Introduction
Legacy authentication remains one of the most common security gaps in small and mid-sized organizations.
Even strong passwords cannot prevent attackers from using legacy authentication to bypass modern security controls like Multi-Factor Authentication (MFA). This makes it a popular entry point for cybercriminals.
In this blog, we explain what legacy authentication is, why it is risky, and how businesses can turn it off safely.
What Is Legacy Authentication?
Legacy authentication means using older sign-in methods that do not support modern security features, such as:
- Basic authentication (username + password only)
- POP, IMAP, SMTP AUTH
- Older Office apps
- Outdated third-party email clients
Since these methods do not support MFA, attackers can guess passwords without setting off advanced protections.
Why Attackers Love Legacy Authentication
Legacy authentication:
- ❌ Bypasses MFA
- ❌ Is easy to brute force
- ❌ Often goes unnoticed in logs
- ❌ Is widely enabled by default
Security reports show that most cloud account breaches involve legacy authentication.
Real-World Attack Scenario
- An attacker obtains a leaked password
- They attempt thousands of login attempts using SMTP or IMAP
- MFA is never triggered
- Account access is granted
- Email data is exfiltrated or used for phishing
All of this can happen without raising any immediate alarms.
How to Check If Legacy Authentication Is in Use
In Microsoft Entra ID, administrators can:
- Review sign-in logs
- Filter by legacy authentication clients
- Identify users and applications still relying on it
It is important to have this visibility before making any changes.
How to Disable Legacy Authentication (Safely)
Step 1: Identify Dependencies
Email scanners
Old mobile devices
Line-of-business applications
Step 2: Enable Modern Authentication
Ensure users are using:
Outlook (modern versions)
Web-based access
OAuth-based apps
Step 3: Create a Conditional Access Policy
Block legacy authentication while allowing modern sign-ins.
Step 4: Roll Out in Phases
Start with:
- Test users
- IT admins
- After that, roll out the changes to everyone in your organization tenant-wide.
Common Business Concerns (Answered)
“Will email stop working?”
Only if outdated clients are still in use.
“Is MFA enough without disabling legacy auth?”
No. MFA can be bypassed if legacy auth is enabled.
“Is this required for cyber insurance?”
Increasingly, yes.
Final Thoughts
Turning off legacy authentication is one of the quickest and most effective ways to improve security for any organization.
It is free, reduces your risk right away, and makes every account in your environment stronger.
If your organization has not done this yet, it is only a matter of time before someone takes advantage of the gap.
Jaspreet Singh Author @ ITBlogs.ca Identity & Cloud Security (Hands-on, not theoretical)