What You Are Actually Defending Against
SMS gateways carry OTPs, transaction alerts and personal data, which makes them a target on three fronts: attackers who steal API credentials to send fraud messages on your account, organised SMS pumping schemes that drain your OTP budget, and data exposure of the phone numbers and message content you store. Twitter famously estimated it was losing around 60 million dollars a year to SMS pumping fraud before restricting SMS 2FA in 2023 — and Indian OTP endpoints see the same attack pattern daily at smaller scale.
Here are the security features that actually matter when you evaluate or configure an SMS gateway, and why each one exists.
1. Transport and Storage Encryption
- TLS 1.2 or 1.3 on every API endpoint. Any provider still accepting plain HTTP API calls is exposing your API key and message content to interception. Check this in one minute with a curl request to the http:// version of the API URL — it should redirect or refuse.
- AES-256 encryption at rest for stored messages and contact lists on the provider side.
- PII masking in logs and dashboards: phone numbers displayed as 98XXXXXX21 and OTP values never written to plain-text logs. Ask your provider how long full message content is retained; 30–90 days is typical, and shorter is better for OTP traffic.
2. API Credential Controls
- Separate API keys per application so a leaked key from a test project cannot send from your production sender ID, and revoking one does not break everything else
- Key rotation without downtime — you should be able to issue a new key, migrate, then revoke the old one
- IP whitelisting so API calls are only accepted from your servers. This single feature neutralises most stolen-key incidents, because the attacker's requests come from their infrastructure, not yours.
- Scoped permissions: a key used only for OTP sending should not be able to export contact lists or view account billing
3. Portal Access Security
The web dashboard is attacked more often than the API, usually via reused passwords. Non-negotiables:
- Two-factor authentication on login
- Role-based access control — campaign managers should not see API keys, and finance users should not send messages
- Session timeouts and forced logout on password change
- Login history visible to the account owner, with alerts for new devices or locations
4. OTP Abuse and SMS Pumping Protection
SMS pumping works like this: fraudsters use bots to trigger thousands of OTP requests to premium or revenue-sharing number ranges they control, and pocket a cut of the termination fee while you pay for every message. Defences worth insisting on, on both the gateway side and in your own code:
- Rate limiting per phone number: no more than 3–5 OTPs per number per hour, with exponential cool-down
- Rate limiting per IP and per session on your signup and login forms, plus a CAPTCHA after the first retry
- Velocity alerts: automatic flagging when traffic to a specific number series or circle suddenly spikes against your baseline
- Destination controls: if you only serve Indian customers, block international destinations at the account level — most pumping traffic targets foreign ranges
- Short OTP validity (3–5 minutes) and a maximum of 3 verification attempts per code
A gateway that offers per-account spending caps and anomaly alerts turns a potential lakh-rupee fraud incident into a capped, contained one. These controls come built into our OTP service, where abnormal request patterns are throttled before they burn your balance.
5. Webhook and Callback Integrity
Delivery reports arrive at your server as HTTP callbacks. If your endpoint accepts any POST that looks right, an attacker can forge delivery statuses or probe your systems. Look for signed webhooks (an HMAC signature header you verify with a shared secret) or at minimum a source-IP allowlist for callback traffic, and always serve the callback URL over HTTPS.
6. Compliance as a Security Layer
TRAI DLT Registration
India's DLT framework is often treated as red tape, but it is genuinely a security control: only registered headers can send, only approved templates pass scrubbing, and every message is traceable to a verified entity. TRAI's traceability mandate (enforced from late 2024) requires the full path from sender to operator to be declared, which killed off many grey routes overnight. If your entity, headers and templates are not registered, start with DLT registration — unregistered traffic is simply dropped.
DPDP Act 2023
India's Digital Personal Data Protection Act treats phone numbers as personal data. Practical implications for SMS senders: collect and store proof of consent for promotional messaging, honour opt-outs promptly, and have a data-deletion process. Penalties for serious breaches run up to ₹250 crore, so "we kept every number we ever collected" is now a liability, not an asset.
Certifications That Mean Something
ISO 27001 (audited information security management) and SOC 2 Type II (controls verified over months, not a point-in-time snapshot) are the two worth asking for. A certificate PDF on request is a reasonable test of any provider's transparency.
7. Audit Logs You Can Actually Use
When something goes wrong, you need to answer "who sent what, when, from where" in minutes. A useful audit trail records every message submission with API key and source IP, every login and permission change, every template and sender ID modification, and every export of contact data — retained for at least 12 months and exportable, so your own security team can ingest it.
Frequently Asked Questions
What is SMS pumping fraud and how do I know if it is happening to me?
SMS pumping is artificially inflated traffic: bots trigger OTP or verification SMS to number ranges the fraudster profits from. Warning signs are a sudden spike in OTP requests with an unusually low verification rate, clusters of sequential destination numbers, and traffic to circles or countries where you have no users. Per-number rate limits, CAPTCHAs and destination blocking stop most of it.
Is OTP over SMS still secure enough in 2026?
For most consumer applications in India, yes — provided you use short expiry codes, limited attempts, and rate limiting. SMS OTP is vulnerable to SIM-swap attacks in targeted, high-value scenarios, which is why banks layer it with device binding. For typical login and transaction verification, a well-configured SMS OTP flow with WhatsApp or voice fallback remains the most accessible second factor for Indian users, including the large feature-phone base.
Does DLT registration improve security or is it only compliance?
Both. Because DLT scrubbing rejects any message whose header and template are not registered to a verified entity, it prevents third parties from spoofing your sender ID through legitimate routes, and it gives regulators traceability when fraud is reported. Combined with your gateway's access controls, it materially reduces brand-impersonation risk.