Why do SMS-based verification flows create fraud and cost risk?

Why This Matters for Security Teams

SMS verification looks simple, but the security control is also a billable transport. When attackers can trigger messages at scale, the cost is no longer incidental. It becomes an abuse path tied directly to authentication traffic, which makes fraud teams, IAM owners, and application security teams share the same failure domain. This is why current guidance increasingly treats verification flows as an availability, fraud, and abuse-prevention problem, not just a login convenience issue.

The broader lesson mirrors what NHI programmes already show: identity systems become high-value cost and risk surfaces when they are easy to trigger and hard to govern. NHIMG’s Ultimate Guide to NHIs notes that 80% of identity breaches involved compromised non-human identities such as service accounts and API keys, which is a useful reminder that identity abuse often follows the path of least resistance. The same pattern appears in SMS flows: the mechanism that proves reachability can also be weaponised as a spend generator. The NIST Cybersecurity Framework 2.0 reinforces the need to manage identity-related abuse as a governance issue, not only a technical one. In practice, many security teams discover SMS flooding only after finance flags the bill rather than through deliberate abuse monitoring.

How It Works in Practice

SMS-based fraud usually starts with automation. An attacker scripts sign-up, password reset, or second-factor requests against a target phone number list, then uses message volume, carrier fees, and premium-rate routing to turn authentication into a cost sink. Even if the attacker never receives the code, the organisation still pays to send it. That is why SMS abuse is often classified as an authentication abuse pattern and a telecom cost-control problem at the same time.

Good defensive design reduces both the trigger rate and the economic value of each trigger. Practitioners increasingly use layered controls rather than relying on SMS alone:

• Rate-limit by account, device, IP range, ASN, and destination number.
• Add bot detection and risk scoring before sending any message.
• Challenge suspicious flows with CAPTCHA, proof-of-work, or step-up checks.
• Block known premium-rate, high-risk, or geographically inappropriate destinations.
• Use short-lived verification windows and suppress repeated resend attempts.
• Monitor message volume, spend anomalies, and carrier-level delivery patterns in real time.

For organisations modernising identity controls, the OWASP NHI Top 10 is useful because it frames identity abuse as an operational risk surface, not a point-in-time authentication event. The same governance mindset applies here: if a verification flow can be triggered without strong anti-abuse controls, it will eventually be used as a resource-exhaustion channel. The Top 10 NHI Issues also highlights how identity weaknesses compound when they are left unbounded and poorly monitored. These controls tend to break down in consumer apps with high-volume onboarding or reset traffic because legitimate peak activity and malicious automation look very similar without strong behavioural signals.

Common Variations and Edge Cases

Tighter SMS controls often increase friction for legitimate users, so organisations must balance fraud reduction against conversion loss and support burden. That tradeoff is especially visible in retail, fintech, and travel flows where users expect fast verification and churn rises quickly when step-up checks feel invasive.

There is no universal standard for when SMS should be removed entirely. Current guidance suggests treating it as a fallback, not a primary trust anchor, especially for high-risk actions. Some organisations keep SMS for account recovery but pair it with device binding, velocity controls, and stronger primary factors such as passkeys or authenticator apps. Others restrict SMS to low-risk notifications and reserve stronger checks for money movement, profile changes, or reset events.

Edge cases also matter. Prepaid SIMs, number recycling, roaming, shared family plans, and VOIP routing can distort risk signals and create false positives. In high-volume environments, organisations should align SMS abuse monitoring with fraud analytics and carrier telemetry rather than relying on IAM logs alone. NHIMG’s Key Challenges and Risks section is relevant here because it shows how identity governance fails when lifecycle, visibility, and revocation are incomplete. SMS becomes safest when it is one signal among many, not the control that carries the entire trust decision.

Related resources from NHI Mgmt Group

• Why do OTP-based verification flows attract traffic pumping fraud?
• Why do automated SMS verification attacks create outsized financial risk?
• Why do SMS-based MFA flows create more risk than TOTP in custom auth systems?
• Why do browser-based identity attacks create more risk than browser exploitation in many enterprises?