NON-TIME-BOUND AND REUSABLE OTPs

OTPs are often seen as a way to enhance security, as they are typically valid for a short period and can only be used once. A critical vulnerability arises when OTPs are neither time-bound nor one-time usable, allowing them to be used multiple times and exposing the application to brute-force attacks.




VULNERABILITY

Some applications may fail to implement a mechanism to enforce OTP expiry. The OTP might remain valid indefinitely, allowing attackers to attempt multiple logins or transactions using the same OTP. Developers may also neglect to ensure OTPs are marked as "used" after the first successful authentication, allowing malicious users to reuse them multiple times.

IMPACT

When OTPs are not time-bound or one-time usable, they can be reused or guessed easily by an attacker. This makes the authentication process less secure. Specifically, the following problems arise:
Navigating the Web Application Security Landscape

Account Compromise:

Attackers can also exploit this vulnerability by using intercepted or previously obtained OTPs to access users' accounts, resulting in unauthorized access, data theft and potential financial loss.

Increased Attack Surface:

The lack of time-bound limitations makes the OTP more susceptible to replay attacks, where an attacker intercepts an OTP and reuses it after the initial transmission.

Brute-Force Attacks:

When an attacker has no time limit or single-use restriction on the OTP, they can try a large number of guesses without any consequence, potentially gaining access to an account. This vulnerability is compounded if OTPs are predictable (e.g., using simple sequences or short codes).

SOLUTION

To prevent the exploitation of OTP vulnerabilities, it's critical to enforce proper security measures that align with best practices. Below is a step-by-step guide to fixing the vulnerability:

1. Set Expiry Time for OTPs:

OTPs should be time-bound, typically expiring after a short window (e.g., 5 to 10 minutes). This ensures that an OTP cannot be used after its intended window of validity has passed. Every time an OTP is validated, check whether the current time is within the permitted window.

2. Allow OTPs to Be Used Only Once:

An OTP should be one-time usable. Once it is used successfully for authentication, it should be invalidated immediately.

Track OTP Usage:

Maintain a record of OTPs and whether they have been used. Once an OTP is consumed, mark it as expired or invalid.

Invalidate OTP Post-Use:

Ensure that the backend logic prevents the same OTP from being accepted for multiple authentication attempts.

3. Add Rate Limiting and Lockout Mechanisms:

To prevent brute-force attacks, rate-limiting should be applied to the validation attempts of OTPs.

Limit Attempts:

Allow a maximum of 3-5 OTP validation attempts per user within a short time frame (e.g., 1 minute). After reaching this limit, temporarily lock the account or introduce a delay.

CAPTCHA or Two-Factor Authentication:

Integrate additional mechanisms like CAPTCHA or two-factor authentication (2FA) to further complicate brute-force attacks.

Account Locking:

If an OTP is incorrectly entered a certain number of times, temporarily lock the user account and require manual intervention or a longer cooldown before further attempts are allowed.

4. Use Strong OTP Generation Mechanisms:

Ensure that OTPs are generated using strong, cryptographically secure algorithms.

Use Random, Complex Codes:

OTPs should be sufficiently long and random (e.g., 6-8 characters using a combination of numbers and letters) to prevent guess ability.

Use HMAC or TOTP:

Implement OTP generation using standards such as HMAC (Hash-Based Message Authentication Code) or TOTP (Time-based One-Time Password), which are secure methods that ensure OTP uniqueness and strength.

Encryption:

Ensure that OTPs are transmitted over encrypted channels (e.g., HTTPS) to prevent interception.

5. Monitor for Suspicious Activity:

To detect potential attacks early, implement logging and monitoring for suspicious authentication attempts.

Track Unusual Patterns:

Look for patterns like repeated failed OTP attempts or rapid, sequential OTP requests from the same IP address.

Notify Users of Suspicious Activity:

Send alerts to users when suspicious login attempts are detected.
Author Avatar

Radhika Lad

Cyber Security Analyst

Location: Pune, India

Radhika is a web and network Pentester and ethusiast in cyber security domain. Her primary focus is on Vulnerability Assessment and Penetration testing of corporate networks, firewalls, web and cloud apps, mobile apps. Coming from finance and education background, she has a passion to get into the world of IoT and OT Cyber security. She is always on the path of learning and trying new things in the domain she likes.