoft servers and is delivered through legitimate infrastructure, it bypasses standard email authentication (SPF, DKIM, DMARC) and looks legitimate to both employees and security tools. This is the first psychological breach: a trusted brand masks a hostile environment.
Phase 2: The Entry — Security Left Behind
Once a user clicks Accept, something critical happens:
Their account temporarily leaves the protection of their own tenant and enters the attacker’s ecosystem.
Inside this guest environment, the attacker dictates all rules:
No Safe Attachments scanning
No Safe Links inspection
No Defender anti-malware policies
No threat monitoring or alerting by the victim’s SOC
Every message, file, and link now flows through the attacker’s infrastructure. The attacker is free to send:
Malware-laced file uploads
Credential phishing URLs
Social engineering payloads masquerading as internal requests
Data exfiltration prompts disguised as collaboration
The user’s organization sees nothing — there is no detection, no logging, no containment.
Phase 3: The Theft — Data Walking Out the Door
This architectural blind spot directly exposes:
Corporate identities and authentication tokens
Credentials shared in chat or through malicious links
Internal documents, financial information, personally identifiable data
Sensitive communications between employees or departments
All theft occurs completely outside the organization’s perimeter, leaving defenders blind until damage is discovered too late.
The victim believes they are chatting inside their workplace — but they are standing alone, in the intruder’s domain.
Phase 4: Covering Tracks — Nothing Triggered, Nothing Blocked
Because the communication occurs outside the organization’s tenant:
No SIEM logs are captured
No EDR alerts fire
Email gateways never see malicious content
SOC teams lack any trace to investigate
The attacker has successfully created what Ontinue researchers call a “protection-free zone.” It’s collaboration turned into a covert access channel.
How to Keep Thieves Out
Organizations must treat guest access as a privileged capability — not a collaboration convenience by default.
Recommended controls:
Restrict guest access only to vetted, trusted domains
Enable Azure AD cross-tenant access controls for conditional policies
Monitor external collaboration behavior and guest activity logs
Disable or limit unsolicited guest invites for Teams
Train users to distrust unexpected collaboration requests — even those delivered by Microsoft
Guest access should not become an escape route for corporate data.
The rise of external collaboration has erased traditional perimeter boundaries — but has not removed attacker creativity. Teams guest access reveals a reality many organizations overlook:
If security does not follow the user, the user walks into danger unprotected.
Attackers are no longer breaking in — they’re being invited. Organizations must ensure that trusted communication platforms do not become unguarded doors where defenders have zero visibility and attackers stage their entire operation unnoticed.
When collaboration extends beyond your walls, your security must extend with it.
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