SMS passcodes
SMS passcodes also known as SMS OTPs (one-time passcodes), are verification codes delivered through text messages to authenticate users during sign-in or before sensitive actions.
These codes form a vital component of many authentication systems, particularly in two-factor authentication (2FA) implementations where additional security beyond passwords or email verification is required.
How SMS passcodes work
When a user attempts to sign in or perform a sensitive action, the authentication system generates a unique, time-limited code and sends it via SMS to their registered phone number. This process creates a verification layer, as users must demonstrate have possession of their mobile device to receive the code.
As per NIST 800-63B, the guidelines for digital identity management produced by the United States government, the passcode shall remain valid for 10 minutes, and contain at least 6 decimal digits (approximately 20 bits of entropy).
Once received, users input this code into the authentication system interface. The system then validates the entered code against the generated one, considering factors like expiration time and previous usage. Only after successful validation does the system grant access or authorize the requested action.
Security implications
SMS codes present distinct advantages for authentication security. Their temporary nature means that even if intercepted, they become useless. The requirement for physical device possession also creates a barrier for remote attackers who might have obtained password credentials through data breaches or phishing attempts.
However, SMS-based authentication faces several security obstacles. The SMS protocol lacks end-to-end encryption, making messages vulnerable to interception. SIM swapping attacks, where criminals convince mobile carriers to transfer a victim's phone number to a new SIM card, also pose a significant threat. These attacks can bypass SMS-based security entirely by redirecting verification messages to attacker-controlled devices.
Network reliability issues can also affect SMS authentication effectiveness. Users in areas with poor cellular coverage or those traveling internationally may experience delays or failures in receiving verification codes. This dependency on cellular infrastructure can create accessibility problems and user frustration.
Implementation best practices
Effective SMS passcode implementation requires careful attention to several key aspects. First, verifying the users phone number verification during registration prevents delivery failures in the future. Clear, concise verification messages improve user understanding and reduce confusion. Implementing rate limiting and exponential backoff for code requests prevents abuse while managing costs.
Security-conscious implementations should incorporate additional protective measures. Device fingerprinting can help identify suspicious login attempts, while IP-based restrictions and geolocation checks add extra security layers. Supporting alternative verification methods, such as authenticator apps or backup codes, ensures users maintain account access even when SMS delivery fails.
Cost management becomes particularly important at scale. Keeping messages concise reduces expenses, while implementing intelligent retry logic prevents unnecessary message sends. International implementations should consider varying SMS costs and regulations across different regions.
SMS toll-fraud attacks
SMS toll-fraud attacks, also called SMS pumping, occur when bad actors generate unauthorized charges by requesting passcodes be sent to premium-rate text messages recipients.
These attacks typically involve a malicious application or script that request a high volume SMS passcodes, often without the applications knowledge, to a premium service number, resulting in substantial charges on the application's SMS bill. The attacker then receives commission (or “kickbacks”) for generating the premium rate SMS messages.
To block these attacks, it is important to implement rate limiting and bot detection, or otherwise leverage a service like Twilio Verify or Prelude with built-in protection.
SMS passcodes and Clerk
Clerk provides strong support for SMS codes as part of its comprehensive authentication solutions. By leveraging Clerk, developers can integrate SMS-based verification into their applications without the hassle of building these systems from scratch. Clerk handles the intricacies of generating, sending, and validating codes, enabling developers to focus on core features.
Clerk’s implementation includes advanced features such as rate limiting and retries to guard against abuse. Developers also benefit from easy integration with other Clerk services like like social sign-ons and other multi-factor authentication options, creating a flexible and secure user authentication experience.
Clerk also prioritizes user experience by ensuring verification messages are straightforward and comprehensible. By providing fallback options, including backup codes, passkeys, or time-based one-time passcodes (TOTPs), Clerk ensures users always have a way to access their accounts even when SMS delivery issues arise.
Future-proofing authentication systems
While SMS passcodes remain widely used, authentication technology continues to evolve. Modern systems often combine SMS verification with other security measures like biometrics, push notifications, or hardware security keys. Simplifying authentication and user management for developers requires staying current with security best practices while maintaining compatibility with existing systems.
Applications implementing SMS Passcodes should plan for future authentication needs. This includes designing systems that can incorporate new verification methods and phase out older ones as security requirements change. Regular security audits and updates ensure authentication systems remain effective against new threats while continuing to serve user needs.