What Is OTP Authentication? How It Works, Risks & Alternatives (2026)

One-Time Passcodes (OTPs) have been a cornerstone of online security for over two decades. If you’ve ever received a six-digit code by text message to log into your bank, you’ve used one. But as cyberattacks grow more sophisticated, the question organizations and security teams are asking has changed from “should we use OTPs?” to “are OTPs still safe enough?”

This article explains what OTP authentication is, how the different types work, the specific security risks each carries, and why biometric face verification is increasingly replacing OTPs as the authentication method of choice for high-assurance use cases.

What is OTP? Meaning and Definition

An OTP is a computer-generated code that is valid for a single authentication session and expires after a short window – typically 30 to 120 seconds, or immediately after use.

Unlike a static password, which remains the same until a user changes it, an OTP is unique each time. This eliminates the risk of credential replay attacks, where an attacker reuses an intercepted password. OTPs are delivered via SMS text message, email, an authenticator app, or a hardware token device.

OTP authentication is commonly used as a component of multi-factor authentication (MFA), satisfying the possession factor – the ‘what you have’ layer of assurance. The assumption is that only the legitimate user has access to the device or inbox receiving the code.

How Does OTP Work?

The basic OTP process follows three steps:

1. The user attempts to log in or complete a transaction.
2. The system generates a unique, time-limited code and delivers it to the user’s registered device or email address.
3. The user enters the code to authenticate. The session is granted if the code matches and has not expired.

OTPs are generated using one of two underlying algorithms:

• HOTP (HMAC-Based One-Time Password): the code is generated based on a counter that increments with each authentication event. The code remains valid until it is used, rather than expiring by time.
• TOTP (Time-Based One-Time Password): the code is generated based on the current time and expires after a fixed interval, typically 30 seconds. Authenticator apps like Google Authenticator use TOTP.

In the case of SMS OTP, neither algorithm is used — the code is simply generated server-side and transmitted over the mobile network, with no cryptographic binding to the user’s device.

The Main Types of OTP Authentication

• SMS OTP:  a one-time code sent to your mobile number via text message. The most widely used format, but the most vulnerable.
• Email OTP: a code delivered to your registered email inbox. Convenient, but dependent on the security of the email account.
• TOTP (Time-Based, App-Generated): a rolling code generated by an authenticator app (Google Authenticator, Microsoft Authenticator, Authy). More secure than SMS because it is not transmitted over the phone network. Typically valid for 30 seconds.
• HOTP (Counter-Based):  a code that increments with each use rather than expiring by time. Used in some hardware token implementations.
• Hardware tokens: a dedicated physical device (not a smartphone) that displays a one-time code. Common in enterprise and financial services environments.

OTP Authentication: What’s The Issue?

Mela Abesamis had always considered herself careful when it came to cybersecurity. She was well aware of phishing attacks and other common fraud tactics, and how to spot them. She was also used to receiving One-Time Passcodes (OTPs) from her bank, sent by SMS to her mobile device to authenticate her identity.

But then in December 2021, Mela received a message from her bank, saying that 50,025 Philippine pesos (around $950) had been moved from her account to a Mark Nagoyo. This was the first and only indication she received that money had been transferred. She had never heard of Mark Nagoyo, she certainly hadn’t made the payment, and she had not received a one-time passcode during the transaction.

She wasn’t alone. Over 700 account holders were affected by the fraud. In Filipino, the word “nagoyo” means to make a fool out of someone.

Five individuals were eventually indicted for the scam. The fraudsters used phishing techniques to harvest bank login credentials. They were then able to bypass the bank’s OTP security controls entirely – victims reported that transactions were processed without any OTP being triggered – and drain people’s bank accounts.

The bank repaid the affected amounts, but this didn’t fully offset the traumatic experience for the victims.

This is just one example of how compromising OTP authentication is becoming less and less of a challenge for increasingly sophisticated attackers. This article examines exactly how those attacks work — and what more secure alternatives look like.

Are OTPs Safe?

OTPs are more secure than static passwords alone. Because each code is unique and expires quickly, intercepting one does not give an attacker reusable credentials.

However, OTPs have a fundamental structural limitation: they only satisfy the possession factor of authentication – “what you have.” Whatever device or inbox receives the code can be compromised. A stolen phone, a hijacked phone number, or a phished code all defeat OTP authentication entirely, without the attacker needing to break any cryptography.

The consensus among security researchers and regulators has shifted. NIST’s Digital Identity Guidelines (SP 800-63B) now explicitly restrict the use of SMS OTP for high-assurance authentication due to its susceptibility to interception. The UK’s National Cyber Security Centre similarly advises organizations to move away from SMS-based second factors where alternatives are available.

So: OTPs are better than passwords alone, but they are not safe enough for high-assurance use cases — and the attack techniques targeting them are becoming easier to execute at scale.

Can OTP Be Hacked?

Yes – and there are several well-documented methods attackers use to intercept or bypass OTP authentication without needing physical access to the target’s device.

SIM Swap Attacks

The most prevalent SMS OTP attack. The fraudster harvests personal details about the target — through phishing, data breaches, or social engineering — and uses them to convince the victim’s mobile carrier to transfer the phone number to a SIM card under the attacker’s control. All subsequent SMS messages, including OTPs, are then delivered to the attacker.

SIM swap fraud is accelerating sharply. UK fraud watchdog Cifas recorded a 1,055% surge in unauthorized SIM swaps in 2024, rising from 289 cases in 2023 to nearly 3,000. In the US, the FBI’s Internet Crime Complaint Center recorded $26 million in reported SIM swap losses in 2024, and a single case resulted in a $33 million arbitration award against T-Mobile after a swap enabled a cryptocurrency theft.

The SS7 Protocol Flaw

SS7 (Signaling System No. 7) is the decades-old protocol that primarily underpins 2G and 3G mobile networks, including SMS routing. It contains a design flaw that allows attackers with access to the SS7 network, typically sophisticated criminal groups or state actors, to intercept calls and text messages in transit, including OTPs.

In Germany, fraudsters exploited the SS7 protocol via a foreign network operator to intercept SMS OTPs sent to O2 Telefonica customers at scale, enabling them to drain bank accounts belonging to an undisclosed number of customers.

Real-Time Phishing and OTP Interception

A growing and particularly dangerous attack class. Adversary-in-the-middle toolkits — including widely available frameworks like Evilginx — allow attackers to create convincing proxy versions of legitimate login pages. When a target enters their credentials and OTP, the attacker’s server captures both in real time and replays them to the genuine site before the OTP expires. The attacker gains full session access despite the victim having correctly completed MFA.

This attack requires no SIM access, no SS7 exploit, and no malware – just a convincing phishing page and an automated relay. It defeats SMS OTP, email OTP, and TOTP equally.

TOTP-Specific Risks

App-based TOTP is more resistant to interception than SMS, but still has exploitable weaknesses:

• Device theft or compromise. If an attacker gains access to a device containing an authenticator app — and that same device is used to complete the transaction — the “two factors” collapse into one. The MFA classification becomes questionable.
• Backup code exposure. Most authenticator apps generate recovery codes during setup. If those codes are stored insecurely (in a notes app, email, or cloud backup), they become a single point of failure.
• Social engineering. Attackers increasingly call targets in real time, impersonating bank fraud teams, and pressure them into reading out their TOTP code during an “account verification” call.

What Are The Security Risks Of OTP Authentication?

The fundamental issue with OTP authentication is that it only meets the possession factor of authentication. What you have — be it your cellphone or hardware token — can be lost, stolen, compromised, or impersonated at the carrier level. Below is a summary of the main risk vectors by OTP type.

SMS OTP Security Risks

As shown in the Philippines case, attackers don’t need to steal your phone to defeat SMS OTP. Text messages are unencrypted and tied to a phone number rather than a specific device. The main attack vectors are SIM swapping, SS7 exploitation, and real-time phishing – all covered in detail above.

TOTP Security Risks

TOTP and app-based authentication offer more security than SMS, as the code is generated locally and not transmitted over a carrier network. But as noted, device compromise, real-time phishing, and social engineering all remain viable attack paths. TOTP also still only satisfies the possession factor – raising questions about whether it genuinely constitutes multi-factor authentication when both the transaction and the authentication happen on the same device.

Does OTP Authentication Offer An Outdated User Experience?

The pandemic accelerated demand for seamless, remote-first user experiences. OTP authentication lags behind on several dimensions.

Active Process

OTP authentication requires action from the user: retrieving a device, opening an app, or switching between screens. TOTP’s 30-second expiry window means users who don’t move quickly enough face authentication failures, adding friction and sometimes causing abandonment entirely.

A Lack of Inclusivity

OTP authentication assumes users have a mobile device, a working phone number, and the ability to follow multi-step instructions across different screens. Not everyone does. For users with certain disabilities, older demographics, or those without consistent mobile access, OTP-based security creates genuine exclusion barriers. Biometric verification also requires a device but eliminates the need for a working phone number and reduces cognitive steps to a single, passive action, aligning with WCAG 2.2 AA.

OTP vs Biometric Authentication: A Comparison

Whereas OTP authentication satisfies the possession factor (“what you have”), biometric authentication satisfies the inherence factor — “what you are.” Your face cannot be lost, stolen, or ported to another number.