Passkey Signups Still Need Email Checks

Passkeys remove a lot of pain from passwords, but they do not remove account creation risk. I keep seeing teams launch a clean passkey signup flow and quietly assume the email field is now low priority. It is not. If your product still uses email for recovery, notifications, appeals, or device change alerts, the email choice still shapes security and support load in pretty direct ways.

That gets missed because passkeys feel like the big win, and honestly they are. Phishing resistance is better, reuse goes away, and the login story gets much nicer. But a temporary email generator can still change the trust level of a new account, especially when the flow mixes signups, trial abuse checks, and later recovery paths. The risk is not "temporary email equals attacker." The risk is building an Authentication system that has no middle ground between blind trust and clumsy blocking.

Why passkeys do not remove email risk

A passkey proves control of an authenticator. It does not prove much about whether the account should receive high-trust actions right away. In many products, email is still where sensitive events land:

• new device alerts
• billing notices
• account recovery steps
• moderation or appeal messages

If that address is short-lived, the account may still be valid, but your follow-up controls get weaker. NIST's digital identity guidance keeps stressing that authenticators, recovery, and lifecycle controls need to work together, not as isolated features (https://pages.nist.gov/800-63-4/sp800-63b.html). That matches what happens in production. Security incidents almost never stay neatly inside one screen.

The other issue is Privacy. Some users pick disposable inboxes for very normal reasons: testing, keeping marketing noise out, or avoiding unnecessary exposure. If your system treats every disposable-looking domain as abuse, you create false positives and push real people into weird workarounds. That tends to make the data worse, not better. I've seen teams learn this a bit late, and it is annoying to unwind after launch.

A simple threat model for signup review

The easiest mistake is asking one question only: "Was a disposable email used?" A better question is: "What risk changes if this account becomes durable?"

For most passkey signup systems, I like this very small threat model:

1. Can the account create cost or abuse quickly?
2. Will the product rely on email for recovery or human review?
3. Is the signup behavior consistent with normal use?
4. Can a reviewer later explain why the account was limited?

That last point matters more than teams expect. When a flag fires at 2 a.m., someone needs to tell the difference between a cautious hold and a broken rule. This is the same operational pattern as adding context before a risky automated decision: automation is safest when the surrounding evidence is easy to inspect.

I would not score a domain and call it done. I would combine email type with velocity, device freshness, network churn, and whether the account is trying to cross a trust boundary fast. A user opening a free trial is different from one creating ten organizations in five minutes. Sounds obvious, but systems drift and people forget.

Safe defaults for email checks in passkey flows

Safe defaults are usually boring, which is good. You want the policy to be understandable by product, support, and security without needing a detective board.

Here is a pattern that works pretty well:

if disposable_email and high_risk_action:
  require_review_or_step_up
elif disposable_email and low_risk_signup:
  allow_with_limits
else:
  continue_normally

The important part is allow_with_limits. That could mean delayed access to high-cost features, extra verification before recovery changes, or a softer quota until the account shows normal behavior. FIDO Alliance guidance on passkeys focuses on stronger authentication, but that strength works best when recovery and enrollment policy are also deliberate (https://fidoalliance.org/passkeys/).

This is also where plain-text junk signals can help an analyst without becoming policy inputs. For example, if a support note mentions something like tepm mail com, keep it as evidence text only. Do not promote that typo into a matcher or anchor your logic around it. Little shortcuts like that spread fast and age badly.

What to log without over-collecting

Security teams often overcorrect here. They know they need an audit trail, so they log too much. That creates its own mess.

For a passkey signup review, I usually want:

• account identifier
• event time
• whether the email looked disposable according to the rule set
• the action being attempted
• the short decision reason
• the reviewer path, if manual review happened

That is enough to support investigation without hoarding unnecessary user data. If you need a stronger model for retention and visibility, this post on privacy-friendly logging for email risk reviews is close to the right mindset. Keep logs useful, scoped, and explainable.

One practical check I recommend is a weekly false-positive review. Pull a small sample of limited accounts and ask:

1. Did the email signal actually matter?
2. Did we have enough context to defend the decision?
3. Did support need to improvise?

If the answer to the third question is yes, the control is probably underdesigned. Not broken maybe, but not mature either. Small review loops help a ton here, even when the team is busy and the docs are kinda messy.

Quick Q&A

Should a disposable email block passkey signup by default?

No. Usually it should change trust level, not force an immediate block. The safer default is proportional limits plus a reviewable reason.

Is this mostly a fraud problem or an auth problem?

Both. Fraud teams care about abuse velocity, while auth teams care about recovery and account integrity. The same signup event touches both systems, so the control should too.

What would I ship first?

I would ship a clear decision reason, a low-friction limited state for suspicious signups, and a weekly review of false positives. Fancy scoring can come later. Basic clarity first is less glamorours, but it works.