SMS has become an essential part of everyday digital communication. Businesses use SMS for OTP verification, account registration, login authentication, payment alerts, and important notifications.

However, the growing use of SMS has also brought an unwanted problem: SMS bombing.

SMS bombing occurs when someone repeatedly triggers SMS messages to be sent to a person's mobile number without their consent. This can result in hundreds of unwanted OTPs, verification messages, and notifications arriving within a short period.

It can be frustrating for individuals and can also create operational and financial problems for businesses.

SMS bombing, sometimes called SMS flooding, involves repeatedly triggering SMS requests to a mobile number through online services, applications, or other messaging systems.

A victim may suddenly start receiving:

• • Multiple OTP and verification messages.
• • Repeated account registration notifications.
• • Unwanted login and password-reset codes.
• • Notifications from different websites and applications.
• • Large volumes of unexpected promotional or transactional messages.

Even when individual messages are legitimate, receiving them repeatedly can disrupt normal mobile communication.

SMS bombing typically involves abusing publicly accessible forms, applications, and SMS-enabled services that allow users to request OTPs, verification codes, or notifications.

Attackers may exploit these services in several ways:

1. Abusing OTP Request Forms

Many websites and applications allow users to request OTPs for login, registration, password recovery, and mobile-number verification. Attackers repeatedly submit a target's mobile number through these forms, triggering multiple SMS messages.

2. Automating Repeated Requests

Instead of submitting requests manually, attackers may use automated scripts or tools to send repeated requests to SMS-enabled websites and applications.

3. Exploiting Multiple Websites and Services

Some SMS bombing attempts involve triggering messages from multiple websites and applications. Since each service sends messages independently, the victim may receive SMS messages from many different sender IDs.

4. Using Multiple IP Addresses or Networks

Attackers may distribute requests across different IP addresses, networks, or devices to make detection more difficult and bypass basic IP-based restrictions.

5. Targeting Weakly Protected SMS APIs

Applications with insufficient request validation, authentication, or rate limiting may allow automated systems to trigger excessive SMS requests.

SMS bombing is not just an inconvenience for mobile users. Businesses operating websites, mobile applications, and OTP-based authentication systems can also be affected.

Some common consequences include:

1. Increased SMS Costs

Repeated OTP requests can increase SMS consumption and create unexpected messaging expenses.

2. OTP Abuse

Automated requests may repeatedly trigger OTP generation, putting unnecessary pressure on authentication systems.

3. Service Disruption

A large number of requests can affect application performance, API availability, and legitimate customer registration or login workflows.

4. Customer Experience Issues

Customers may receive too many messages, become confused, or lose confidence in the business.

5. Infrastructure Abuse

Unprotected SMS endpoints can be exploited to generate unnecessary traffic and consume application resources.

Businesses should implement multiple layers of protection rather than relying on a single security mechanism.

1. Implement Rate Limiting

Restrict how frequently an OTP or SMS can be requested by the same mobile number.

For example, introduce a minimum waiting period between consecutive OTP requests and enforce reasonable request limits.

2. Introduce CAPTCHA Verification

Use CAPTCHA or other bot-detection mechanisms before allowing sensitive SMS requests, especially during account registration and password recovery.

3. Apply IP-Based Restrictions

Monitor repeated requests from the same IP address and temporarily restrict suspicious traffic.

However, IP-based restrictions should be implemented carefully because multiple legitimate users may share the same network.

4. Add Temporary Request Blocking

If a mobile number receives an unusually high number of OTP requests, temporarily prevent additional messages from being triggered.

5. Monitor SMS Activity

Monitor unusual SMS volumes, repeated requests, failed authentication attempts, and unexpected increases in delivery costs.

6. Protect API Endpoints

Never expose unrestricted SMS-sending functionality through publicly accessible endpoints. Validate requests on the server, authenticate API calls, and enforce appropriate usage limits.

7. Use Layered Abuse Prevention

Combine rate limiting, CAPTCHA, device or session checks, IP monitoring, and mobile-number-based restrictions. No single mechanism can prevent every form of abuse.

If your mobile number suddenly starts receiving a large number of OTPs, verification codes, and other unwanted SMS messages, there are several steps you can take to reduce the disruption and report the activity.

1. Register Your Number with TRAI DND

In India, you can register your mobile number under the National Customer Preference Register (NCPR), commonly known as the Do Not Disturb (DND) service.

You can:

• • Register your preferences through the TRAI DND app.
• • Call 1909 or send START 0 by SMS to 1909.
• • Choose to block promotional communications completely or select specific categories.
• • Modify your preferences whenever required.

Important: DND primarily controls unsolicited commercial communications. It does not stop legitimate transactional OTPs, authentication messages, or every SMS bombing attempt.

2. Enable Do Not Disturb on Mobile

If you are receiving a large number of unwanted SMS notifications, enable your phone's Do Not Disturb mode to reduce interruptions.

• • Android: Open Quick Settings, then tap Do Not Disturb. If it isn't visible, look under Settings → Notifications → Do Not Disturb.
• • iPhone: Open Control Centre, tap Focus, and select Do Not Disturb. You can also configure it under Settings → Focus → Do Not Disturb.

You can customize exceptions to allow calls or notifications from selected contacts.

Important: Do Not Disturb only silences notifications. It does not stop SMS messages from arriving, block the sender, or prevent SMS bombing. For that, use spam filtering, block/report options, and contact your mobile operator if the messages continue.

3. Enable Spam Protection on Android

If you use Google Messages on Android:

• • Open the Google Messages app.
• • Tap your profile picture.
• • Select Messages settings.
• • Open Protection & Safety → Spam and scam protection.
• • Make sure spam protection is enabled.

You can also report individual conversations:

• • Press and hold the unwanted SMS conversation.
• • Tap Block.
• • Select Report spam and confirm.

Reported conversations are moved to the Spam & blocked folder, and the sender is blocked.

4. Filter Unknown Senders on iPhone

iPhone users can reduce interruptions from unknown senders:

• • Open the Messages app.
• • Tap the Filters option.
• • Open Manage Filtering.
• • Enable Screen Unknown Senders.
• • Review messages from unknown numbers in the filtered section.

You can also report unwanted messages and block individual senders.

Note: Filtering generally moves messages into separate folders rather than preventing every SMS from reaching the phone.

5. Block and Report Suspicious Senders

If you repeatedly receive messages from the same sender or number:

• • Open the SMS conversation.
• • Use your phone's Block or Report Spam option.
• • Report suspicious commercial messages through your telecom operator or the TRAI DND app.
• • Avoid clicking links in unexpected messages.

You can learn more about SMS sender IDs and how to identify them in our SMS Sender ID Guide. For official sender-header information, visit TRAI's Header Information Portal.

6. Report Unsolicited Commercial SMS to 1909

If you receive unsolicited commercial messages that violate your registered DND preferences, report them to your telecom operator.

TRAI provides complaint and reporting facilities through:

• • Calling 1909.
• • Sending an SMS to 1909.
• • Using the TRAI DND app.

TRAI's complaint process specifies reporting within three days of receiving the unsolicited commercial communication. This is for UCC complaints, not necessarily every unwanted OTP or transactional message.

7. Contact Your Telecom Operator During an SMS Flooding Incident

If you suddenly receive hundreds of messages within a short period:

• • Contact your mobile network operator's customer support.
• • Explain that your number appears to be experiencing SMS flooding.
• • Ask whether they can investigate or apply any available network-level filtering.
• • Keep screenshots and timestamps of the incoming messages.
• • Ask for a complaint or service request reference number.

Blocking individual senders may not be effective when messages originate from many different services or sender IDs.

8. Protect Your Accounts

Receiving OTP messages does not automatically mean someone has accessed your accounts. However, repeated unexpected OTPs can indicate attempts to trigger authentication requests.

• • Never share OTPs with anyone.
• • Do not approve login or authentication requests you did not initiate.
• • Check important accounts for unfamiliar login activity.
• • Change passwords if you suspect account compromise.
• • Contact the relevant service provider if OTP requests continue.

Blocking individual senders may not be effective when messages originate from many different services or sender IDs.

An important reminder: DND registration and phone-level spam filters can reduce unwanted communications and interruptions, but neither guarantees protection against SMS bombing. The most effective response combines reporting, phone-level filtering, telecom-operator assistance, and account security.

• TRAI - DND and Unsolicited Commercial Communication

• TRAI - Customer Protection Tips

• TRAI DND App and Other Portals

SMS bombing is a problem that requires attention from individuals, businesses, application developers, and SMS service providers.

For businesses, protecting SMS endpoints is just as important as ensuring successful message delivery. For individuals, understanding OTP security and reporting repeated unwanted messages can help reduce the impact.

As businesses increasingly depend on SMS for authentication and communication, responsible SMS delivery and abuse prevention should be treated as essential parts of digital security, not optional features.

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