SMS send routes are rate-limited

SMS is the most expensive metered communication channel in a typical web stack — Twilio charges $0.0079–$0.02 per outbound message, and there is no free tier for abuse. An SMS route without per-IP rate limiting lets an attacker trigger thousands of messages per minute, reaching four-digit charges before any alert fires. CWE-799 (Improper Control of Interaction Frequency) is the direct mapping, alongside OWASP A05:2021. Beyond direct billing, carriers flag high-volume spammy senders for grey-listing or number suspension, destroying your transactional SMS deliverability for legitimate users. This is a cost bomb that also breaks a security-critical channel (OTP delivery).

Add per-IP rate limiting at the route level before the Twilio client is called. Three sends per hour per IP is generous for legitimate OTP flows and stops abuse cold.

import { Ratelimit } from '@upstash/ratelimit'
import { Redis } from '@upstash/redis'
import twilio from 'twilio'

const limiter = new Ratelimit({
  redis: Redis.fromEnv(),
  limiter: Ratelimit.fixedWindow(3, '1 h'),
})

const client = twilio(process.env.TWILIO_SID!, process.env.TWILIO_AUTH_TOKEN!)

export async function POST(req: Request) {
  const ip = req.headers.get('x-forwarded-for') ?? 'anon'
  const { success } = await limiter.limit(`sms:${ip}`)
  if (!success) return Response.json({ error: 'Rate limited' }, { status: 429 })
  const { to, body } = SMSSchema.parse(await req.json())
  await client.messages.create({ to, from: process.env.TWILIO_FROM!, body })
  return Response.json({ sent: true })
}

• ID: ai-slop-cost-bombs.unrate-limited-spend.sms-rate-limited

• Severity: high

• What to look for: When an SMS library is in package.json dependencies (twilio, @aws-sdk/client-sns, messagebird, vonage, @plivo/server-sdk, bandwidth-rest-api), walk source files for SMS send call sites: .messages.create(, .sendMessage(, client.messages.create(. Count all SMS send call sites and verify each is preceded by a rate-limit check OR is inside a route handler that imports a rate limiter.

• Pass criteria: 100% of SMS send call sites are either gated by a rate limiter OR are part of a transactional flow (1 SMS per user action). Report: "X SMS send call sites inspected, Y rate-controlled, 0 unbounded."

• Fail criteria: At least 1 SMS send call has no upstream rate limiter.

• Skip (N/A) when: No SMS library in dependencies.

• Detail on fail: "1 unbounded SMS handler: app/api/sms/route.ts calls twilio.messages.create() with no rate limit — at $0.01/SMS, an attacker can run up $1,000+ in minutes"

• Remediation: SMS is even more expensive than email. A small rate limit per IP is mandatory:

  import { Ratelimit } from '@upstash/ratelimit'
  import { Redis } from '@upstash/redis'
  
  const limiter = new Ratelimit({
    redis: Redis.fromEnv(),
    limiter: Ratelimit.fixedWindow(3, '1 h'),  // 3 SMS per hour per IP
  })
  
  export async function POST(req: Request) {
    const ip = req.headers.get('x-forwarded-for') ?? 'anon'
    const { success } = await limiter.limit(ip)
    if (!success) return Response.json({ error: 'Rate limited' }, { status: 429 })
    await twilio.messages.create({ to: ..., from: ..., body: ... })
    return Response.json({ sent: true })
  }