Challenging users without CAPTCHAs

This guide outlines some of Castle’s recommended strategies for introducing challenges in response to risky activity. The aim is to prevent abuse while minimizing disruption for real users. Rather than relying on blunt tools like CAPTCHAs, Castle encourages contextual, risk-based methods that preserve user experience.

Why Castle Recommends Avoiding CAPTCHAs

Although widely used, CAPTCHAs are not a reliable defense and often hurt user experience more than they help.

• They’re easily bypassed by attackers using APIs, automation frameworks, or human-solving services.
• They create friction even for legitimate users

Here are a couple of pieces we wrote on the topic:

• What a Binance CAPTCHA solver tells us about today’s bot threats
• CAPTCHAs 101: what they are, how they work, and where they fall short

Instead, Castle customers use their native UX to slow down suspicious traffic based on a holistic risk assessment approach by permitting access, then monitoring and restricting actions:

• If risk signals (e.g. new device, location, spoofed device) are present, temporarily disable sensitive actions (e.g., fund transfers, credential changes) by placing the account in a warning or restricted state, using Lists.

• If user successfully completed a challenge from a device (e.g., verifies identity via email or SMS), trust that user device for a limited time period by adding them to a temporary allowlist to avoid re-challenging them for a set period. Once the allowlist entry expires, they will be challenged again if needed.

  Check out these tutorials for some examples:

  • Challenge logins from a new country or device
  • Trust a user’s device for 14 days

This pattern helps distinguish between legitimate but unusual behavior (e.g. traveling users) and automation or account takeover attempts.

If a user is denied based on the above, there are some general good practices to follow on the UX:

• Show generic error messages (if blocking user from a certain action) that don’t reference risk, security decisions, or Castle.
• Maintain consistency with other error states in your app to avoid revealing Castle involvement.