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How passwordless authentication changes password spraying
Password spraying tests a small set of common or reused passwords against many accounts, often to avoid triggering lockouts from repeated attempts against one account. If an attacker obtains or guesses a password, MFA can block access when the attacker lacks the additional factor. Passwordless authentication goes further by eliminating the password as a credential to spray. CISA describes passwordless systems as eliminating passwords altogether as an attack vector in its Identity and Access Management: Recommended Best Practices for Administrators (December 2023).
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That does not make every passwordless method equally resistant to phishing. If a sign-in method lets a user enter a code or approve a prompt, an attacker may still trick the user into providing or approving it. FIDO/WebAuthn methods are the leading broadly available option identified by CISA for phishing resistance: authentication is bound to the legitimate service rather than a look-alike site. CISA’s Implementing Phishing-Resistant MFA explains the approach and its role in MFA.
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Compare the alternatives
| Method | Password available to spray? | Phishing and replay resistance | Compatibility and user friction | Recovery considerations |
|---|---|---|---|---|
| FIDO2/WebAuthn passkey or security key | No, if the account is configured for passwordless sign-in and password login is disabled or otherwise unavailable. | Phishing-resistant; CISA’s FIDO2 guidance describes resistance to phishing, password stuffing, replay, session hijacking and man-in-the-middle attacks. | The service and user device must support the chosen authenticator. A security key is a physical option; check service and device compatibility before deployment. | Register backup authenticators and secure lost-device replacement. Recovery must not provide a weaker bypass. |
| Passwordless MFA using a cryptographic key, device PIN or local biometric unlock | No, when the service uses the passwordless method instead of a password. | Depends on implementation. A biometric or PIN may unlock a cryptographic key locally; the local unlock is not by itself proof that the service uses phishing-resistant authentication. | May be convenient on supported devices, but device and service support vary. | Protect the underlying account and provide secure replacement if the device or key is lost. |
| Authenticator app with number matching | Usually yes, if a password remains part of sign-in. | Stronger than a basic push approval, but not equivalent to FIDO/WebAuthn phishing resistance. | Requires an authenticator app and user interaction. CISA presents it as an improved interim option. | Plan for a lost or replaced phone and secure account recovery. |
| Authenticator app one-time codes | Usually yes, if a password remains part of sign-in. | Adds a factor, but a real-time phishing proxy may capture and relay a code. | Requires access to the authenticator and entry of a time-based code. | Users need a secure way to restore access if the authenticator is lost. |
| Push approvals | Usually yes, if a password remains part of sign-in. | Conventional approval prompts are not inherently phishing-resistant and can be abused through repeated unwanted requests. Number matching improves this fallback but does not make it FIDO authentication. | Convenient, but users must avoid approving prompts they did not initiate. | Securely replace the registered device and prevent recovery from bypassing stronger controls. |
| SMS or email codes | Usually yes, if a password remains part of sign-in. | Weaker than phishing-resistant methods; CISA ranks text or email codes as the weakest methods in its small-business guidance. | May be available when stronger options are not, but depends on access to the phone number or email account. | Protect the phone number and email account, which can themselves be recovery channels. |
For organizational deployments, CISA’s Require Multifactor Authentication identifies stronger MFA choices and presents number matching as an interim improvement. CISA’s #StopRansomware Guide also recommends passwordless MFA using two or more verification factors, such as a fingerprint, face recognition, device PIN or cryptographic key. The exact security and privacy properties depend on how a service and device implement those factors.
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Choose and roll out a safer sign-in method
1. Prioritize high-impact accounts
Start with email, remote access such as VPNs, administrator accounts and accounts that control critical systems. These can provide an attacker with a route to reset other accounts, reach internal services or change security settings. CISA’s phishing-resistant MFA guidance emphasizes protecting important services, while its ransomware guidance highlights email, VPNs and critical-system accounts.
2. Prefer FIDO2/WebAuthn where available
Check whether each service supports passkeys or security keys and whether administrators can require them. A security key is one way to use FIDO/WebAuthn, not a stand-alone fix: the service must support the protocol and enforce it. Confirm compatibility across the service, user devices and any sign-in workflows before rollout. No particular key model or service compatibility is established here.
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3. Establish identity before enrollment
Decide how the organization verifies that a person is the legitimate account owner before associating an authenticator with their account. Weak enrollment can let an attacker register their own authenticator and defeat the protection before it is in place. CISA’s Hybrid Identity Solutions Guidance addresses authenticator enrollment, lifecycle management and recovery.
4. Register backups and secure recovery
Encourage users to register more than one authenticator where the service allows it. Set a clear process for reporting lost, stolen or damaged authenticators, deactivating them and issuing replacements. Verify identity with safeguards comparable to initial credential issuance: an attacker may target account recovery to get around strong MFA. Avoid making an SMS code, email link or help-desk interaction an easy substitute for the stronger sign-in control.
5. Use the strongest available fallback
If FIDO/WebAuthn is not supported, require the strongest MFA the service offers. Number matching is a better interim choice than a simple approve-or-deny push prompt, but it remains a fallback rather than phishing-resistant FIDO authentication. Treat authenticator codes, ordinary push approvals, SMS and email codes as weaker choices, and use SMS or email only when stronger methods are unavailable.
6. Verify enforcement, not just enrollment
Confirm that users cannot silently fall back to a password or weaker MFA method where policy is intended to require phishing-resistant sign-in. Review exceptions, recovery paths and legacy sign-in routes as part of the rollout. A registered key or passkey provides limited protection if an attacker can simply choose another sign-in path.
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What passwordless authentication does not guarantee
- It does not secure a weak recovery process. Recovery and replacement procedures need protection comparable to normal enrollment.
- It does not make every passwordless prompt phishing-resistant. Codes and push approvals can still be captured, relayed or abused; implementation matters.
- It does not protect accounts where passwords remain usable by default. The service must support and enforce the intended sign-in method.
- It does not make a specific security key universally compatible. Check support for the service and devices in use.
CISA’s guidance supports prioritizing FIDO/WebAuthn and stronger MFA, but it does not establish a percentage reduction in password-spraying attacks attributable to passwordless authentication. The practical result depends on which accounts are covered, whether the stronger method is enforced and how enrollment and recovery are secured.
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