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.
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.
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.
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.
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).
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.
An OTP should be one-time usable. Once it is used successfully for authentication, it should be invalidated immediately.
To prevent brute-force attacks, rate-limiting should be applied to the validation attempts of OTPs.
Ensure that OTPs are generated using strong, cryptographically secure algorithms.
To detect potential attacks early, implement logging and monitoring for suspicious authentication attempts.