The latest wave of phishing emails targeting inboxes is exploiting Microsoft’s trusted infrastructure — and they’re bypassing conventional email defences. These attacks abuse Microsoft’s Entra B2B Collaboration System to send guest invitations that appear completely legitimate, with the added urgency of a PayPal refund notice to prompt recipients into action.
While the emails are delivered from trusted Microsoft domains and IPs, the intent behind them is anything but.
What's happening?
Cybercriminals are spinning up rogue Microsoft 365 tenants and inviting unsuspecting users to join them as guest collaborators. The emails include templated text claiming to confirm a Microsoft 365 subscription renewal — with a high-value transaction (e.g., $699.95 AUD) and a phone number to call for a refund.
These invitations originate from invites(at)microsoft(dot)com and appear under the name of an existing organisation — such as “Základní škola” or “Digite” — giving them a false sense of authenticity.
Figure 1: A Microsoft invitation impersonating PayPal and urging the user to call for a refund
Why is this scam dangerous?
Once a recipient engages with the email, the threat escalates in two ways:
1.) Vishing AttackThe real call to action is not a link — it’s a phone number. Victims who call are met with a fake PayPal support line, where attackers attempt to harvest:
- Login credentials
- Multi-Factor Authentication (MFA) codes, and
- Personal or financial information
This is voice phishing (vishing) – a sophisticated form of social engineering that exploits urgency and trust.
2.) Tenant Compromise & PersistenceIf the recipient accepts the Microsoft guest invitation, they become part of the attacker’s rogue tenant. From there, attackers can:
- Auto-register malicious applications
- Harvest OAuth tokens, and
- Access or exfiltrate data via legitimate collaboration paths
This turns what looks like a harmless invitation into a stepping stone for deeper compromise.
Figure 2: A second variation of the scam using the same tactic but different tenant and callback number
Bypassing traditional email defences
These are especially troublesome and difficult for traditional email security solutions to detect because they:
- Are sent from Microsoft’s own invites(at)microsoft(dot)com address
- Pass SPF, DKIM and DMARC
- Are hosted on Microsoft infrastructure, and
- Include links to legitimate Microsoft services (like myapplications(dot)microsoft(dot)com)
This makes them extremely difficult to detect using conventional filtering techniques — especially those relying solely on sender reputation or domain analysis.
Stay Safe - Know the Signs
MailGuard advises all recipients of these emails to delete them immediately without clicking on any links. Responding or providing personal details can lead to identity theft, data breaches, and financial losses.
Avoid emails that:
- Aren’t addressed to you personally.
- Are unexpected and urge immediate action.
- Contain poor grammar or miss crucial identifying details.
- Direct you to a suspicious URL that isn’t associated with the genuine company.
Many businesses turn to MailGuard after a near miss or incident. Don't wait until it's too late.
Reach out to our team for a confidential discussion by emailing expert@mailguard.com.au or calling 1300 30 44 30.
One Email Is All That It Takes
All that it takes to devastate your business is a cleverly worded email message that can steal sensitive user credentials or disrupt your business operations. If scammers can trick one person in your company into clicking on a malicious link or attachment, they can gain access to your data or inflict damage on your business.
For a few dollars per staff member per month, you can protect your business with MailGuard's specialist, 'zero zero-day' email security. Special Ops for when speed matters! Our real-time 'zero zero-day', email threat detection amplifies our client’s intelligence, knowledge, security and defence. Talk to a solution consultant at MailGuard today about securing your company's inboxes.
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