“MFA Fatigue” Protection: Moving from Push Notifications to Number Matching
/Article summary: Simple push notification MFA has a well-documented weakness: attackers who have your password can flood your phone with approval prompts until someone taps “allow” out of frustration or inattention. Number matching MFA closes this gap by requiring users to type a code shown on their login screen rather than blindly approving a prompt. The switch is quick to enable in Microsoft 365 and Google Workspace as a meaningful step toward stronger authentication.
In September 2022, a hacker with a stolen password needed one more thing to get into Uber’s corporate network: someone on the other end of an MFA push notification to tap Approve. So the attacker sent dozens of prompts. Then dozens more.
Eventually, tired of the alerts, a contractor approved one. The attacker was in.
That’s an MFA fatigue attack. And it works because push-based MFA (multi-factor authentication) asks very little of the person approving. One tap. No context. No confirmation that the login was even initiated by you.
MFA fatigue attack protection starts with understanding that flaw. Building a layered cybersecurity posture means going a step further than simply having MFA turned on.
How MFA Fatigue Attacks Work
An MFA fatigue attack only works if the attacker already has your credentials. They obtained them through phishing, a data breach, or a purchased dump from a dark web marketplace. The password is compromised. The only obstacle left is the second factor.
The attack is simple. The attacker attempts a login with the stolen credentials, which triggers a push notification on the target’s phone. They do it again. And again. The prompts arrive during a meeting, at 2 a.m., mid-commute. There’s no technical bypass involved. The attacker is betting on human exhaustion.
Microsoft tracked over 382,000 MFA fatigue attacks in a single 12-month period and found that 1% of users accept the very first prompt they receive without scrutiny.
At a 50-person company, that’s roughly one person who will approve the first request reflexively. One approval is all an attacker needs.
Why an “Approve” Button Is the Weak Link
Traditional MFA push notifications are fast and frictionless by design. A prompt appears on your phone: "Are you trying to sign in?" You tap Yes. Done.
But that same simplicity is the vulnerability. The prompt gives the user no information about which app triggered the request, where the login attempt originated, or whether they initiated a sign-in at all.
In a prompt bombing campaign, the user is being worn down until judgment fails.
Cisco was breached in 2022 in the same way. An employee received voice calls from someone impersonating IT support while push notifications flooded in, with instructions to just approve one to resolve the issue.
Stolen credentials are the prerequisite.
Sound password hygiene and an understanding of how phishing attacks deliver those credentials reduce exposure at the front end.
But number matching adds a control that holds even after credentials are gone.
What Number Matching Changes
With number matching enabled, the login screen displays a two-digit number. The user opens their authenticator app and must type that specific number to complete sign-in. No number typed correctly, no access granted.
This one change eliminates two failure modes.
First, accidental approvals become nearly impossible. There’s nothing to tap blindly.
Second, it creates a brief but meaningful check: the user must see both their login screen and their phone at the same time, making it obvious if a request wasn’t self-initiated.
In Microsoft 365 and Microsoft Authenticator
Microsoft made number matching the default for all Authenticator push notifications in May 2023.
If your organization uses Microsoft 365, your users are likely already seeing it. Admins can verify the setting is active for all users via Microsoft Entra Admin Center under Security > Authentication Methods > Microsoft Authenticator.
Microsoft also offers two additional context features that can be enabled alongside number matching:
Displaying the name of the application requesting authentication,
Showing the geographic location of the sign-in attempt.
Both help users immediately identify a suspicious prompt before they respond.
In Google Workspace
Google Workspace doesn’t use a number matching system in the same way, but administrators can require stronger authentication methods through the Admin Console under Security > Authentication > 2-Step Verification.
Google’s prompt already shows device name, location, and time of the sign-in request, giving users enough detail to recognize when something looks off.
Is Number Matching Enough?
Number matching MFA is a meaningful upgrade, not a final destination.
CISA explicitly recommends number matching as an interim mitigation for organizations that use mobile push-based MFA and cannot yet implement phishing-resistant authentication. It is described as one of the best available steps short of a full platform change.
The strongest option is phishing-resistant MFA, which uses hardware-bound cryptographic keys (FIDO2 security keys or Windows Hello for Business).
Even if an attacker tricks a user into entering their credentials on a fake login page, the hardware key won’t authenticate against a domain it doesn’t recognize.
For most small businesses, number matching is the right step to take this week. Phishing-resistant MFA is the right goal to plan toward.
What matters most right now: teams should know that an unexpected flood of MFA prompts is a sign of an active attack, not a reason to start tapping Approve.
Train your team to deny unexpected prompts immediately and report them to IT. That response, combined with number matching, closes most of the gap that push-only MFA leaves open.
Upgrade How Your Team Signs In
MFA fatigue attack protection doesn’t require new software or a major rollout. If you’re on Microsoft 365, number matching is likely already on. The question is whether it’s verified, configured correctly, and whether your team understands what to do when prompts arrive unexpectedly.
BrainStomp can help you audit your current MFA setup, enable number matching and additional context, and put a plan in place for stronger authentication over time. Reach out at brainstomp.com/contact.
Article FAQs
What is MFA fatigue?
MFA fatigue (also called push bombing or prompt bombing) is an attack technique where a hacker uses stolen credentials to trigger repeated MFA push notifications on a target’s phone until the user approves one out of frustration or distraction. It requires no technical bypass of MFA itself.
What is number matching in MFA?
Number matching is a security setting that requires the user to type a two-digit code shown on their login screen into their authenticator app before the login is approved. It prevents accidental approvals because the user must actively look at both devices and enter the correct number.
Is number matching the same as phishing-resistant MFA?
No. Number matching is a strong upgrade over simple push notifications, but CISA classifies it as an interim step.