Malicious Connectors Are Becoming Microsoft 365’s Silent Attack Vector

When most people think of Microsoft 365 attacks, they picture phishing emails or stolen passwords. But the truth is, attackers are finding quieter, more persistent ways to break in and stay in. One of the fastest-growing, least understood methods? Rogue Microsoft 365 connectors. 

These connectors, normally designed to manage mail flow in Exchange Online, can be twisted into powerful attack tools. Many admins don’t even know they exist. 

What Microsoft 365 Connectors Are 

In Microsoft 365 (specifically Exchange Online), connectors are configuration rules that control how email flows into and out of your organization’s environment. 

• Legitimate use case: An admin might set up a connector to route email through a third-party security tool, an on-premises mail server, or to allow specific trusted systems to send mail on behalf of the organization. 

• Where they live: They’re managed in the Exchange Admin Center under Mail Flow → Connectors. 

• Default state: Most organizations don’t actually need them, and by default there are none. 

Why Connectors Matter for Security 

Because connectors sit at the server level (not just on a single device or inbox like Outlook rules), they’re incredibly powerful: 

• They can rewrite sender or recipient information. 

• They can reroute mail to external accounts. 

• They can make malicious changes invisible to the end user. 

An attacker who compromises admin credentials can create or modify a connector, essentially giving themselves a backdoor into your mail flow. Even if you change your password, reset MFA, or install Outlook on a new device, the rogue connector may still be there silently doing its work. 

Why Most Admins Miss Them 

• They’re not commonly used, so many IT teams don’t think to check them. 

• Microsoft doesn’t loudly advertise that every tenant has the ability to set them up. 

• Traditional monitoring tools often don’t flag changes to connectors unless specifically configured. 

That combination makes them an ideal, “hidden in plain sight” attack vector. 

How the Attack Works 

Attackers often start the old-fashioned way: with a successful phishing attempt or stolen authentication certificate. From there, they move beyond the inbox and set up malicious rules, forms, or connectors inside Microsoft 365. 

Once a rogue connector is in place, it can: 

• Reroute legitimate emails to attacker-controlled inboxes 

• Delete sent or received mail to erase evidence 

• Redirect invoices or payment instructions to fraudulent accounts 

• Hide all traces of the compromise from both users and admins 

Victims believe they’ve “cleaned up” the threat while attackers continue siphoning off information and money. 

What the Experts Are Saying 

Security evangelist Roger Grimes recently highlighted just how serious this issue is becoming: 

“I have known about the abuse of Exchange connectors for nearly as long as I have known about abused Outlook rules and forms. But until recently, I thought connectors were only a problem to really worry about if you had on-premises Microsoft Exchange servers. It turns out that it applies to every Microsoft 365 account as well. ” — Roger Grimes, KnowBe4 

When Microsoft support engineers immediately ask victims to “check your connectors,” it tells you just how common this attack has become. 

Why This Matters for Your Organization 

Microsoft 365 is used by more than 300 million people worldwide, from small businesses to Fortune 500s. That makes it a prime target, and attackers know the average admin is too busy to monitor obscure configuration risks constantly. 

The complexity of Microsoft 365 means new features, updates, and back-end tools are always being added. That’s good for productivity, but it also means more opportunities for attackers to exploit what you don’t know exists. 

The Solution 

iuvo Guardian is built to give IT leaders proactive visibility into exactly these kinds of hidden risks: 

• Automated Oversight: Continuously scans for risky rules, connectors, and configurations. 

• Proactive Defense: Flags and helps remediate suspicious changes before attackers can exploit them. 

• Near-Perfect Microsoft Identity Secure Score: Keeps pace with Microsoft’s updates so your environment doesn’t fall behind. 

• Expert Support: Backed by a team that lives and breathes Microsoft 365 security. 

Instead of wondering if something like a rogue connector is hiding in your tenant, iuvo Guardian ensures you know, and that it’s addressed before it becomes a costly breach. 

Don’t Let Hidden Threats Stay Hidden 

The rise of malicious connectors is just one example of how quickly Microsoft 365 threats are evolving. Yesterday’s best practices aren’t enough to protect against today’s attacks. 

With iuvo Guardian, you don’t just react,  you stay ahead. 

Ready to see how iuvo Guardian protects your Microsoft 365 environment?  Book a demo today.

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