Key Takeaways
- Security hardening for a gateway server means least-privilege keys, verified webhooks, locked-down hosts — not exploit PoCs.
- Keep Bearers in env/vault; rotate on staff change; never in mobile clients.
- Verify X-SmsGateway-Signature; dedupe event ids.
- The Android phone remains part of the trust boundary: physical access and OEM updates.
- Platform Free pauses at the SMS allowance. You still pay operator airtime.
Security hardening checklist for gateway server is the defensive punch list for hosts that call the Android SMS Gateway API. Security & trust · API keys in env. Docs: Developer Center. Baseline: OWASP.
Devices and send volume. You fund the SIM.
Least privilege first. Clever bypass write-ups do not belong in this checklist.
What “gateway server” means here
Your backend, worker, or automation host that holds the Bearer and posts JSON. Not a guide to attacking the control plane or the handset.
If a step only helps an attacker, delete it. This list is for defenders who ship OTP.
Hardening checklist
| Control | Pass criteria |
|---|---|
| API key storage | Env/vault only; not in git or mobile apps |
| Key rotation | On hire/fire; staging ≠ production |
| TLS | HTTPS to control plane; no cleartext tokens |
| Webhook auth | Verify HMAC signature |
| Idempotency | Stable keys; no retry storms |
| OTP allowlist | Staging numbers until DLR is clean |
| Logging | No OTP digits or Bearers in logs |
| Host patching | OS + deps on a calendar |
API keys and env
One key per environment. Revoke leaked keys immediately. Secure keys guide.
Webhook HMAC
Reject unsigned bodies. Unique-constrain event ids so replays do not flip state twice. Server webhooks.
Network and hosts
Private workers where you can; lock admin panels; no public debug endpoints that echo secrets.
Handset side still matters
Physical custody, trusted APK source, screen lock. A hardened API with an unlocked lobby phone is not hardened. OTP use case.
Next steps
Walk the table with eng + ops; open tickets for every red cell before the next OTP launch. Downloads. Free: 300 SMS lifetime. No Unlimited SMS titles.
Related product pages
Jump to the live product docs for this topic—not another long-form article.
- SMS API documentationLive endpoint reference
- device and SMS volume pricingPlans and allowances
- Security and Trust CenterCompliance and posture
- Android SMS gateway product guideDefinition, product, and how to buy
