What Is SMS Pumping and How Can You Protect Your OTP Budget?

Businesses use one-time passwords to verify phone numbers, register customers, authenticate logins, and support password recovery.

However, every public workflow that can trigger an SMS message may become a target when automated programs are allowed to request verification codes without sufficient controls.

This type of abuse is commonly known as SMS pumping or artificially inflated traffic—AIT. It can increase messaging costs, distort product analytics, consume technical resources, and affect legitimate users.

What Is SMS Pumping?

SMS pumping is the artificial generation of SMS traffic through forms, applications, or APIs that automatically send messages.

An attacker may use bots to submit large numbers of phone numbers to an OTP, registration, login, password-recovery, or app-download workflow.

In some scenarios, the attacker targets numbers or mobile networks from which they can benefit financially. In other cases, the objective may be to exhaust a business’s messaging budget or disrupt its service.

How Does an SMS Pumping Attack Work?

Consider an application that sends an OTP immediately after a phone number is submitted.

An automated script may:

1. Generate or obtain ranges of phone numbers.
2. submit repeated requests to the registration endpoint.
3. Rotate IP addresses, sessions, or device attributes.
4. Trigger thousands of SMS messages.
5. Avoid completing verification or account creation.

The messaging platform processes real send requests, but the traffic produces no legitimate users or transactions.

SMS Pumping Versus OTP Theft

SMS pumping and OTP theft are different problems.

SMS pumping primarily aims to generate traffic and increase cost. The attacker does not necessarily need to read the code or access a customer account.

OTP theft aims to obtain a legitimate user’s verification code through phishing, social engineering, interception, or another method, and then use it to access an account.

Both may target authentication workflows, but their indicators and incident-response procedures differ.

Why Are Registration Workflows Common Targets?

Registration, login, password recovery, and resend-code endpoints often send a message before the user has an established or verified account.

AWS describes AIT attacks as bot activity that particularly targets workflows that trigger immediate SMS sends, including login and sign-up processes. Google also identifies SMS-based two-factor authentication and phone verification as potential targets for SMS toll fraud.

Warning Signs of SMS Pumping

A sudden increase in messages does not automatically prove fraud. It should be investigated alongside other signals, including:

• OTP sends increasing without a similar rise in successful verification.
• Traffic concentrating in one country, carrier, or number prefix.
• Repeated requests from the same IP, device, account, or session.
• High resend activity without code submission.
• Unusual traffic during low-activity hours.
• Increasing errors or invalid numbers in specific ranges.
• Unexpected hourly or daily messaging costs.
• Rapid changes in destination patterns.

Provider-side fraud systems commonly evaluate historical traffic, destination fluctuations, message volumes by number or prefix, and error patterns associated with phone-number ranges.

How SMS Pumping Affects a Business

Direct messaging costs

Every request that reaches the messaging provider may create a billable message, even when it does not represent a real customer.

Friction for legitimate users

Emergency restrictions introduced during an attack may block or delay real customers if they are not carefully targeted.

Distorted analytics

Registration and verification-request numbers may appear to grow while completion rates fall, leading to misleading product and marketing conclusions.

Operational overhead

The attack consumes API capacity, database operations, logs, monitoring resources, and staff time.

How to Protect an OTP Workflow

Effective protection is layered. Application controls, destination policies, risk analysis, provider capabilities, and incident procedures should work together.

Apply rate limits at multiple levels

A limit per phone number is not sufficient because an attacker can rotate across thousands of destinations.

Consider separate limits for:

• Phone number.
• IP address.
• Account.
• Device or session.
• Country.
• Number prefix.
• API endpoint.

Introduce a resend cooldown and maximum hourly and daily requests. Thresholds should be based on the application’s legitimate behavior rather than copied from another service.

Some verification platforms support programmable API-layer rate limits that block a request before channel delivery occurs.

Protect forms from automated traffic

Use bot-risk assessment or an appropriate challenge before the application calls the SMS API. Always validate the result on the server.

Cloudflare’s Turnstile documentation describes a browser-generated token that must be validated by the server. Google’s risk-assessment services can also provide a score that a backend uses when deciding whether to allow an SMS request.

Bot protection should not be the only control. It is stronger when combined with rate limits, geographic restrictions, destination analysis, and behavioral signals.

Restrict destination countries

A Saudi-focused service does not normally need to send OTP messages to every country.

Use an allowlist for supported markets and introduce additional checks for unusual or high-cost destinations. AWS recommends blocking countries where a business does not operate and filtering destinations associated with higher cost or SMS pumping concerns.

Use risk-based decisions

Evaluate the full request rather than treating every user identically.

Higher-risk signals may include a new device, unusual location, rapidly changing phone numbers, repeated requests without successful verification, and a sudden concentration within a specific prefix.

Low-risk requests may proceed normally. Medium-risk requests can receive an additional challenge. High-risk requests can be blocked or manually reviewed.

Control the OTP lifecycle

An OTP should be:

• Valid for an appropriately short period.
• Accepted only once.
• Connected to a specific session or transaction.
• Limited to a defined number of attempts.
• Invalidated after successful verification.

A resend request should not create unlimited new messages. AWS includes a short OTP time-to-live among its SMS security best practices.

Monitor volume, cost, and conversion

Track:

• Messages by hour and day.
• Messages by country, carrier, and prefix.
• Successful verifications divided by messages sent.
• Resend frequency.
• Requests by IP, device, and account.
• Cost against expected budgets.

Create alerts and an emergency circuit breaker for abnormal volume or spend.

Separate OTP traffic

Where possible, use separate API credentials, reports, budgets, and controls for OTP traffic. This makes abnormal verification traffic easier to isolate without affecting order notifications and other operational messages.

What Should You Do During an Active Attack?

Identify the affected endpoint, country, prefix, traffic source, and time window. Then apply targeted controls:

1. Tighten rate limits.
2. Disable unused countries.
3. Challenge suspicious requests.
4. Temporarily block repeated IPs, devices, and prefixes.
5. Apply an emergency volume or spending limit.
6. Preserve message IDs, timestamps, and request logs.
7. Contact the messaging provider for destination analysis.
8. Monitor the impact on legitimate customers.
9. Conduct a post-incident review.

Avoid disabling all OTP traffic unless necessary. Isolating the affected destination or workflow can preserve access for legitimate users.

Common Mistakes

Common mistakes include limiting only by phone number, applying controls in the browser rather than on the server, allowing all countries by default, and generating a new message for every resend click.

Businesses should also avoid relying on CAPTCHA alone, discovering fraud only after reviewing the invoice, or using one unrestricted API credential for every messaging use case.

Developer Checklist

Before launching an OTP workflow, confirm that it includes:

• Phone-number normalization and validation.
• A country allowlist.
• Limits by number, IP, device, account, and session.
• A resend cooldown.
• A maximum attempt count.
• Short-lived, single-use codes.
• Server-validated bot protection.
• Send-to-verification conversion monitoring.
• Volume, destination, and cost alerts.
• Audit logs for blocks and configuration changes.
• A documented incident-response process.
• Security testing after every major workflow update.

How OurSMS Supports OTP Integration

OurSMS provides OTP API and SMS API services for applications and websites, together with API credentials, delivery-status reporting, and Webhooks. Businesses can use this delivery information alongside their internal verification results to build more accurate monitoring.

Preventing SMS pumping remains a shared responsibility across the application, the messaging provider, and the security and operations teams.

Before launching a high-volume OTP workflow, discuss supported countries, expected traffic, access controls, reporting, and escalation procedures with the OurSMS team.

Conclusion

SMS pumping is not simply an increase in message volume. It is the abuse of a verification workflow in a way that can consume budget, distort data, and affect legitimate customers.

Protection begins before the SMS API is called. Validate the request, enforce multiple limits, restrict destinations, monitor conversion, and prepare an incident-response plan.

The earlier suspicious traffic is identified, the fewer fraudulent messages are sent and the easier the incident becomes to contain.